Raisis, A. and Musk, G. (2012) Emergency neuroanaesthesia ... · disease is to preserve neuronal...

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Raisis, A. and Musk, G. (2012) Emergency neuroanaesthesia. In:

Platt, S. and Garosi, L., (eds.) Small Animal Neurological Emergencies. Manson Publishing Ltd, London, pp. 535-555.

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INTRODUCTION

This chapter discusses the considerations and techniquesfor anaesthetizing animals with neurological disease.The chapter will cover intracranial disease, spinal diseaseand NM disease. A thorough understanding of thepathophysiology of diseases affecting these regions of thenervous system is essential to select the most appropriateanaesthetic technique and drug combination.

INTRACRANIAL DISEASE

ConsiderationsThe main aim of anaesthesia in animals with intracranialdisease is to preserve neuronal function. Normal neuro-nal function depends on maintaining adequate CBF.

CBF regulation is complex and will not be describedin detail in this chapter (for more details see Furtherreading). Put simply, CBF is maintained if CPP is main-tained. The CPP represents the difference betweenMAP and ICP. The considerations for maintaining CBFand minimizing neuronal injury during anaesthesia willbe discussed with reference to the stages of the anaes-thetic procedure (stabilization, induction, maintenanceand recovery from anaesthesia). This is summarized inTable 99 (p. 537).

Stabilization prior to anaesthesiaAny patient with increased ICP (404), regardless of thecause, requires stabilization before considering anaes-thesia or sedation for further diagnostics.

Correction of hypoxaemia, hypercapnia and poorperfusion are the most important strategies for reducingICP and stabilizing the patient prior to anaesthesia.(For specific details on supporting the respiratory andcardiovascular systems see Chapter 2.)

Specific management of increased ICP is indicated ifdeterioration of neurological status occurs rapidly orcontinues despite normal oxygenation, ventilation andperfusion. (For management of increased ICP seeChapter 20.)

Sedation In animals with clinical signs of intracranial disease, the performance of procedures under heavy sedation is generally avoided. Heavy sedation may lead to excessivedepression of cardiovascular and respiratory function,

535

SPECIFIC MANAGEMENT ISSUES Chapter 29

! 404 Animals with intracranial disease are at risk ofincreased ICP and subsequent brain herniation (as shownon this sagittal T2-weighted MR image [arrow]) ifappropriate stabilization is not performed prior toanaesthesia. (Photo courtesy Victoria Johnson)

EMERGENCY NEUROANAESTHESIA

Anthea Raisis & Gabrielle Musk

which will exacerbate secondary neuronal injury (405). In addition, heavy sedation may interfere with accurateassessment of the neurological status of the animal anddelay initiation of appropriate therapy.

In these cases, anaesthesia performed carefully with a good understanding of how to minimize detrimentaleffects is preferable, as this allows protection of theairway and control of ventilation. In addition, manyanaesthetic agents, such as propofol and barbiturates,have the added benefit of reducing cerebral metabolicrate, which helps reduce neurological injury.

Premedication The aims of premedication are to:• Reduce stress and anxiety. • Decrease the amount of anaesthetic induction and

maintenance agents required. • Provide analgesia.

Agents used for premedication should have minimaleffects on cerebral perfusion and ICP. The advantagesand disadvantages of different agents in animals with

intracranial disease are detailed in Table 100 (p. 539).Drugs such as acepromazine and medetomidine are bestavoided in animals with intracranial disease.

Opioids provide analgesia and varying degrees ofsedation without affecting cerebral perfusion or ICP.Adverse effects associated with a decrease in heart rateand respiratory depression can be avoided by using con-servative doses. Morphine administration is best avoidedin patients with high ICP given its significant potentialfor causing vomiting (and transient increases in ICP).

Benzodiazepines may be useful as anxiolytics in criti-cally ill animals. These agents should be used cautiouslyin animals with mild disease/minimal decreases in men-tation, as the effects can be unreliable and excitement,dysphoria and disinhibition can occur.

Phenobarbital is the drug most often used to controlseizures. The authors have found that the administrationof phenobarbital at 2–3 mg/kg intramuscularly can be auseful premedicant in anxious dogs when used in con-junction with an opioid 30 minutes prior to induction ofanaesthesia.

Adjustment of dose rates of sedative andanaesthetic agents In animals with intracranial disease, damage to theblood–brain barrier and concurrent CNS depression dueto the neurological injury will serve to exacerbate theeffects of a given dose of anaesthetic or analgesic agent.As a result, the doses used in these patients should belower than those used in healthy patients. As the agentshave a rapid onset and short duration of action, the doserate can be adjusted incrementally (i.e. titrated to effect)until the desired analgesic effect is achieved, while minimizing side-effects.

Induction of anaesthesia A smooth and high-quality induction of anaesthesia isessential. Minimizing stress and struggling and main-taining oxygenation and ventilation during induction ofanaesthesia is necessary to prevent adverse effects on theCNS. The induction of anaesthesia using intravenousagents minimizes struggling and allows rapid control ofthe airway.

Pre-oxygenation is performed by mask, if tolerated,or by flow-by (406) for 5–10 minutes prior to inductionto minimize hypoxaemia during and immediatelyfollowing induction of anaesthesia.

SP E C I F I C MA N A G E M E N T IS S U E S536

! 405 Heavy sedation, as seen in this Bull Terrier, isgenerally avoided in patients with intracranial disease, dueto the risk of excessive cardiovascular and respiratorydepression.

EM E R G E N C Y NE U R O A N A E S T H E S I A 537

CONSIDERATION MANAGEMENT

Maintain adequate CPP (CPP = MAP – ICP)

Maintain haemodynamic stability

Ensure adequate ventilation and normocapnia (PaCO2 35–40 mmHg)

Maintain adequate oxygenation

Decrease CMR

Ensure adequate venous drainage

MAP = mean arterial blood pressure; ICP = intracranial pressure; CBF = cerebral blood flow; CMR = cerebral metabolic rate;PaCO2 = arterial carbon dioxide partial pressure.

Maintain MAP between 80 mmHg and 100 mmHg. Maintain normalcirculating blood volume. Minimize depressant effects of anaesthetic agentson cardiovascular function. Avoid/use carefully anaesthetic drugs thatinterfere with CBF autoregulation (e.g. volatile anaesthetics)

Avoid sudden increases in MAP and associated increases in ICP caused bystress, pain, surgical stimulation and laryngeal stimulation. Provide adequateanalgesia. Ensure adequate depth of anaesthesia before intubation

Avoid hypercapnia (PaCO2 >40 mmHg) and associated increased ICP.Use positive pressure ventilation during anaesthesia. Avoid hyperventilation(PaCO2 <30 mmHg) except in an emergency to avoid brain herniation. Donot decrease PaCO2 below 30 mmHg

Avoid hypoxaemia (PaO2 <80 mmHg) by providing oxygen supplementationduring induction, maintenance and recovery from anaesthesia

Select anaesthetic agents that decrease CMR (e.g. propofol). Avoid increasein CMR by preventing or controlling seizures, maintaining normothermia andavoiding anaesthetic agents that increase CMR (e.g. ketamine)

Avoid interference with jugular venous blood flow and associated venouscongestion and increased ICP. Avoid jugular obstruction, excessive airwaypressure during ventilation and fluid overload. Mild head elevation (15–30degrees) will encourage venous drainage

" 406 ‘Flow-by’ oxygen therapy is useful for providingoxygen supplementation to animals that will not tolerate amask over their face.

