General Anesthesia

All inhalational anesthetics produce dose-dependent respiratory depression by suppressing central respiratory drive in the medullary respiratory centers and blunting chemoreceptor responsiveness. The resulting breathing pattern is rapid and shallow — tidal volume falls while respiratory rate may increase, producing net hypoventilation and hypercapnia under spontaneous breathing conditions. This pattern is in contrast to opioid-induced respiratory depression, which primarily slows rate while preserving tidal volume.

The ventilatory response to rising carbon dioxide is progressively blunted in a dose-dependent manner. At 1 minimum alveolar concentration, this response is reduced by approximately half; at 2 minimum alveolar concentration, it is nearly abolished. Controlled mechanical ventilation is therefore standard practice during general anesthesia for most surgical procedures. The hypoxic ventilatory response — the increase in ventilation triggered by falling arterial oxygen tension, mediated by carotid body chemoreceptors — is even more sensitive to volatile agents and is substantially impaired at subanesthetic concentrations. This explains the rationale for supplemental oxygen and pulse oximetry monitoring throughout the recovery period.

All volatile halogenated anesthetics produce bronchodilation through direct relaxation of bronchial smooth muscle, independent of the autonomic nervous system. Sevoflurane and halothane are the most potent bronchodilators and are preferred in patients with asthma or reactive airways disease. Desflurane at high concentrations is an airway irritant that can paradoxically trigger reflex bronchoconstriction in reactive airways, making it unsuitable in this population.

Hypoxic pulmonary vasoconstriction is the physiological reflex that diverts blood flow away from poorly ventilated lung regions, preserving arterial oxygenation. All volatile halogenated agents inhibit this reflex in a dose-dependent manner, worsening intrapulmonary shunting and contributing to hypoxemia — particularly during one-lung ventilation for thoracic surgery. Propofol does not inhibit hypoxic pulmonary vasoconstriction and is preferred for total intravenous anesthesia during thoracic procedures requiring one-lung ventilation.

Malignant hyperthermia is a potentially lethal pharmacogenetic disorder of skeletal muscle calcium homeostasis. It is caused primarily by mutations in the ryanodine receptor type 1 gene, which encodes the sarcoplasmic reticulum calcium release channel in skeletal muscle. These gain-of-function mutations produce a channel that is pathologically sensitive to triggering agents, causing uncontrolled calcium release when exposed to them. Inheritance is autosomal dominant with variable penetrance — first-degree relatives of an affected individual have a 50% probability of carrying the mutation.

All volatile halogenated anesthetic agents — halothane, isoflurane, sevoflurane, desflurane, and enflurane — are triggering agents. Succinylcholine, the depolarizing neuromuscular blocking agent, is also a trigger and is often the first drug given in an anesthetic sequence. Nitrous oxide is not a triggering agent for malignant hyperthermia — this distinction is critical for planning anesthesia in susceptible patients.

Exposure to a triggering agent in a susceptible individual causes the mutant ryanodine receptor to open uncontrollably, flooding the myoplasm with calcium from the sarcoplasmic reticulum. The resulting massive, sustained skeletal muscle contracture drives a hypermetabolic crisis: adenosine triphosphate is consumed faster than it can be produced, generating heat, carbon dioxide, and lactic acid at extraordinary rates. Rhabdomyolysis releases myoglobin, potassium, and creatine kinase into the circulation.

The earliest and most sensitive sign of malignant hyperthermia is a rapid, unexplained rise in end-tidal carbon dioxide that cannot be explained by changes in ventilation. This precedes temperature elevation by minutes to hours. Other early signs are tachycardia and masseter muscle rigidity (jaw rigidity after succinylcholine is a warning sign). As the crisis evolves: generalized muscle rigidity, hyperthermia (temperature may rise 1 to 2 degrees Celsius per minute, reaching above 40 to 41 degrees Celsius), metabolic and respiratory acidosis, rhabdomyolysis, hyperkalemia, and cardiovascular collapse follow.

Treatment of malignant hyperthermia is a medical emergency requiring immediate action on several fronts simultaneously.

All volatile anesthetics transiently reduce renal blood flow and glomerular filtration rate through hemodynamic effects — reduced cardiac output and renal vasoconstriction. These effects are clinically insignificant in patients with normal renal reserve and reverse with emergence. In patients with pre-existing renal impairment, any additional reduction in renal perfusion during anesthesia may contribute to perioperative acute kidney injury.

Beyond hemodynamics, two specific renal toxicity mechanisms are important:

Inorganic fluoride generated by volatile agent metabolism can cause a vasopressin-resistant (nephrogenic) diabetes insipidus — a high-output renal failure syndrome — when intrarenal fluoride concentrations are sufficiently high. This mechanism was first characterized with methoxyflurane, a now-discontinued volatile agent that underwent approximately 50% metabolism, generating very high systemic and intrarenal fluoride concentrations. Enflurane generates lower systemic fluoride concentrations and may cause mild, subclinical reductions in urinary concentrating ability after prolonged use.

Sevoflurane generates systemic fluoride concentrations that transiently exceed the threshold associated with methoxyflurane toxicity, but clinically significant nephrotoxicity has not been demonstrated. The key difference: methoxyflurane was substantially metabolized within the kidney itself, producing high local fluoride concentrations at the site of tubular toxicity. Sevoflurane is metabolized primarily in the liver, so intrarenal fluoride generation is insufficient to cause tubular injury despite elevated systemic levels. The fluoride threshold concept from methoxyflurane does not translate directly to sevoflurane nephrotoxicity risk.

Sevoflurane degrades in contact with carbon dioxide absorbents at low fresh gas flows to produce compound A, a vinyl ether that causes nephrotoxicity in rats. Human nephrotoxicity from compound A has not been demonstrated in clinical trials, likely because the metabolic pathway responsible for nephrotoxic activation in rats is quantitatively less active in humans. Caution is nevertheless advised in patients with pre-existing chronic kidney disease undergoing prolonged procedures at very low fresh gas flows.