General Anaesthesia

·         Stages of anaesthesia.

·         Anaesthetic practice.

Stages of General anaesthesia

1.       Stage I – Analgesia:

a.       Characterized by analgesia and amnesia.

b.       Analgesia is partial until stage 2 is about to be reached.

c.       Consciousness and sense of touch are retained.

d.       Sense of hearing is increased.

2.       Stage II – Delirium:

a.       Patient is unconscious, but automatic movements may occur.

b.       Laryngospasm may develop.

c.       Sudden death due to vagal inhibition of the heart may occur in a violent second stage.

3.       Stage III – Surgical anaesthesia:

a.       Surgical anaesthesia occurs with progressive depression of ascending pathways in the reticular activating system and the suppression of spinal reflex.

b.    This is divided into four planes and the required depth differs according to the kind of operation to be performed.

c.       Depth is determined by noting characteristic changes in respiration, pupils, spontaneous eye-ball position, reflexes and muscle tone.

4.       Stage IV – Medullary depression: overdose at this stage leads to cardio-respiratory collapse.

Before Surgery

1.       Assessment of:

a.       patient’s physical and psychological condition.

b.    any intercurrent illness.

c.    the relevance of any existing drug therapy.

2.       Anxiolysis and amnesia:

a.    A patient who is going to have a surgical operation is naturally apprehensive, and it is kind to attempt to reduce this by explanation, reassurance and drugs.

b.       Benzodiazepines promote desirable anxiolysis and amnesia for the immediate presurgical period; temazepam, midazolam or diazepam are appropriate.

3.       Analgesia:

a.       Analgesia may be needed when there is existing pain or as a supplement to an anaesthetic agent having analgesic effect, e.g. nitrous oxide.

b.    If postoperative pain is expected, however, an analgesic may be given both before and at the end of the operation.

4.       Inhibition of secretions:

a.    It may be helpful to limit bronchial and salivary secretions which may collect in the lung and predispose to infection and to reduce any tendency to bronchospasm.

b.       Atropine, hyoscine or glycopyrronium may be used.

c.       Hyoscine can cause confusion in the old.

5.       Gastric contents:

a.    A single dose of a gastric antacid may be given before a general anaesthetic as prophylaxis against aspiration of acid gastric contents in vulnerable patients.

b.    A histamine H2 receptor blocker is an alternative, to reduce secretion volume as well as acidity.

c.       Metoclopramide hastens gastric emptying, usefully increases the tone of the lower esophageal sphincter and is an antiemetic.

During surgery

1.    The aim is to induce unconsciousness, analgesia and muscular relaxation often with separate drugs.

2.       Balanced anaesthesia:

a.    Pre-anaesthetic medication for sedation and analgesia, e.g. benzodiazepines and opioids.

b.    the use of both inhalational and intravenous anaesthetics.

c.    the use of neuromuscular blocking drugs during anaesthesia, e.g. pancuronium.

3.    A typical general anaesthetic consists of induction of general anaesthesia followed by its maintenance.

4.       Induction:

a.    With thiopentone, etomidate, methohexitone or propofol i.v.

b.    The airway is rendered secure with a face mask or laryngeal mask.

c.       Insertion of an endotracheal tube requires brief neuromuscular block with succinylcholine.

5.       Maintenance:

a.       Usually with nitrous oxide and oxygen plus a volatine agent, e.g. halothane or isoflurane.

b.    Less often, with nitrous oxide and oxygen plus i.v. analgesic, e.g. fentanyl, morphine, pethidine, plus a competitive neuromuscular blocking agent if muscle relaxation is required for abdominal surgery.

c.       Where neuromuscular block is used there is risk of paralyzed patient regaining consciousness.

d.    If a neuromuscular blocking drug is not used, muscle relaxation can be provided by deep anaesthesia with an inhalation agent.

6.       Special techniques:

a.       Dissociative anaesthesia: a state of analgesia and light hypnosis is useful where modern equipment and staff are lacking and at scenes of major accidents and wars.

b.       Neuroleptanalgesia, in which a patient is in a state of analgesia but is cooperative (produced by droperidol + fentanyl) is used as a supplement to general anaesthesia.

c.       Sedation and amnesia without analgesia is provided by diazepam and midazolam i.v. used alone for procedures causing discomfort but not pain, e.g. endoscopy, and with a local anaesthetic where pain is expected, e.g. dental procedures.

d.       Patient-controlled analgesia, e.g. with nitrous oxide / oxygen mixtures (Entonox) is effective for brief procedures.

