Inhalational agents

These are the vapours which in clinically useful concentrations help to maintain anaesthesia and decrease awareness. In Britain they are generally added to the fresh gas flow by passing a fraction of the carrier gas (N2O/O2 or oxygen enriched air) through a plenum vaporizer.

Halothane

This gas has little analgesic effect. It decreases cardiac output (vagal tone ↑, leading to bradycardia, vasodilation, and hypotension). It sensitizes the myocardium to catecholamines (beware in patients with arrhythmias; surgical infiltration with local anaesthetic and adrenaline/ epinephrine). Halothane has been replaced by safer inhalational agents, due to the rare but serious complication of postoperative hepatitis.

Isoflurane

This is a halogenated ether. Theoretically induction should be quick, but isoflurane is irritant, so coughing, laryngospasm, or breath- holding may complicate the onset of anaesthesia.

Sevoflurane

This is a halogenated ether which is well-tolerated. It is the agent of choice for inhalation induction of general anaesthesia with low blood:gas solubility.

Desflurane

is another halogenated ether with a pungent smell, rapid onset of anaesthesia, and quick recovery. Meta-analyses of trials in which the duration of anaesthesia was h indicate that patients receiving either desflurane or sevoflurane did not have significant differences in time to be discharged from the recovery unit, or in nausea/vomiting frequency. Patients receiving desflurane followed commands, were extubated, and were oriented 21 minute earlier than those receiving sevoflurane.1 But this may not be significant clinically. Also, desflurane needs a special vaporizer.

Stopping inhalation reverses all the above effects except for hepatitis resulting from drug metabolism.

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Local anaesthesia

All regional techniques may lead to loss of consciousness or loss of airway, and so require the same facilities, expertise, and precautions as for general anaesthesia (eg full resuscitation facilities and patient fasted).

Local anaesthesia is used either alone or to supplement general anaesthesia, the aim is to prevent or reduce nerve conduction of painful impulses to higher centres (via the thalamus), where the perception of pain occurs. (Action is by a membrane-stabilizing effect, impairing membrane permeability to sodium, so blocking impulse propagation.)

Types of local anaesthesia (LA) Amides:

• Lidocaine (=lignocaine) t 2h; max dose in typical healthy adult=3mg/kg up to 200mg (500mg with epinephrine, but use with caution).
• Prilocaine t 2h. Moderate onset. The dose is 3-5mg/kg. (400mg is the maximum dose in adults; 300mg if used with felypressin.) Very low toxicity, so it is the drug of choice for Bier's block (IV regional anaesthesia).
• Bupivacaine (levobupivocaine1) t 3h. Slow onset and prolonged duration. Contraindicated in intravenous regional anaesthesia (Bier's block). (NB: levobupivacaine is a newer less cardiotoxic local analgesic, otherwise similar in action to bupivacaine.) For dose see BNF.
• Ropivacaine t 1.8h. Dose: see BNF. Less cardiotoxic than bupivacaine. Less motor block when used epidurally. Contraindicated for regional anaesthesia and paracervical block in obstetrics.

Note: 0.5% solution=5mg/mL. 1% solution=10mg/mL. So for a 70kg man, the max dose of lidocaine is 20mL of 1% or 10mL of 2% solution. NB: lidocaine injections are less painful if they are warm, or at lower concentrations.

Certain commercially available preparations contain adrenaline (=epinephrine); these should be used with extreme caution, as systemic effects from the adrenaline may arise and prove hazardous, especially in CVS disease orBP.

Adrenaline (epinephrine) is ABSOLUTELY contraindicated in digital or penile blocks, and around the nose or ears. (Ischaemia produced may cause gangrene.)

Esters

(Infrequently used now.)

• Cocaine. Very high toxicity. Short duration of action. Used as a paste preparation for anaesthesia and vasoconstriction prior to nasal intubation or nasal surgery.
• Tetracaine t 1h. Slow onset. High toxicity. Drops for topical anaesthesia to eye (the eye must be covered with a patch following use), and now topically as an alternative to EMLA.

Toxicity

From excess dose, too rapid absorption, or direct IV injection.

Features

Perioral tingling; numb tongue; anxiety; lightheadedness; tinnitus; seizures; apnoea; collapse; direct myocardial depression; coma.

Treatment

Ensure oxygenation.

• Hypotension: try IV fluids first, then consider ephedrine 5mg IV as a slow bolus (may be repeated every 3-4min to a total of 30mg).
• Convulsions: thiopental (=thiopentone) sodium, or benzodiazepines, eg IV Diazemuls 2.5-5mg, or midazolam 5mg boluses. Intubation/IPPV will probably also be needed.
• CVS collapse: full resuscitation, ie intubation, adrenaline (=epinephrine), atropine, and cardiac massage as required.

Anaphylaxis

Occurs more commonly with the esters, but can occur with amides. A 2mL test dose can detect possible allergic reactions, but not anaphylaxis (and can warn of inadvertent intravascular injection).

