The Premature Infant & Anaesthesia
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Transcript of The Premature Infant & Anaesthesia
Anaesthetic consideration in premature infantDalila Hussain
Introduction
Premature babies are defined as those born before 37 weeks
Categorization of prematurity by gestational age & birth weight
Gestational age
Birth weight
36-37 week Borderline/ near term
<2500g LBW
31-36 week Moderately premature
<1500g VLBW
24-31 week Severely premature
<1000g ELBW
Physiology Airway :
- They are obligatory nose breathers due to high resistance to airflow through oral passage .
- large & lax tongue- easily fall back & obstruct airway
- Trachea length is 4cm
• Infant’ s larynx is higher in neck (C2-C3) compared to adult’s (C4-C5)- glottis @ C3 level in premature infant
• Large , floppy & ohmega (Ω) shaped epiglottis , make visualisation of glottis difficult during intubation
Funneled shape of larynx;-Narrowest part of infant’s larynx is @ cricoid ring with an approximate 14mm2 (ETT that passes easily through the glottic opening may be tight @ level cricoid ring)
Tight fitting ETT may cause
oedema
Closing volume relatively large & encroaches normal tidal volume--predispose to:
Airway close
Atelectasis
Increase O2 consumption ( 6ml/kg/min)to meet high metabolic rate
Tidal volume is relatively fixed due to anatomic structure
Unable to increase tidal volume--compesating for increase respiratory demands through raised respiratory rate -- lead to early fatigue
Minute alveolar ventilation is more dependent on increase resp. rate than on TV
Reduce FRC which is smaller than the closing capacity --- rendering them dependent on PEEP & prone rapid desaturation
Chemoreceptor responses are blunted
Extension of head does not facilitate intubation but even obstruct airway
External laryngeal pressure facilitate intubation by bringing anterior larynx into view
- Changes in head position result in ETT movement due to shortness of trachea & large tounge
- Flexion of the neck moves the tip of tube farther out of trachea
- Extension moves the tube farther into the trachea
Sniffing position
Neck flexed forward, head extend on the
neck & jaw held forward is
required for effective
ventilation
Respiratory system
- Fetal diaphragm contain only 10% Type 1 respiratory muscle ( high oxidative & resistant to fatigue)-- contributing to apnoea during physiological stress & fatigue more quickly
- Alveoli form @ 17-28 weeks gestational age
- Pulmonary capillaries form @ 28-36 weeks gestational age
- Lung maturation @ 36 week
- Lung surfactant produce at 32-34 weeks. Surfactant function to :
Lower surface tension in alveoli
Facilitate alveolar opening
Prevent alveolar collapse at end of expiration
Prone to apnoea (pauses in breathing > 20 sec/ loss effective breathing associated with bradycardia)
Central apnoea Obstructive apnoea Mixed apnoea
- Diminishedhypercapnicresponse
- Hypoxic ventilatorydepression
- Active inhibitory reflexes
- Opposition of hypopharyngealsoft tissues
- Nasal occlusion
-obstruction followed by central pauses
Risk factor for apnoea
Hypoglycaemia Hypoxia Anaemia
Hypothermia Sepsis
Post operation, apnoea are frequent in 1st 12 hour & can continue until 48-72 hour
Premature infant exposed to mechanical ventilation & > 28 days on O2 supplementation likely to develop bronchopulmonary dysplasia (BPD)
Sx of BPD :
Increase O2 requirements
Reduced lung compliance
Reversible airway obstruction
Cardiovascular system
The ductus arteriosus often remain patent ( PDA)
Other pathology eg: atrial / VSD can cause significant :
Left to right shunting
Progressively increased pulmonary flow
Congestive cardiac failure
Hypoxia is a potent pulmonary vasoconstrictor
Raised pulmonary vascular resistance (PVR)
Lead to right to left shunting, exacerbating hypoxia & acidosis
Prolonged raise PVR lead to:
Poor right ventricular fx
Impaired cardiac output
Limited oxygen delivery
Pulmonary oedema
Sudden death
Haematology
Term baby has 18-20g/dL of Hb
Premature baby : 13-15g/dL of Hb
70-80% of which is HbF
HbF has reduced ability to release oxygen
Target haematocrit 40-45%---facilitate O2 delivery
Estimation of blood volume in premature baby: 95ml/kg
If less than target, may necessitate preop blood transfusion
Renal Ability to retain Na+ @ 32 week GA
The distal tubular response to aldosterone is low until 34 week
ADH level are high
Impaired ability to concentrate urine
Drug excretion delayed due to immature renal system
Total body water : 75-85% of body weight . Inversely related to GA
Marked transepidermal permeability & large body surface area accelereate H2O loss
Evaporative H2O increase 15 fold during 1st few days of life compare with term babies.