Table 99 Considerations for anaesthetizing animals with intracranial disease

Manual ventilation with a close-fitting mask duringinduction of anaesthesia (407) may be necessary to ensurenormocapnia until the anaesthetic depth is sufficient tominimize reflex responses (i.e. cough, increased heartrate and MAP) to endotracheal intubation. Once an oralETT has been positioned and secured in place, ventila-tion via the tube is commenced. (Note: When ventilatingvia a mask, oxygen can be forced into the stomach,leading to gastric distension. Should this occur, astomach tube can be passed once the animal is adequatelyanaesthetized and the stomach deflated.)

To maintain adequate cardiovascular and respiratoryfunction during induction of anaesthesia, the use ofshort-acting agents that can be carefully titrated to effect without excitation is preferred. Drugs that minimally interfere with regulation of cerebral perfusionare also preferred. The advantages and disadvantages ofdifferent intravenous anaesthetic agents in the neurolog-ical patient are provided in Table 100. Propofol is theagent the authors most frequently use in animals with

intracranial disease. The administration of ‘co-induction’agents with fewer depressant effects on the cardio-vascular and respiratory systems can be used to reducethe dose of the more depressant induction drugs. Co-induction agents (see Table 101, p. 540) are administeredimmediately prior to the induction agent and are usuallygiven intravenously.

To minimize coughing in response to endotrachealintubation, it is important to ensure that the depth ofanaesthesia is adequate. The depth of anaesthesiarequired to prevent a response to intubation is compara-ble to that required for major surgery. It is common prac-tice in cats to apply topical local anaesthetic (lidocaine) tothe larynx before intubation (408). This is particularlyeffective for preventing the autonomic response to intu-bation and it is useful for canine patients as well. Incanine patients, use of co-induction agents such as lido-caine (1–2 mg/kg) and opioids (e.g. fentanyl, 1–5 µg/kg)intravenously can also help reduce stimulation of thelarynx during intubation.


" 407 Ventilation via a tight-fitting mask may benecessary during induction to ensure adequate CO2removal and delivery of oxygen.

# 408 Application of topical lidocaine to the cat’s larynx,after an adequate depth of anaesthesia has been achieved.


Table 100 Intravenous sedatives and induction agents for use in animals with central nervous system disease

AGENT CBF DIRECT CMR ICP SEIZURE COMMENT DOSEREGULATION CARDIOVASCULAR ACTIVITY RATE*

EFFECTS

Acepromazine

Alpha-2 agonists

Opioids

Benzodiazepines

Lidocaine

Thiopentone

Propofol

Ketamine

CBF = cerebral blood flow; CMR = cerebral metabolic rate; ICP = intracranial pressure; NR = not reported; BP = blood pressure;HR = heart rate; CO = cardiac putput; VD = vasodilatation; VC = vasoconstriction.

* Note: These dose rates are based on those used in normal animals and lower doses may be required in animals with CNS disease.

NR

! Flow-metaboliccoupling

Normal

Normal

Normal

Normal

Normal

Normal

!! BP

" BP then !BP

! HR +/– ! BP

No directvascular effects

! BP: VD and ! CO

! BP: VD and!CO

! BP: VD and!CO

" HR and BP

NR

!

!

!

!

!!

!!

""

"

-

!

!

!

!!

!!

"

"

"

!

!!

Lowdose: !

Highdose: "

!

!

"

Avoid in intracranialdisease. Useful anxiolyticin spinal disease.Contraindicated in hypo-volaemic patients

Avoid in intracranialdisease. Useful in fractiousanimals. Care in hypovolaemic patients

Reduce the required doseof induction andmaintenance agents.Reduce response tointubation and surgicalstimulus

Possible sedative/anxiolytic. Reduceinduction agent.Potentiate respiratorydepression of other agents

Reduce response tointubation and extubation.Decrease seizures.Analgesia (see Chapter30)

Excitement in unsedatedanimals. Accumulates withrepeated dosing

Excitement-free induction.Suitable for maintenanceof anaesthesia in dogswith intracranial disease

Avoid in animals withintracranial disease.Avoid in animals at risk ofseizures (e.g. postmyelography)

0.01–0.05mg/kg IM

Dogs: 2–10µg/kg IM Cats: 5–20µg/kg IM

See Table 102andChapter 30

Diazepam,0.1–0.5 mg/kg IV;midazolam,0.1–0.5 mg/kgIV or IM

Co-induction:1 mg/kg IV

Up to 10mg/kg IV

Induction:up to 2–4mg/kg IV.Maintenance:0.2–0.4 mg/kg/minute

1–2 mg/kg IV


Table 101 Co-induction agents for use in animals with central nervous system disease

AGENT DOSE* AND TIMING COMMENTS

Butorphanol

Fentanyl

Diazepamor Midazolam

Lidocaine

*Lower end of dose range is recommended in critically ill animals.

0.1–0.2 mg/kg IV 1–2 minutesprior to induction

1–5 µg/kg IV 5 minutes prior toinduction

0.1–0.2 mg/kg IV immediatelyprior to induction

1–2 mg/kg IV 1–2 minutes prior toinduction

Potent antitussive. Mild analgesia only: avoid in surgicalor animals in pain. Antagonize effects of mu agonists

Bolus administration can cause significant bradycardia.Exacerbates respiratory depression of induction agent.Capnography and appropriate manual or mechanicalventilation should be commenced. Also useful forreducing the autonomic response to endotrachealintubation

Negligible cardiovascular depression. Can cause disinhibition and paradoxical excitement: follow withinduction agent immediately. May exacerbate respiratorydepression of induction agent. Capnography andappropriate manual or mechanical ventilation should becommenced

DO NOT USE IN CATS. Can exacerbate cardiovasculardepression of other agents. Also useful for reducing theautonomic response to endotracheal intubation

Table 102 Opioids used intraoperatively to control pain and stabilize anaesthesia

OPIOID ADVANTAGES DISADVANTAGES COMMON DOSE RATES*

Fentanyl

Alfentanil

Remifentanil

* Doses are a guide only and should be titrated on an individual patient basis.

Potent analgesia (full mu agonist).Short acting after bolus administration (15–20 minutes).Suitable for infusion

Potent analgesia (full mu agonist).Fast onset: 1 minute. Short durationafter bolus administration (5 minutes). Suitable for infusion

Potent analgesia (full mu agonist).Very short acting (1–2 minutes).Suitable for infusion. Duration ofaction is constant regardless ofduration of infusion

Marked respiratory depression athigher doses. Duration of actionincreases with duration of infusion

Marked respiratory depression atdoses used intraoperatively.Duration of action increases withduration of infusion

Marked respiratory depression atdoses used intraoperatively. Veryrapid recovery: additionalanalgesia required beforestopping infusion

Bolus: 1–2 µg/kg IVq15–20 minutes.CRI: 0.2–0.7 µg/kg/min

0.5–2 µg/kg/minute

0.2–0.7 µg/kg/minute

Maintenance of anaesthesia Following induction of anaesthesia it is essential to selectdrugs and apply techniques that will either decrease orminimally increase ICP.

