Dr Khulood Alsaraf

Is essential to surgical practice, because it render patients analgesic, amnesic & unconscious

It causing muscle relaxation & suppression of undesirable reflexes

No single drug is capable of achieving these effects both rapidly & safely

General anesthesia

Selection of drugs that provide a safe & efficient anesthetic regimen based on the nature of the surgical or diagnostic procedure, as well as on the patient’s physiologic, pathologic & pharmacologic state.

A.Status of organ systems B. Concomitant used of drugs

Patient factors in selection of anesthesia

Status of organ systems

 

Liver & kidney – because the liver & kidney not only influence the long-term distribution & clearance of anesthesia agents but can also be the target organ for toxic effects.

Respiratory system – specially if inhalation anesthetic are indicated

C.V.S – where as the hypotensive effect of most anesthesia is sometimes desirable, ischemic injury of tissues could follow reduced porfusion pressure.

Nervous system – the existence of neurological disorders (e.g.:- epilepsy, or myasthenia gravis) influences the selection of an anes.

Pregnancy – some precautions should be kept in mind when anes. & adjunct drugs are administered to a pregnant woman.

Concomitant used of drugs

Pre-anesthetic medication serves to calm the patient, relieve pain & protect against undesirable effects of the subsequently administered anesthetic & the surgical procedure.

e.g. on these drugs:-Anticholinergics, Antiemetics,

Antihistamines, Barbiturate, Benzodiazepines, muscle relaxants & opioids.

Induction, Maintenance & Recovery from Anesthesia

Anesthesia can be divided into 3 stages :-

1- Induction : is defined as the period of time from the onset of administration of the anesthetic to the development of effective surgical anesthesia in the patient . G.A is normally induced with I.V anesthetic

2- Maintenance : is the period during which the patient is surgically anesthetized.

3- Recovery : is the time from discontinuation of administration of anesthesia until consciousness & protective physiologic reflexes are regained.

Depth of anesthesia Is divided into 4 sequential stages

1- stage I – analgesia: Loss of pain sensation result from interference with sensory transmission in the spinothalamic tract.

2- stage II – excitement: Patient experiences delirium & violent combative behaviour, irregularity in blood pressure.To avoid this stage of anes. , a short-acting barbiturate, such as thiopental, is given I.V before inhalation anes. is administered.

3- stage III – surgical anesthesia: Regular respiration & relaxation of the skeletal muscles occur in this stage. Surgery may proceed during this stage.

4- stage IV – medullary paralysis: Sever depression of the respiratory & vasomotor center occur during this stage. Death can rapidly occur unless measures are taken to maintain circulation & respiration.

Inhalation Anesthetics

Inhaled gases are the mainstay of anesthesia & used primarily for the maintenance of anesthesia after administration of an I.V agent.

No one anesthetics is superior to another under all circumstances.

Inhalation Anesthetics have a benefit that is not available with I.V agents, bec. The depth of anes.

can be rapidly altered by changing the conc. of the drug. Inhalation Anesthetics are also reversible, bec. most are rapidly eliminated from the body by exhalation.

Effect of different tissue types on anesthetics uptake

a.brain, heart, liver, kidney, & endocrine glands : Highly perfused tissues rapid attain a steady-state.

b. Skeletal muscles : Poorly perfused, prolonged the time required to achieve Ss.

c. Fat : Poorly perfused, large capacity to store anesthesia, so prolonged the time required to achieve Ss.

d. Bone, ligament & cartilage : Poorly perfused, low capacity to store anesthetics

Mechanism of action

No specific receptor has been identified as the locus of G.A action Initial research focused on identifying a single biological site of action for these drugs.In recent years this “unitary theory” of anesthetic action has been supplanted by a more complex picture of molecular targets located at multiple levels of CNS. For e.g the G.A increase the sensitivity of the Gamma-amino-butyric acid [ GABAa ] receptors to the neurotransmitter, GABA, at clinically effective conc. of the drug. This causes a prolongation of the inhibitory chloride ion current after a pulse of GABA release, post synaptic neuronal excitability is thus diminished. Other receptors are also affected by volatile anes. for e.g : The activity of the inhibitory glycine receptors in the spinal motor neurons is increased. In addition, the inhalation anes. block the excitatory post-synaptic current of nicotinic receptors, the mechanism is not understood.