Temperature regulation Increase surface area to body weight ratio
Decrease brown fat store- limit heat production
Non keratinised skin , extreamly susceptible to heat loss
Inability to shivering
Thermoneutral environment for unclothed preterm baby is 34 degree celcious
@ this temp, O2 demand is minimal
Hypohermia induced stress can lead to:
Hypoglycaemia
Apnoea
Metabolic acidosis
Glucose homeostasis
Have fewer glycogen stores
Underdeveloped gluconeogenesis pathway
Prone developing hypoglycaemia during starvation
Hyperglycaemia should be avoided as a hyperosmolar state can lead to IVH, osmotic diuresis & dehydration
Gastrointestinal system
Gastro-oesophageal reflux is common resulting from underdeveloped and incompetent lower oesophageal sphinter.
This lead to:
-Laryngospasm - Chronic cough
-Laryngitis -Tracheitis
-Apnoea -Otitis media
-asthma
NEC with bowel wall necrosis and perforation can lead to systemic sepsis
Drug metabolism is immature
Central nervous system
Pain receptors develop by 20 weeks GA
Pain pathways develop by 26 weeks.
A foetus of 26 weeks may demonstrate a flexion withdrawal reflex in response to pain stimulation
Descending inhibitory pathways are immature leading to greater pain sensitivity
Risk factor for IVH or later neurodevelopmental delay include :
- RDS
- Hypotension / fluctuating blood pressure,
- the use of hypertonic infusions
- aggressive volume expansion
Pre anaesthetic assessment Consult parent regarding risk of deterioration of
pulmonary function & necessitating postoperative ventilatory support
Particular points to ascertain are:
1) Gestational age at birth and the current gestational age
2) Weight
3) Periods of mechanical ventilation, CPAP and oxygen therapy and the duration
4) Apnoeas – frequency, duration, possible triggers
5) Co-morbidities, particularly cardiac
6) General health, growth and development
7) Previous operations
8) Medications
Airway assessment
If already intubated- check the ETT size & length
Blood investigations
If anticipated blood loss greater than 10% of blood volume, crossmatch should be taken
Echocardiogram must be performed before surgery
Minimise starvation times to prevent hypoglycaemia and dehydration.
Intraoperative management
The ambient temperature minimum 27 degree celciousfor reception of the baby
Effective warming device eg: warming matress, warming air blanket
Inspired gases should be heated & humidified.
Fluids , blood, blood products & irrigation fluid must be warmed.
If 360 o access to baby is required by the surgeon( eg: laser surgery for retinopathy), access can be improved by removing the head & foot ends of operating table.
GA machine
ETCO2 detector
Tapes for securing
ETT
Laryngoscope handle
& blades: 00, 0 (premature)
ETT: portexpeadiatricsize 2.5, 3.0 & 3.5mm
Connector to fit between ETT tube & ventilation bag, circuit
Check all the
equipment
ETT size & lengthBaby weight
( kg)Tube size
(mm)Oral tube
length @ lip ( cm)
Nasal tube length @ nose (cm)
Suction tube size ( Fr)
<1.0 2.5 5.5 7.0 6
1.0 2.5-3.0 6.0 7.5 6
2.0 3.0 7.0 9.0 6
3.0 3.0 8.5 10.5 6
3.5 3.0-3.5 9.0 11.0 8
4.5 3.5 9.0 11.0 8
An alternative is to assess ET tube length by the rule of 6.