After surgery

1.    The anaesthetist ensures that the effects of neuromuscular blocking agents and opioid-induced respiratory depression have either worn off or have been adequately reversed by an antagonist.

2.       Relief of pain:

a.       Since opioids constipate, may cause vomiting and depress cough and respiration, they are not recommended, e.g. after operations on the bowel and chest and for day-case surgery.

b.       Pethidine neither constipates nor suppresses spontaneous cough significantly, but it can be given i.v. to reduce cough from an endotracheal tube.

3.       Postoperative vomiting:

a.       Avoidance of causative drugs, e.g. thiopentone, nitrous oxide.

b.       Antiemetics can be effective, e.g. metoclopramide, ondansetron, prochlorperazine.

Pharmacology of Anaesthetics

1.       Classification:

a.       Inhalation: nitrous oxide, halothane, enflurane, isoflurane.

b.       Intravenous: thiopentone, methohexitone, ketamine, etomidate, droperidol, fentanyl and benzodiazepines.

2.       Mechanism of action:

a.    Act on the brain, primarily on the midbrain reticular activating system and the cortex.

b.       Increase the cellular threshold of firing and decrease neuronal activity.

c.    A principal site of action is along the neuronal lipid bilayer membrane, which is disordered by the drugs so that cation movements through the ion channels are obstructed.

3.    An ideal anaesthetic should:

a.       induce anaesthesia smoothly and rapidly.

b.       allow rapid recovery from anaesthesia.

c.    have a wide margin of safety.

d.    be devoid of adverse effects.

Drugs that affect anaesthesia

Obstetric analgesia and anaesthesia

1.       Choice of drug:

a.       Relieve pain without making patient confused or uncooperative.

b.    Must not interfere with uterine activity nor must it influence the fetus.

c.       Respiratory depression is chief disadvantage.

2.       Generally, strong analgesic drugs should not be started before uterine contractions are well advanced as they can arrest labor if started sooner.

3.       General guide:

a.       Onset of labor and up to three-quarter dilatation of cervix: non-inhalational tranquilizers and analgesics, e.g. pethidine.

b.       From three-quarter dilatation of cervix till birth: inhalation analgesia, e.g. nitrous oxide/oxygen to avoid respiratory depression of the fetus.

4.       Pethidine:

a.       Seldom causes serious respiratory depression.

b.       Mother may experience drowsiness and anusea.

c.    May impair infant feeding for up to 48h.

d.       Delay gastric emptying and carry hazard of vomiting.

5.       Benzodiazepines:

a.    As tranquilizer during labor.

b.    As anticonvulsant in pre-eclampsia and eclampsia.

6.       Problems of GA during labor:

a.       Regurgitation and aspiration are a particular risk.

b.       Safety of fetus must be considered as all anaesthetics and analgesics cross the placenta in varying amounts.

c.       Gallamine is best avoided as it crosses the placenta.

Anaesthesia in special groups

1.       Respiratory disease and smoking: predispose patient to postanaesthetic pulmonary collapse and pneumonia.

2.       Cardiac disease:

a.       Avoid circulatory stress caused by struggling, coughing, laryngospasm and breath holding.

b.       Drugs given i.v. should be injected slowly to avoid hypotension.

3.       Hepatic and renal disease: can lead to increased drug effects.

4.       Malignant hyperthermia:

a.       Occurs in about 1:20 000.

b.    An inherited muscle disorder involving a sudden rise in release of stored intracellular calcium, stimulating contraction and a hypermetabolic state.

c.       Occur during or within several hours of anaesthesia and is precipitated by almost any drug but especially halothane and succinylcholine.

d.    A life-threatening emergency treated by dantrolene.

5.       Raised intracranial pressure:

a.       Made worse by inhalation agents, e.g. halothane, by hypoxia or hypercapnia in response to intubation.

b.       These patients are liable to respiratory failure with CNS depressants.

6.    The elderly are liable to become confused by cerebral depressants, e.g. hyoscine.