Inhaled Anesthetics

Volatile and gaseous anesthetics, both of which are administered by inhalation.Volatile anesthetics: halothane, enflurane, isoflurane, desflurane, sevoflurane. [they are liquid at room temp.]Gaseous anesthetics: nitrous oxide and xenon.Used for the maintenance of anesthesia after administration of IV agent.They have very narrow therapeutic index

Halothane

It is the prototype It is ability to induce the anesthetic state rapidly & to allow quick recovery Because. of it is adverse effects & the availability of other anes. that cause fewer complication. Halothane is largely being replaced by others G.A.

Adverse effects

1-Cardiac effect : Halothane is vagomimetic & cause atropine-sensitive bradycardia, cardiac arrhythmias, & conc. – dependent hypotension. 2-Malignant hyperthermia

Enflurane

This gas is less potent than halothane, but produces rapid induction & recovery Enflurane anes. exhibits the following differences from Halothane anes. :- Fewer arrhythmias Lesser sensitization of the heart to catecholamineGreater potentiating of muscle relaxants due to a more potent curare-like effect.

Disadvantage CNS excitation & for this reason, it is not used in patients with seizure disorders.

Sevoflurane

Rapid induction without irritating the air way Suitable for inhalation induction in pediatric patients, it is replacing halothen Has a rapid onset of action and recovery due to low blood solubility

Nitrous oxide

N2O called [ laughing gas ]

- It is potent analgesic but a weak G.A.

For e.g :- N2O is frequently employed at conc. of 30% with O2 for analgesia, particularly in dental surgery .

N2O is poorly soluble in blood & other tissues, allowing it to move very rapidly in & out of the body.

N2O can concentrate the halogenated anes. in the alveoli when they are concomitantly administered bec. of its fast uptake from the alveolar gas. This phenomenon is known as [ second gas effect ]

N2O does not depress respiration, and does not produce muscle relaxation.

It has moderate to no effect on the CVS or on increasing cerebral blood flow, & it’s the least hepatotoxic of inhalation anes.

Intravenous Anesthetics

-I.V anesthetics are used for the rapid induction of anesthesia, which is then maintained with an appropriate inhalation agent. -They rapidly induce anes. & must therefore be injected slowly. -Recovery from I.V anes.is due to redistribution from sites in the CNS.

1-Barbiturates

Thiopental is a potent anesthetic but a weak analgesic.

It is an ultra-short acting barb. & has a high lipid solubility. The short duration of anes. action is due to decrease of its conc. in the brain to a level below that necessary to produce anes.

the barb. are not significantly analgesic & some time combined with analgesic to avoid objectional changes in blood pr. & autonomic function. Thiopental has a minor effects on the CVS, but it may contribute to sever hypotension in hypovolemic or shock patients.

All barb. can cause apnea, coughing, chest wall spasm & bronchospasm.

2- Benzodiazepines - Used in conjunction with anes. to sedate the patient.

-The most commonly employed is midazolam.-Diazepam and lorazepam are alternatives.

-All three facilitate amnesia which causing sedation

3- Opioids  

-Because of their analgesic property, opioids are frequently used together with anes. for e.g., the combination of morphine and N2O provides good anes. for cardiac surgery.

-The most frequently employed opioids is fentanyl, because it induce analgesia more rapidly than does morphine.

-Opioids are not good amnesics and they can all cause hypotension, respiratory depression and muscle rigidity as well as post-anesthetic nausea and vomiting.

-They administered either I-V, epidurally, or intrathecally. 

4- Etomidate

-It is used to induce anes.-It is hypnotic agent but lacks analgesic activity.-Induction is rapid, and the drug is short-acting.

-It is only used for patients with coronary artery disease or CV dysfunction such as shock.

-Little to no effect on the heart and circulation.-Its adverse effect include a decrease in plasma cortisol and aldosterone levels, which can persist for up to 9 hr. also venous pain can occur

5-Ketamine

-a short acting -it provides sedation, amnesia & immobility -it stimulates the central sympathetic out flow, stimulation of the heart & increased B.Pr & cardiac output. This is useful in hypovolemic or cardiogenic shock,& asthmatic patients. -its used mainly in children & young adults-its not widely used, bec. It increases cerebral blood flow & induces postoperative hallucination [ night mares ] particularly in adults.

6-Propofol

 -I-V sedative/hypnotic used in the induction or

maintenance of anes.-onset is smooth and occurs within about 40 sec,

combined with narcotics for analgesia, it decreases blood pressure without depressing the myocardium.

-it also reduces intracranial pressure, it is widely used and has replaced thiopental as the first choice for anes. induction and sedation, because it produces a euphoric feeling in the patient and does not cause post-anes. nausea and vomiting.