Total intravenous anaesthesiaTotal intravenous anaesthesia (TIVA) with suitable agentsprovides the best conditions for the maintenance of anaes-thesia in the neurological patient, providing normocapnia(P’ETCO2 30–35 mmHg [4–4.7 kPa]) and systemic BPare maintained. TIVA can be achieved with variable rateinfusions of propofol (0.2–0.4 mg/kg/minute) or targetcontrolled infusion (TCI) of propofol (2.5–3.5 µg/ml ofblood) using specialized infusion equipment. Propofol isfrequently infused in combination with short-actingopioids (409) (see Table 102) to allow the dose of propofoland, therefore, its associated side-effects to be reduced.Additional information on TCI can be found in theFurther reading list for this chapter.

The use of opioids (e.g. fentanyl (0.2–0.7 µg/kg/minute, remifentanil 0.2–0.7 µg/kg/minute) in combina-tion with propofol allows the dose of propofol requiredfor maintenance of anaesthesia and, in turn, the cardio-vascular side-effects, to be reduced.

TIVA is the authors’ preferred technique for canineneurosurgical patients and unstable patients requiringdiagnostic imaging. The TIVA protocol utilizing propo-fol in cats is less well established and less commonly prac-tised. Cats are inefficient metabolizers of propofol,predisposing to prolonged recoveries. Their RBCs arealso more prone to the oxidative effects of the propofol(this may cause a Heinz body anaemia). Alfaxalone mayprove to be a suitable alternative, as it has a similar phar-macokinetic profile to that of propofol, thus making itideal for infusion. Infusion rates of alphaxalone currentlyused clinically in healthy animals range from 0.07–0.1mg/kg/minute. However, appropriate dose rates for usein neurological patients have not been determined andthe quality of recoveries in these patients is not known.

Inhalation anaesthesiaInhalation anaesthesia can be used for short anaesthetics instable neurological patients. It is preferred by many anaes-thetists for maintenance of anaesthesia in cats given theconcerns about using propofol by infusion. The character-istics of inhalation agents are summarized in Table 103(next page).

Sevoflurane and isoflurane have the least effect onICP providing the dose is minimized and the animals areventilated to normocapnia. As described below, infusionof short-acting opioids is a useful technique for reducingthe required dose of inhalation agent. Despite efforts toprevent herniation of the brain, there are still anecdotalreports of this life-threatening complication occurringwhen these agents are used to maintain anaesthesiaduring intracranial surgery in small animals. Othervolatile agents, such as halothane, desflurane and N2O,have a marked effect on ICP and are best avoided.

Maintain adequate ventilation and oxygenationTo maintain adequate ventilation and ensure normocap-nia (PaCO2 35–40 mmHg [4.7–5.3 kPa]; P’ETCO230–35 mmHg [4–4.7 kPa]), the use of IPPV and meas-urement of end tidal CO2 concentration breath by breathare essential.

To maintain oxygenation during diagnostic proce-dures and surgery it is not uncommon to use highinspired concentrations of oxygen during anaesthesia.For long-term ventilation the FiO2 is ideally adjusted tothe minimum required to maintain PaO2 >80 mmHg(10.7 kPa) and minimize the risk of lung damage. Inanimals with concurrent pulmonary pathology, ventila-tion strategies employing PEEP may be required tomaintain oxygenation. (For more details on IPPV andPEEP see Chapter 2.)


! 409 Propofol can be administered by variable rateinfusion where the rate is adjusted by the operator.

Monitoring pulmonary function during anaesthesia isessential to ensure normocapnia (PaCO2 35–40 mmHg[4.7–5.3 kPa]) and adequate oxygenation (PaO2 >80mmHg [10.7 kPa]). For short anaesthetic procedures,such as for diagnostic imaging or CSF sampling, capnog-raphy and pulse oximetry are adequate for monitoringventilation and oxygenation. For unstable patients oranimals undergoing long procedures, such as surgery,analysis of serial arterial blood gas samples is essential.(For details on monitoring techniques, see Chapter 2.)

Maintain perfusion and cerebral perfusion pressureTo maintain CPP in patients with increased ICP, it is rec-ommended that mean BP is maintained between 70 and80 mmHg and systolic BP above 100 mmHg. For shortprocedures in stable patients undergoing MRI or CT,non-invasive BP monitoring is adequate. For unstablepatients or for monitoring during surgical procedures,invasive, direct monitoring of arterial BP and CVP ispreferred. (For details on BP and CVP monitoring seeChapter 2.)


Table 103 Inhalation agents for use in animals with central nervous system disease

AGENT CBF DIRECT CMR ICP SEIZURES COMMENTREGULATION CARDIOVASCULAR

EFFECTS

Halothane

Isoflurane

Sevoflurane

Desflurane

Nitrous oxide

CBF = cerebral blood flow; CMR = cerebral metabolic rate; ICP = intracranial pressure; VD = vasodilation; MAP = mean arterial pressure;IPPV = intermittent positive pressure ventilation.

Autoregulation: !!Flow-metabolismcoupling: !Chemical regulation:!

Autoregulation: !Flow-metabolismcoupling: !Chemical regulation:normal

Autoregulation: !Flow-metabolismcoupling: !Chemical regulation:normal

Autoregulation !!Flow-metabolismcoupling: !Chemical regulation:normal

Not reported

Cerebral VD !! MAP

Cerebral VD!! MAP

Cerebral VD !! MAP

Cerebral VD !! MAP

Potent cerebralvasodilator.Minimal systemiceffects

!

!

!

!

Noeffect

"""

"

"

""

"

None

None

Reportedin humans

None

None

Avoid in neurologicalpatients

IPPV required. Rapidrecovery. Use balancedanaesthesia to reducedose and side-effects inneurological patients

IPPV required. Veryrapid recovery. Usebalanced anaesthesiato reduce dose andside-effects in neuro-logical patients

IPPV required.Very rapid recovery.Use balancedanaesthesia to reducedose and side-effects inneurological patients

Avoid in patients withincreased ICP

Reduce cardiovascular depression associated withmaintenance agents As most anaesthetic agents cause dose-dependentdecreases in BP, it is preferable to combine short-actinganaesthetic agents that can be titrated to effect withshort-acting opioids that cause minimal cardiovasculardepression. This will reduce the required dose of theselected maintenance agent in a dose-dependent fashion.The doses of opioids commonly used for maintenance ofanaesthesia are provided in Table 102. Animals withsevere neurological impairment may require lower doses.

An expected side-effect of administration of thesepotent opioids is respiratory depression. Even at lowdoses, significant hypoventilation may occur and IPPVmay be required to maintain normocapnia. Both alfen-tanil and fentanyl will accumulate after a period of infu-sion, so it may be prudent to terminate or reduce theinfusion rate prior to the end of anaesthesia to ensureadequate ventilation on recovery. Remifentanil doesnot accumulate and has a duration of action of approxi-mately 3 minutes regardless of the duration of infusion.

Maintenance of normal fluid balance Providing animals have normal fluid and electrolytebalance prior to anaesthesia, fluid therapy during anaes-thesia initially consists of isotonic, polyionic crystalloidsadministered at 10 ml/kg/hour. Subsequent infusionrates and types of fluid will depend on losses and cardio-vascular performance during anaesthesia. Measurementof CVP, arterial BP and urine output (UOP) is the bestway to assess the response to fluid therapy. (For details onselection of fluids and rates for varying conditions seeChapter 31; for details of techniques for monitoring ofBP, CVP and UOP see Chapter 2.)