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Some complications of anaesthesia

The commonest respiratory complication is airway obstruction from loss of muscle tone in the soft palate

Atelectasis and pneumonia

Atelectasis is best seen on CT (not CXRs). It starts within minutes of induction, and is partly caused by using 100% O2.

[prescription take]

good pain relief aids coughing. Arrange physio + antibiotics.

Laryngospasm

The cords are firmly shut. Treat with 100% oxygen. Attempt to ventilate. It may be necessary to paralyze and intubate.

Bronchospasm

Ensure oxygenation. If intubated, check tube position (carina stimulation may be the cause: withdraw tube slightly). Check for pneumothorax. (Increase concentration of volatile agent if he is light.) Salbutamol 250 eg by IV, aminophylline hyperventilation with 100% O2. If persistent, give hydrocortisone 100mg IV and consider epinephrine (=adrenaline, 1-10mL up to 0.1mL/kg of 1:10,000 IV).

Shivering hypothermia

Preventable with space blankets, and may be treated by radiant heat and pethidine for shivering (0.3mg/kg slowly IV). Be aware that shivering increases O2 consumption 5-fold.

Scoline apnoea

(Abnormal cholinesterase) ventilate until power returns.

Malignant hyperpyrexia

(Autosomal dominant). Rapidly rising temperature (>2°C/h) is precipitated by eg halothane or suxamethonium. Masseter spasm may be the first indication, then tachycardia, mottling, hypercarbia, hyperkalaemia, arrhythmias, hypoxaemia, and acidosis.

Treatment

• Get senior help; stop all precipitants and stop surgery.
• Hyper- ventilate with O2.
• Active cooling (ice packs, cold IVI).
• Take to ITU
• Dantrolene 1mg/kg IV bolus; repeat as needed till up to 10mg/kg given.
• Mannitol for myoglobinuria.
• Correct acidosis (sodium bicarbonate).
• Treat any hyperkalaemia (OHCM p 825)
• Encourage diuresis
• Insulin
• Steroids, eg hydrocortisone 100mg IV.
• Phenotype relatives (muscle biopsy response to halothane and caffeine).
• Get a Medicalert bracelet, and a letter to give to future anaesthetists.

Awareness

This is most distressing for patients. Neuromuscular blockade makes its diagnosis difficult; the BIS monitor (above) may incidence.

Prevalence

Much less than 1% even in emergency/obstetric anaesthesia (here doses used are minimized, to avoid shock). It is still rarer in other contexts (eg 0.2%).

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Sedation

Sedation is an induced reduction in conscious level, during which verbal contact is maintained with the patient.

Doctors in many specialties may be required to administer sedation. The doctor giving the sedation must not be also responsible for performing any procedure (such as manipulation of a dislocated joint). His sole responsibility is to ensure that the sedation is adequate, and to monitor the patient's airway, breathing, and circulation. Sedation is not a short cut to avoid formal anaesthesia, and it does not excuse the patient from an appropriate work-up or reasonable fasting. Monitoring is mandatory, and should include pulse oximetry.

It is easy for sedation to become general anaesthesia, with its attendant risks (see p 628). The loss of the eyelash reflex (gentle stroking of the upper eyelashes to produce blinking) is a good guide to the onset of general anaesthesia.

In the elderly,cognition after GA may persist for months: it is unknown whether lesser forms of anaesthesia can obviate this.

Agents

• Midazolam: Initial adult dose 2mg IV over 1min (1mg if elderly). Further 0.5-1mg IV as needed after 2min. Usual range 3.5-7.5mg (elderly max 3.5mg). SE: psychomotor function.18
  In some circumstances (eg manipulation of large joint; painful dressing changes) a narcotic analgesic may be used in addition (eg morphine in 1-2mg aliquots IV, or shorter acting opioids such as fentanyl), or small incremental doses of ketamine may be administered.
• Diazepam (as emulsion = Diazemuls: initial adult dose 2.5-10mg IV over 4min via a large vein.

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Recovery from anaesthesia

• Ensure same monitoring as during anaesthesia.
• 40% O2 via a face mask for >15min to assist with any respiratory depression or ventilation/perfusion mismatch.1
• Monitor pulse and BP.
• Keep the patient warm.
• Look for hypoventilation (inadequate reversal check with nerve stimulator; narcosis reverse opiates with naloxone cautiously to minimize pain; check for airway obstruction, eg from bleeding tonsil). Ensure adequate analgesia.
• Return the patient to the ward when you are satisfied with his cardiovascular and respiratory status and pain relief.
• Give clear instructions on post-operative fluid regimens, blood transfusions, oxygen therapy, pain relief, and physiotherapy.
• Post-op vomiting is partly preventable by a 5-HT3 antagonist eg granisetron, or dexamethasone.

Metoclopramide is less good.

Epidural local anaesthetics GI paralysis compared with systemic or epidural opioids, with comparable pain relief.

Epidurals may also post-op risk of respiratory failure.

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