Describe Adverse Effects of Inhalational Agents in detail.

Overview

Although modern inhalational agents are relatively safe, they share dose-dependent cardiovascular and respiratory depression and have specific toxicities. Awareness of these guides agent choice and monitoring.

Cardiovascular & Respiratory Effects

Volatile agents cause dose-dependent myocardial depression and vasodilatation (falling blood pressure), and respiratory depression (reduced tidal volume, blunted response to CO₂). Halothane notably causes bradycardia and sensitises the myocardium to catecholamines (arrhythmias); the newer agents cause less arrhythmia. All reduce the ventilatory drive, so ventilation is usually supported.

Halothane Hepatitis

Halothane hepatitis is a rare but potentially fatal immune-mediated hepatic necrosis associated with halothane, more likely after repeated exposure over a short interval, in obese middle-aged women, and with a previous unexplained reaction. It presents as fever and jaundice with markedly deranged liver function days after anaesthesia. Because of this, halothane is avoided where alternatives exist and not repeated within a short interval.

Malignant Hyperthermia

All the potent volatile agents (and suxamethonium) can trigger malignant hyperthermia in susceptible individuals — a rare, inherited, life-threatening hypermetabolic crisis of skeletal muscle (rising CO₂ production, tachycardia, rigidity, hyperthermia, acidosis) treated with dantrolene and removal of the trigger. A personal or family history mandates a trigger-free (‘clean’) anaesthetic — total intravenous anaesthesia with a vapour-free machine.

Postoperative Nausea & Shivering

Among the less dramatic but common adverse effects of inhalational anaesthesia are postoperative nausea and vomiting, to which the volatile agents and nitrous oxide both contribute, and postoperative shivering. These are important because they distress patients, delay recovery and discharge, and — in the case of shivering — increase oxygen consumption, which matters in patients with limited cardiorespiratory reserve. They are managed with prophylactic anti-emetics in at-risk patients, sometimes by choosing a total intravenous technique (propofol being anti-emetic), and by maintaining normothermia and using active warming to reduce shivering.

• halothane — catecholamine-sensitised arrhythmias and immune hepatitis
• all potent volatiles — dose-dependent cardiorespiratory depression and the malignant-hyperthermia trigger (treated with dantrolene)
• nitrous oxide — vitamin B₁₂ inactivation and air-space expansion. Nitrous oxide and the intravenous agents do not trigger malignant hyperthermia.

Trigger-free Anaesthesia

For a patient known or suspected to be susceptible to malignant hyperthermia, the entire anaesthetic is planned to be trigger-free: all potent volatile agents and suxamethonium are avoided, anaesthesia is provided by total intravenous techniques (for example propofol with a non-triggering opioid and a non-depolarising relaxant), and the anaesthetic machine is prepared to be vapour-free by removing or disabling vaporisers and flushing the circuit. Dantrolene is confirmed to be available, and the patient is monitored closely, because recognising and avoiding the triggers is far more effective than treating an established crisis.

Malignant hyperthermia is triggered by all volatiles and suxamethonium.

• All volatiles: dose-dependent myocardial depression, vasodilatation, respiratory depression.
• Halothane: bradycardia, catecholamine sensitisation (arrhythmias), and halothane hepatitis (immune, repeated exposure).
• Potent volatiles (+ suxamethonium) trigger malignant hyperthermia → dantrolene; N₂O/IV agents don’t.
• Susceptible patients need a trigger-free (TIVA) anaesthetic.
• Rising ETCO₂ + rigidity + hyperthermia on a volatile = MH until proven otherwise.