Avoid sudden/marked increases in blood pressureVarious stimuli during anaesthesia, including nocicep-tion from surgical stimulation, can cause increases in BP,which in a diseased brain may result in an increase inCBF and ICP. To minimize sudden or marked increasesin BP during anaesthesia, the continuous infusion of anopioid (as described above) can help minimize this sym-pathetic stimulation and the haemodynamic responses tosurgery. This in turn will contribute to the maintenanceof stable BP and CBF.

Maintain venous drainage from the headDiagnostic imaging procedures and craniectomy areinvariably performed with the patient positioned insternal recumbency with the head level with the spine(410). This position is excellent for ensuring adequatelung expansion and also encourages venous drainagefrom the head. However, it is important to ensure thatthe jugular veins are not occluded when the animal isplaced in this position, as this will lead to venous conges-tion within the brain and marked increase in ICP. Foranimals in lateral recumbency, mild head elevation(15–30 degrees) is also recommended to encourage cere-bral venous drainage.

Measurement of CVP is generally performed via acatheter inserted into the jugular vein. This may increasethe risk of disturbance to venous return and increasedICP. Methods for reducing the interference withvenous return are described in Chapter 2.

The use of IPPV may also impede venous returnfrom the head during the inspiratory phase of ventilation.To minimize this adverse effect, the inflation pressuresrequired to maintain normocapnia can be reduced byadministration of NM blockade using drugs such asatracurium. Atracurium is initially administered at0.2–0.5 mg/kg IV. This is followed by increments of0.1 mg/kg, which is administered according to NM activ-ity assessed using a nerve stimulator.


! 410 In animals with intracranial disease, venousdrainage from the head is maintained by positioning insternal recumbency with the head at the same level as the spine.

Maintain body temperature Body temperature should be maintained as close tonormal as possible. Hypothermia has numerous adverseeffects on the patient including: • Cardiovascular system depression with bradycardia

and hypotension.• Suppression of the immune system and an increased

risk of infection.• Delayed healing.• Intra- and postoperative coagulopathy and

increased blood loss.• Slow recovery.• Shivering on recovery, which increases oxygen

demand when oxygen delivery may becompromised.

• Prolonged hospital stays.

Hyperthermia, on the other hand, increases CMR,which increases CBF and can lead to increases in ICP andfurther reductions in CPP.

It is important to remember that head trauma (espe-cially if it involves the hypothalamus) can result inimpaired thermoregulation. Close monitoring of thetemperature in these animals is imperative and appropri-ate therapy should be initiated when abnormalities arise.Methods for maintaining normal body temperature inanimals with neurological disease will be discussed inmore detail in the NM disease section below.

Recovery from anaesthesiaThe aims during the recovery period are to achieve asmooth emergence from anaesthesia with minimalexcitement, coughing or straining, and adequate ventila-tion (411). The timing of extubation is a compromisebetween ensuring the animal can protect its own airway,breathe spontaneously and maintain normocapnia, whileavoiding stimulation that can lead to increases in arterialBP and coughing.

Ensure adequate ventilationReduce rates of opioid infusionsIf high infusion rates of opioids have been used duringsurgery, it is important that the infusion rate is reduced inpreparation for recovery. Once the patient is extubatedand IPPV can no longer be delivered, it is imperative thatthe patient can breathe spontaneously. Extubationshould therefore only be performed when the patient canspontaneously ventilate adequately (check the capno-graph or blood gases).

When using either an alfentanil or fentanyl infusion,the rate should be reduced approximately 30 minutesprior to extubation. This will depend to some extent onthe duration of infusion and the total amount of drug thathas been delivered (the higher the dose and the longerthe infusion, the more time required to reduce serumconcentrations).

Remifentanil does not accumulate and activity rapidlydisappears after infusion is stopped, allowing promptreturn to spontaneous ventilation. The disadvantage isthat the analgesic activity is also rapidly terminated. Ifcontinued analgesia is required, another opioid must beadministered prior to turning off the remifentanil infu-sion. This may be in the form of a long-acting opioidsuch as methadone. Alternatively, infusion of short-acting opioids at a lower dose (see Chapter 30) can beused to provide postoperative analgesia so long as thepatient can breathe spontaneously.


! 411 Following brain surgery, animals should berecovered in a controlled, quiet manner. Extubation is abalance between ensuring normal ventilation andpreventing coughing when the endotracheal tube isremoved.

Assess adequacy of spontaneous ventilationperiodicallyWhen preparing to extubate, the ETT tie is undone andthe cuff left inflated. As the depth of anaesthesia decreas-es, trial periods of apnoea are performed. If spontaneousventilation and maintenance of normocapnia occur, thenthe animal can be extubated; otherwise IPPV is reintro-duced before P’ETCO2 exceeds 45 mmHg. Once extu-bated, the patency of the airway needs to be assessed,particularly in animals with brachycephalic airway syn-drome. It is important to have ready access to an induc-tion agent in case immediate reintubation is required.

Minimizing coughing and hypertension on recoveryTo minimize laryngeal stimulation, the administration ofagents such as fentanyl or lidocaine can be used asdescribed for intubation. At the end of anaesthesia, thedrugs are given just prior to expected extubation. Alter-natively, in animals requiring ongoing analgesia, a con-tinuous infusion of fentanyl (2–5 µg/kg/minute) can alsohelp reduce coughing on extubation.

Hypertension in the recovery period despite ade-quate analgesia can be treated by administration of #receptor antagonists or blockers (e.g. esmolol, 50–200µg/kg/minute CRI). More details on hypertension inneurological patients can be found in Chapter 2.

Minimizing agitation in the perianaesthetic periodAgitation is not uncommon in animals with intracranialdisease. Administration of opioid analgesics will ensurethe agitation is not caused by pain. If it persists, thensedative agents such as acepromazine or dexmedetomi-dine will be required, but as these agents have adverseeffects on the CNS and the cardiovascular system, theirbenefits for controlling agitation and calming the patientneed to be weighed against the adverse effects that mayoccur. If administration of these agents is deemed neces-sary, low doses should be used and blood volume and BPshould be normalized before administration. Forexample, the authors have used acepromazine (5–10µg/kg IM) to control agitation in dogs post craniectomywhen other methods of controlling the agitation havefailed and the animal is considered at risk of injury fromthe agitation. Trazodone hydrochloride, a triazolopyri-dine derivate and member of the phenylpiperazine classof drugs, can also be considered as an anxiolytic in thesesituations (see Further reading).

SPINAL DISEASE

Considerations Anaesthesia in animals with spinal disease should bedesigned to maintain spinal perfusion and minimizefurther neurological injury. As a result, many of the prin-ciples of anaesthesia for patients with intracranial diseaseare relevant to patients with spinal disease. Considera-tions for anaesthetizing animals with spinal disease areoutlined in Table 104 (next page).

Stabilization Animals with spinal trauma frequently have otherinjuries. Patients with spinal disease may have hadreduced access to water due to reduced mobility (412).Stabilization of pulmonary and cardiovascular functionsand correction of fluid and electrolyte deficits should beperformed prior to anaesthesia. A full clinical examina-tion should be performed with particular attention paidto the function of the cardiovascular and respiratorysystems.