Oral tube length(cm) =: 6 + wt (kg)
Nasal tube length(cm) = 6 + (1.5 x wt)
appropriate position must always be comfirmed
Monitoring:
-BP using an appropriate sized cuff
-ECG
-capnography
-temperature
- SpO2 ( 2 oximeter probes recommended)
- pt with PDA, 1 probe placed on the right hand ( pre-ductal) & the other on lower limb ( post-ductal)
Inhalational induction is often preferred
Moderate concentration of volatile can be used to ;
- Minimized increase in PVR
- Avoid decrease in systemic arterial presure
Newer shorter acting agent, desflurane may useful for recovery in a preterm infant.
Sale, in their study on premature babies under 37 weeks gestation & under 47 week undergoing inguinal herniotomy suggested that infant wake faster from GA when maintained with desflurane as compared with sevoflurane, but no difference in postoperative respiratory events was demonstrated
A range of uncuffed tubes should be available
– a neonate of <1200g may need a 2.5mm tube
- 2 kg baby has tracheal tube positioned @ gum margin @ 8cm mark
Anticipate difficult intubation if premature infant has undergone prolong ventilation-- possibility subglottic stenosis
Exposure to high O2 levels is associated with increase morbidity & mortality.
Tacycardia & HTN can be detrimental in presence of underdeveloped cerebral autoregulation.
- Careful titration of anaesthetic & narcotic agent is necessary
Multimodal analgesia should be use for pain relief
(eg: local anesthesia, paracetamol, opioid (fentanyl)
Intaoperative fluid management
Estimated maintenance fluid requirement: 100ml/kg/24hour
Maintenance fluid should be isotonic.
Premature infant often receive a glucose-containing solution to maintain normoglycaemia.
- This should continue intraoperative
Replace on going loss:-Superficial surgery: 1-2ml/kg/hour-Thoracotomy : 4-7ml/kg/hour-Abdominal surgery :5-10ml/kg/hour
Blood / blood products Required volume
Pack cell ml = desired increment in Hb(g/dL) x weight (kg) x 3
Platelet 10-20ml/kg
FFP 10-20ml/kg
cryoprecipitate 5-10ml/kg
Sign fluid depletion: Hypotension, tacycardia, increase core peripheral temperature, delayed capillary refill time, reduce heart sound
Do not introduce air bubbles into
circulation which may transverse
right to left shunts
Post operative care
Remain intubated??? Or extubate???
- Decision should consider the preoperative state of the baby as well as the type of surgery performed.
- If plan for extubate, baby should fully awake & managing adequate TV without support
Post operation, apnoea are frequent in 1st 12 hour & can continue until 48-72 hour
Monitor in high dependency unit for at least 12 hours post operatively and for a further 12 hours following any apnoeic period.
References Anaesthesia for the preterm infant – Anaesthesia UK
Kawshala Peiris, David Fell,..The Prematurely Born Infant and Anaesthesia, Oxford Journal: Volume 9, Issue 3, Pp 73-77
Sale SM, Read JA ,..Prospective Comparison of Sevoflurane and Desflurane in Premature Infant undergoing inguinal herniotomy, Br J Anaesth.2006 Jun; 96 (6): 774-8. Epub 2006 Apr 28
Guy Bayley, Special consideration in the premature and ex-premature infant, Anaesthesia and Intensive care medicine 12:3, 2010 Elsevier Ltd
Bharti Taneja, Vinish Srivastava, Physiological and anaestheticconsideration for the preterm neonate undergoing surgery ,2012; 1:14
Neonatal Handbook