! 412 Spinal patients may have reduced access to waterdue to immobility. This may predispose to dehydration ifsupplemental fluid is not provided. Dogs, such as the onein this photograph, will need to be administered fluidsintravenously or frequently ‘by hand’.

Guidelines for sedating and anaesthetizinganimals with spinal diseasePremedicationPremedication is important not only to provide analgesiaand minimize the dose of induction agent required, butalso to increase the ease of handling without the need forexcessive physical restraint, which may be dangerous forthese patients.

Choosing drugs and drug combinationsOpioids form the basis for premedication of animals withspinal disease and are generally administered on theirown to patients with other systemic abnormalities.A variety of other agents can be used in conjunction withopioids to provide additional sedation if required. Thedoses and advantages and disadvantages of these agentsare outlined in Table 101. Acepromazine is useful inanxious but otherwise stable animals. Medetomidine can


Table 104 Considerations for anaesthetizing animals with spinal disease

CONSIDERATION MANAGEMENT

Maintain perfusion

Pain

Anxiety

Mechanical instability

Respiratory insufficiency

Maintain airway

Blood loss

Correct deficits in circulating blood volume and body water. Use agents/techniques thatminimally depress cardiovascular function

Most spinal diseases are painful and effective analgesia (e.g. full mu agonists) is necessary

Anxiety is common in paralysed animals and decreases the pain threshold. Administeringanxiolytics is an important part of pain management in spinal patients

Animals need to be moved carefully to minimize further trauma to the spinal cord,particularly when a fracture or luxation is suspected. Intubation of animals with suspectedinstability of the cervical spine should also be performed carefully. Flexion and extension ofthe head should be avoided

Cervical spinal lesions can interfere with innervation of the diaphragm and intercostalmuscles, leading to inadequate ventilation and respiratory arrest. Sternal recumbencyduring surgery restricts movement of the diaphragm and thus mechanical ventilation isrecommended during surgery to ensure adequate ventilation

Ventral approaches to the cervical spine require retraction of the trachea and may partiallyor completely obstruct the endotracheal tube

Blood loss during surgery can be significant and needs to be monitored closely by weighingswabs and measuring fluid in suction bottles. Transfusion is indicated if >20% ofcirculating blood volume is lost or if signs of hypovolaemia (increased heart rate withoutincreased MAP, pale mucous membranes) are observed

be used in extremely anxious or fractious animals that arenormally hydrated and cardiovascularly stable. Benzodi-azepines are unpredictable and unreliable sedatives inhealthy dogs and cats, but may provide useful sedation incritically ill animals. Medetomidine and benzodiazepinesboth cause skeletal muscle relaxation and should beavoided in animals with unstable spinal fractures (413).Ketamine is another option in normovolaemic cats withnormal renal function in combination with opioids, ace-promazine or benzodiazepines. However, ketamine isextremely painful when injected intramuscularly. Thismay cause additional discomfort to animals already inpain. In addition, sudden uncontrolled movement inresponse to the injection may be detrimental in animalswith an unstable spinal fracture.

InductionMinimize further damage to spinal cord A rapid and controlled induction of anaesthesia withminimal struggling is best achieved with intravenousagents. Endotracheal intubation should be performedcarefully with adequate support of the head and neck,particularly in animals with cervical spinal cord injury(414). Intubation is facilitated by the use of a laryngo-scope (415). Excessive extension of the neck should beavoided in dogs with caudal cervical lesions, while exces-sive flexion should be avoided in animals with AA sublux-ation or other cervical fractures.

Select induction agents that minimally interfere withspinal perfusionSelection of an appropriate induction agent is based onthe same basic principles as those described for animalswith intracranial disease. The characteristics of the intra-venous agents and suggested doses are described inTable 100. To decrease the required dose of intravenousinduction agent and to minimize cardiovascular depres-sion, concurrent administration of a potent opioid, suchas fentanyl or a benzodiazepine, can be used duringinduction (Tables 101 and 102).


! 413 Handling and choice of premedication should beperformed carefully in animals with unstable spinalfractures, such as that noted on this lateral thoracolumbarradiograph (arrow).

! 414 For animals with cranial spinal lesions, such as theC2/C3 disc extrusion seen on this sagittal MR image(arrow), intubation should be performed with support ofthe head and neck.

! 415 Intubation is best performed with the animal inlateral recumbency. Use of a laryngoscope will aid visuali-zation of larynx.

Maintenance of anaesthesiaSelect agents that maintain spinal perfusionMaintenance of anaesthesia is usually performed withinhalation agents. As autoregulation of perfusion to thespinal cord and chemoreceptor response to carbondioxide are better maintained with isoflurane and sevo-flurane than halothane, these are the preferred inhalationagents. Nitrous oxide is reported to increase ICP as aresult of cerebral vasodilation, so is best avoided inpatients with intracranial disease. Whether the same pre-cautions are warranted in animals with spinal cord injuryand compression is not known.

Infusion of short-acting opioids can also be used in conjunction with inhalation agents. These agentsprovide analgesia, which is essential in most animals withspinal disease, and will help reduce the dose and thus theamount of cardiovascular depression observed withinhalation agents. Details of the use of these opioids aredescribed in the section on intracranial disease.

Maintenance of anaesthesia can also be performedwith a TIVA technique as described for intracranialdisease. TIVA is especially useful for dogs requiringsurgery of the cranial cervical spinal cord where manipu-lation of the cervical spinal cord and/or brainstem mayoccur (e.g. repair of AA instability).

Maintain adequate ventilation IPPV is recommended during anaesthesia in spinalpatients for several reasons. Firstly, the detrimentaleffects of inhalation agents on spinal blood flow regulation can be minimized by maintaining normo-capnia. Secondly, surgical access frequently requires thatthe animal is positioned in sternal recumbency, whichcan interfere with diaphragmatic excursions and impairventilation. Finally, the dose rates of the opioid agonistsrecommended for intraoperative use produce markedrespiratory depression and therefore necessitate IPPV.

Maintain adequate perfusionIntravenous fluid therapy is essential during anaesthesiain all spinal cases to maintain fluid balance, adequate BPand perfusion of the spinal cord. In hypovolaemicanimals, the volume deficit should be replaced beforeanaesthesia. Intraoperative blood loss can be unpre-dictable and surprisingly high during spinal surgery.Blood loss should be estimated regularly by counting

blood-soaked swabs, weighing swabs (1 ml of bloodweighs approximately 1.03 g) or measuring the volume offluid in suction bottles (taking into account dilution fromirrigation fluids).

Blood loss can also be estimated from the PCV offluid in the suction bottle. This technique requires anaccurate measurement of the patient’s PCV at the timeof blood loss. The PCV prior to anaesthesia may not berepresentative of the PCV during anaesthesia. The PCVmay decrease due to splenic sequestration of RBCs inresponse to anaesthetic agents (propofol, barbiturates)and dilution by intravenous fluid therapy. If an accurateestimate of the PCV of the patient prior to haemorrhageis known, the amount of blood in the bottle can beestimated using the following:

Amount of blood lost (ml)

= PCV of flush in bottle $ total volume of fluid in bottle PCV of patient at time of haemorrhage

A blood transfusion is indicated when blood loss exceeds20% of the circulating blood volume or haemoglobinconcentration is <80 g/l (<8 g/dl). Blood loss of less than20% can be managed by administrating crystalloids +/– colloids such as hetastarch (maximum dose20 ml/kg/day). (For more details on fluid therapy seeChapter 31.)


! 416 Heat and moisture exchange devices can be placedbetween the endotracheal tube and the breathing circuitto help reduce heat and moisture loss from animal.

Maintain body temperatureHeat loss can be a problem, particularly when spinal cordinjury causes sympathetic nervous system imbalance andperipheral vasodilation. Monitoring core body tempera-ture should be performed during anaesthesia. Heat losscan be prevented during anaesthesia by heat pads, warmwater beds or warm air blowers and ‘blankets’. Heat andmoisture exchange devices (416) can be placed betweenthe endotracheal tube and the breathing system topromote warmth and humidification of inspired gases.

MonitoringDuring diagnostic imaging and surgery in animals withspinal disease, non-invasive monitoring of cardiopul-monary function with electrocardiography, non-invasiveBP measurement, capnography and pulse oximetry isgenerally adequate. In animals where cardiopulmonarydysfunction (e.g. cranial cervical surgery, traumainvolving multiple organ systems) or excessive blood lossis expected, invasive BP measurement is recommended.Monitoring techniques have been discussed in detail inChapter 2.

Recovery It is essential that the provision of analgesia is continuedinto the postoperative period to ensure a calm and com-fortable recovery. In some cases the use of low-dose seda-tives, such as acepromazine (0.01–0.02 mg/kg IM or IV)or dexmedetomidine (0.5–1.0 µg/kg IV bolus or infusionof 0.5–1.0 µg/kg/hour), may be required in extremelystressed or agitated animals exhibiting signs of subopti-mal emergence. However, these drugs should only beused in animals with normal cardiovascular function.Trazodone (see p. 545) can also be used to treat postoper-ative anxiety. (For details on possible postoperative anal-gesia see Chapter 30.)

NEUROMUSCULAR DISEASE

ConsiderationsConsiderations for anaesthetizing animals with NMdisease are outlined in Table 105. The general principlesfor anaesthetizing these patients are similar, with somedifferences depending on the type of NM diseasepresent. These principles are described in more detail inregard to premedication, induction, maintenance andrecovery of anaesthesia.

Stabilization Fluid and electrolyte abnormalities are common in thesepatients due to immobility and an inability to eat anddrink. These deficits need to be corrected prior toanaesthesia when possible. Fluid and acid–base abnor-malities associated with toxicities, such as metaldehyde,will also need to be corrected prior to anaesthesia (seeChapter 28).

Animals with aspiration pneumonia should be stabi-lized as much as possible prior to anaesthesia. Antibioticand oxygen therapy forms the basis of symptomatic treat-ment in these animals.

Premedication The use of premedication in animals with peripheralNM disease will depend on the type of disease that ispresent, how painful that disease is and how urgent theneed for anaesthesia is (e.g. an animal with airwayobstruction requires immediate anaesthesia). In anxiousanimals, the judicious use of sedatives may be needed tofacilitate a smooth, stress-free induction. (Note: Sedationcan interfere with the maintenance of a patent airway andincrease the risk of upper respiratory tract obstructionand aspiration. If used, low doses are recommended andanimals should be constantly monitored for any adverseeffects.)

In animals with painful NM disease (which is uncom-mon), premedication should include an opioid, eitheralone or in combination with other agents. Opioid pre-medicants should also be given to animals requiringmuscle and nerve biopsies and animals undergoingpainful diagnostics such as electromyography.


Induction of anaesthesiaMaintain oxygenation The patient should be pre-oxygenated for 5–10 minutesprior to induction when possible. If an animal objects tothe placement of a face mask, the use of flow-by oxygen isrecommended. Flow-by oxygen is preferred in animalspredisposed to hyperthermia, because masks encouragere-breathing of warm expired gases. Pre-oxygenationincreases the concentration of oxygen in the func-tional residual capacity of the lung and delays the onset ofhypoxaemia in the event of a difficult intubation or inpatients with cardiovascular or respiratory compromise.

Prevent regurgitation and aspirationInduction of anaesthesia should be performed with thepatient in sternal recumbency (417). If there is anincreased risk of regurgitation (e.g. megaoesophagus),cricoid pressure should be applied. Cricoid pressure ismaintained until the ETT is placed and secured in theairway with the cuff inflated. Suctioning of the pharynx,oesophagus and stomach should be performed as soon asthe airway is secure to minimize the risk of aspirationduring anaesthesia (if the cuff becomes deflated). If vom-iting or regurgitation occurs during induction ofanaesthesia and before the airway is protected by thepresence of an ETT, the animal should immediately bepositioned with its head over the edge of the table toallow gravity-assisted drainage of the pharynx. Thepharynx and oesophagus should be suctioned beforeintubation is performed.

Induction of anaesthesia should be performed withshort-acting intravenous agents that facilitate rapidcontrol of the airway. Furthermore, a prompt recoveryfrom anaesthesia is important in order for the patient toregain control of its airway as soon as possible. Agentssuch as propofol and alfaxolone allow rapid recovery;however, in animals at risk of obstruction, titration ofthese agents to effect can delay intubation. Furthermore,bolus administration can lead to marked decreases in BP. To reduce the dose and thus the side-effects, co-induction agents (fentanyl + short-acting benzo-diazepines) can be administered immediately prior to anintravenous induction agent. This combination of agentswill invariably cause apnoea and therefore animals shouldbe ventilated as soon as intubation is performed.Ketamine should be avoided in animals with tetanus andincreased muscle activity. Thiopentone allows rapidcontrol of the airway, but the recovery period is likely tobe prolonged.

Maintenance of anaesthesiaSelection of agentAgents with a short duration of action, thus allowingrapid recovery, are preferred. Inhalation agents, such asisoflurane, sevoflurane and desflurane, all have physico-chemical properties that ensure a rapid clinical responseto changes in vapourizer settings. In addition, recoveryfrom anaesthesia is relatively rapid, with prompt returnof airway reflexes.


! 417 Animals that are at risk of regurgitation duringinduction should be maintained in sternal recumbencywith cricoid pressure applied until the endotracheal tubeis placed and cuff inflated.

Ensure adequate ventilation Mechanical ventilation is recommended in all animalswith peripheral NM disease as there is likely to be a com-ponent of respiratory muscle involvement. IPPV shouldbe delivered with close monitoring of CO2 concentrationin the expired gas with a capnograph or serial arterialblood gas analyses. Monitoring of the haemodynamicconsequences of ventilation is also prudent.

Maintain normal body temperatureAnimals with NM disease may have difficulty maintain-ing normal body temperature (see Table 105). It is there-fore essential to monitor core body temperatureperioperatively in these animals. Mild decreases in bodytemperature can be managed with passive warming(e.g. warm air blankets or warm water beds). It is easier toprevent hypothermia than to treat it, so all efforts to pre-serve body temperature should be made. Although it isunusual for animals to develop hyperthermia underanaesthesia, increases in body temperature should bemanaged by passive cooling.

Neuromuscular relaxationIn animals with peripheral NM disease requiring surgeryfor other reasons (e.g. thoracotomy for thymomaremoval in animals with MG), NM relaxation may berequired to improve surgical access. Non-depolarizingmuscle relaxants can be used, but extreme care is requiredas prolonged duration of skeletal muscle weakness canoccur. Depolarizing muscle relaxants such as suxametho-nium should be avoided.

Non-depolarizing NM blocking agents should beadministered in incremental doses with careful monitor-ing of peripheral nerve blockade with a nerve stimulator.There must also be facilities to provide either mechanicalor manual IPPV and assessment of adequacy of ventila-tion (capnography or blood gas analysis). Shorter-actingNM blocking agents, such as atracurium or vecuronium,administered at one tenth of the usual dose, are the pre-ferred agents. Infusions allow more precise control of thedegree of NM blockade than boluses, which create peaksand troughs in plasma concentration and thus cause relative overdose and relative underdose, respectively.


Table 105 Considerations for anaesthetizing animals with neuromuscular disease

CONCERN CONSIDERATIONS

Pain

Dysphagia

Airway obstruction

Impaired ventilation

Impaired oxygenation

Impaired thermoregulation

Dehydration and electrolyte abnormalities

Presence and severity of pain will vary with the disease. Painful conditions includepolyradiculoneuritis, some myopathies and muscle and nerve biopsy patients

Weakness of pharyngeal muscles results in difficulty swallowing and predisposes to aspiration

Laryngeal paresis or paralysis. Laryngeal spasm (e.g. tetanus)

Mechanical ventilation frequently required due to:• Weakness of respiratory muscles (e.g. snake envenomation, myasthenia gravis,

polyradiculoneuritis, tetrodotoxin, botulism).• Spasm of diaphragm and intercostals (e.g. tetanus)

Dysphagia, megaoesophagus and inability to protect the airway predisposes to aspiration.Recumbency predisposes to atelectasis. Supplemental oxygen recommended in peri-operative period

Laryngeal dysfunction and impaired ventilation reduce ability to pant and predispose tohyperthermia during exposure to warm environments. Muscle fasciculations and tetanyincrease metabolic rate and predispose to hyperthermia. Generalized weakness preventsshivering and predisposes to hypothermia during exposure to cold environments

Recumbent animals may have restricted access to water. Dysphagia impedes ability to eatand drink. Regurgitation associated with megaoesophagus increases loss of water andbicarbonate (from saliva)

RecoveryMaintain adequate oxygenation and ventilationAs the animal recovers from anaesthesia, it is essential tomonitor end-tidal CO2 to ensure spontaneous ventila-tion is sufficient to maintain normocapnia. Theseanimals will invariably have some degree of pulmonarypathology due to aspiration or atelectasis, so oxygenationshould be monitored throughout recovery and supple-mental oxygen provided until the animal can maintainSpO2 >95% when breathing room air. Initially, thiscan be performed via the ETT; however, followingextubation, oxygen can be provided by mask, oxygencage or nasal catheters. Where oxygenation is expectedto be poor for prolonged periods (e.g. animals with pneu-monia or animals expected to be recumbent followinganaesthesia), nasal catheters should be placed before theend of anaesthesia to provide a smooth stress-free transi-tion from oxygenation via the ETT to the nasophar-yngeal catheters. (Note: Animals that develop upperrespiratory tract obstruction [e.g. laryngeal paralysis orlaryngeal spasm] following extubation may require a tra-cheostomy to maintain adequate oxygenation and venti-lation [see below].)

Prevent aspirationSuctioning of the oesophagus and pharynx should beperformed prior to recovery in order to minimize the riskof regurgitation at extubation. The animal is best posi-tioned in sternal recumbency with the head elevated tomaximize chest excursions and respiratory function. Inaddition, elevation of the head will help prevent passiveregurgitation. Should vomiting or regurgitation occurduring recovery, the animal’s head must be positionedover the edge of the table to allow fluid or stomach con-tents to flow out of the mouth. If the animal is still suffi-ciently anaesthetized, the pharynx and mouth can becleared by suctioning and swabbing. To prevent oeso-phagitis associated with regurgitation of gastric contents,the oesophagus should ideally be carefully lavaged untilthe fluid retrieved is clear. It is essential that the ETT issecured in place with adequate cuff inflation when lavageis performed.

Maintain a patent airwayAnimals with neuromuscular weakness or post-gastric lavageIn these animals extubation is delayed for as long as pos-sible to ensure upper and lower respiratory muscle func-tion is adequate. Recovery should be performed in aquiet, dimly lit environment to minimize stimulation onrecovery. The cuff of the ETT=endotracheal tube is leftinflated until the animal is ready to be extubated. Ade-quate analgesia must always be provided in animals withpainful diseases to optimize conditions for a smoothemergence from anaesthesia. Low-dose infusions ofshort-acting opioids can also help reduce stimulationfrom the ETT and help maintain a patent airway forlonger.

Animals with tetanusAlthough tetanus does not cause NM pathology, it isassociated with severe muscle spasms. Extubation inpatients with tetanus can stimulate laryngospasm. Thesafest approach is to recover these animals with a tra-cheostomy in place. If a tracheostomy is not performed,laryngospasm can be minimized by extubating early, aslong as the patient is ventilating spontaneously. Topicallidocaine applied to the larynx may also help. In anycase, it is essential to be prepared to perform an emer-gency tracheostomy should laryngospam occur and if reintubation is too difficult. Preparing the site before-hand is recommended to save time should a tracheo-stomy be required. The ability to provide oxygensupplementation via an intratracheal needle or cathetershould also be available in case upper respiratory tractobstruction occurs (see Chapter 2). (Note: This methodof providing oxygen is only suitable for short periods, asthere is no concurrent ventilation, and overinflation ofthe lungs can occur because there is no outflow for theinsufflated oxygen.)


SEDATION/ANAESTHESIA FOR CHRONICINTUBATION AND MECHANICAL VENTILATION

The choice of agent used to maintain anaesthesia inanimals requiring ventilation will depend on the indi-cations for ventilation, the expected duration of ventila-tion and whether the animal has an oral ETT or atracheostomy. Ventilation techniques are discussed inChapter 2. Examples of agents that can be used toprovide sedation or anaesthesia for ventilation are listedin Table 106.

Dose ratesMaintenance of anaesthesia has ‘lighter’ requirementsthan does surgical anaesthesia, and in some cases sedationonly may be needed. In animals intubated via tracheo-stomy, the depth of sedation/anaesthesia will be evenlower, as the stimulus associated with oral intubation isabsent. Animals that are weak, paralysed or sufferingfrom CNS depression will also require much lower doses than those with normal CNS activity. Therefore,

short-acting agents that can be titrated to achieve therequired level of sedation or anaesthesia in each individ-ual are preferred.

Additional considerations for paralysed animals The use of sedation or anaesthesia for ventilating para-lysed animals warrants special mention. Sedation oranaesthesia is not required to tolerate the presence of anETT; however, as they recover from paralysis, sedationor anaesthesia will be required to maintain intubation.The reason for this is the differential recovery of differ-ent skeletal muscles from paralysis, allowing theseanimals to move before being able adequately to ventilateor protect their own airway. As a result, these animals canstart to struggle. Furthermore, it must be rememberedthat even when a patient is fully paralysed they are con-scious and responsive to their environment, thereforesome sedation and/or analgesia is required to minimizestress and discomfort.

Nursing and airway management of the chronicallyventilated patient are covered in Chapter 2.


Table 106 Possible agents for sedating or anaesthetizing animals during long-term ventilation

DRUG DOSE COMMENTS

Inhalation agents:isoflurane/sevoflurane

Propofol

Midazolam

Fentanyl

1 minimum alveolar concentra-tion equivalent or less

0.05–0.4 mg/kg/minute

0.05–0.2 mg/kg/hour (do notdilute drug with Hartmann’s)

0.2–0.7 µg/kg/minute

Not recommended for long-term ventilation in animalswith intracranial disease. Isoflurane can be irritant toairway and therefore is best avoided. May have adverseeffects in airway disease or prolonged anaesthesia

Ideal for animals with intracranial disease. Can be usedfor any animal requiring sedation or anaesthesia forventilation

Can be used in animals with intracranial disease thatrequire sedation for intubation. Dysphoria observedwith prolonged infusion may cause stressful recoveries.Use cautiously in animals with neuro-muscular weakness

Can be used alone in paralysed animals or inconjunction with other agents to provideanalgesia/sedation

SPECIFIC CONSIDERATIONS FOR DIAGNOSTIC PROCEDURES

MyelographyMyelography is associated with a risk of seizures onrecovery from anaesthesia and a variety of cardiopul-monary abnormalities that can occur during anaesthesia,particularly during/after contrast injection (418).

Seizure activitySeizure activity is a recognized adverse effect of injectionof contrast agents into the subarachnoid space. The riskof seizures is influenced by several factors including thevolume and rate of contrast injection, the site of injec-tion, the size of the animal, duration of anaesthesia afterinjection and the position of the animal during injection.Seizures are more commonly observed after CMC myel-ography compared with lumbar myelography. Animalsweighing >20 kg are also observed to have a higher inci-dence of seizures, possibly due to the relatively highervolume of contrast injected.

To reduce the risk of seizures, the dose rate should becalculated from surface area rather than body weight, thespeed of injection should be slow and the head shouldbe elevated as soon as injection is complete to promotethe flow of contrast away from the head. The use of atilting table allows head elevation while keeping theanimal’s spine straight and supported.

Pharmacological agents that decrease the seizurethreshold should be avoided. Acepromazine, ketamineand medetomidine have been previously reported todecrease the seizure threshold, and most of the literaturerecommends that these agents are not used in animalsundergoing myelography. However, the associationbetween use of acepromazine and seizure activity hasbecome increasingly unclear and its effect may dependon the cause of the seizure activity. The authors recom-mend that its use be avoided whenever possible and ifrequired for sedation/anxiolysis, it is used carefully and atlow doses.

Seizures have been reported to occur up to 6 hoursafter contrast injection, so these animals should beclosely monitored during this time. If seizures occur,administration of diazepam (0.2–1.0 mg/kg IV) is recom-mended as the first-line treatment.

Cardiopulmonary disturbanceCardiopulmonary side-effects during or immediatelyafter the injection of contrast have been observed andinclude apnoea, tachypnoea, bradycardia, tachycardia,arrhythmias, hypotension and hypertension. Many ofthese effects are associated with the discomfort or pain ofinjection and can be minimized by slowing the injectionrate and ensuring an adequate depth of anaesthesiaduring injection. Transient increases in ICP may also beresponsible, particularly with cisternal contrast injec-tions. Careful monitoring of cardiopulmonary functionis necessary during myelography to detect any problemsearly and treat accordingly.

Magnetic resonance imagingThe main considerations for anaesthetizing patients withintracranial and spinal disease for MRI are described inthe relevant sections earlier in this chapter. In addition,there are several important considerations unique toanaesthetizing a patient within a magnetic field.


! 418 Myelography is frequently performed to diagnoseintervertebral disc disease (as seen here). Performance of amyelogram requires special considerations for anaesthesia.(Photo courtesy Victoria Johnson)

Equipment hazardsFerromagnetic objects can become dangerous projectilesand may result in injury and/or death to the patient orpersonnel within the scanning room. It is essential thatthese objects remain outside the 5 gauss line. Anaesthet-ic machines are required to be as close to the patient aspossible to minimize the length of the breathing systemand should be composed of non-ferromagnetic materi-als. If this is not possible, a non-rebreathing anaestheticcircuit (e.g. Bain) can be used, as the length of the inspi-ratory and expiratory tubes (which are coaxial) may beeffectively infinite. Non-ferromagnetic objects within amagnetic field (e.g. ECG leads) have the potential toinduce electric currents, leading to heating and burns.The risk of burns can be minimized by insulating thewires, separating the wires from the skin by padding,avoiding large loops of wire that allow the induction ofcurrents and applying sensors as far away from theimaged area as possible.

Monitoring Monitoring of the anaesthetized patient during MRI hasinherent limitations. Equipment used for monitoringmust be MRI safe and ideally MRI compatible. Equip-ment that is MRI safe has been demonstrated to presentno additional risk to the patient. Equipment that is MRIcompatible has been demonstrated to be both MRI safeand to not reduce significantly the diagnostic quality ofthe imaging procedure nor have its operation affected bythe scanning procedure. MRI compatible equipment iscurrently available that allows distant monitoring ofanimals during MRI. Where cost is limited, some moni-toring equipment, such as capnography, oesophagealstethosope and oscillometric methods of non-invasiveBP measurement, can be used if the electrical compo-nents are outside the 5 gauss line.

Cerebrospinal fluid collectionThe collection of CSF may be performed by CMC orlumbar puncture. CMC puncture requires flexion of theneck, which can cause inadvertent kinking of the ETTand respiratory obstruction. In addition, flexion of theneck can obstruct jugular veins, impair venous drainageand contribute to increased ICP. IPPV is essential duringCMC puncture to ensure adequate ventilation andnormocapnia.

ETTs reinforced with coiled wire resist kinking andcan be used to prevent airway obstruction when the neckis flexed for collection of CSF. As these tubes containmetal, they are not suitable for use in animals undergoingconcurrent MRI imaging.

In animals with increased ICP the collection of CSFcarries the risk of parenchymal herniation. When sam-pling is essential for the diagnosis and treatment of theanimal, pre-emptive use of mannitol (30 minutes prior toCSF collection) and reduction of PaCO2 to 30 mmHg (4 kPa) by hyperventilation during the sampling periodmay reduce the risk of herniation.

ElectroencephalographyElectroencephalography records the spontaneous elec-trical activity within the brain and may be performed in animals to identify areas of abnormal electricalactivity responsible for seizures. Performance of electro-encephalography in conscious animals is difficult, asmuscle movement causes artefacts, which affect the diag-nostic quality. Inhalational and intravenous anaestheticagents also alter the electrical activity within the brain ina dose-dependent manner, thus limiting the amount ofuseful information that can be obtained from electro-encephalography in anaesthetized animals. A sedativeregimen used to perform electroencephalography inconscious animals has been described. This regimen wasreported to limit spontaneous movement in consciousanimals, while reducing the effects of deep sedation orgeneral anaesthesia on the recorded EEG. However, thisreport describes the use of high doses of acepromazine, adrug that may decrease seizure threshold. As discussedabove, the association between seizures and acepro-mazine is still unclear and its use in animals with patho-logical causes of seizure activity should be performedcautiously.


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