Isoflurane (Forane)

MAC Value - 1.2% in 100% O2 (age-adjusted). Reduces with nitrous oxide, opioids, and increasing age. Use end-tidal monitoring to titrate depth.

Cardiovascular Depression - Causes dose-dependent hypotension primarily through vasodilation rather than myocardial depression. Usually well tolerated but watch your elderly and volume-depleted patients. Treat with fluid or vasopressors if needed.

Coronary Steal - Theoretical risk in patients with coronary artery disease (CAD) — isoflurane dilates coronary vessels and may steal flow from already-compromised areas. Clinically debated, but worth considering in high-risk cardiac cases.

Airway Irritability - More pungent than sevoflurane; not ideal for inhalation induction. Can trigger breath-holding, coughing, or laryngospasm, especially in kids or light patients. Better suited for maintenance than induction.

Bronchodilation - Despite airway irritability on induction, isoflurane is actually a good bronchodilator once the patient is deep enough. Useful in patients with reactive airway disease intraoperatively.

Malignant Hyperthermia - All volatile agents, including isoflurane, can trigger MH in genetically susceptible patients. Know your patient's history. Have dantrolene immediately available.

Increased ICP - At higher concentrations, cerebral vasodilation increases ICP. Use with caution in patients with intracranial hypertension; keep concentration low and maintain normocarbia or mild hypocarbia.

Drug Interactions - Potentiates neuromuscular blockade — you'll need less NMB and should monitor TOF closely. Additive CNS and respiratory depression with opioids, benzos, and propofol. Avoid or use extreme caution with epinephrine (arrhythmia risk, though less than with halothane).

Pediatric Implications - Not preferred for mask induction in kids due to its pungent smell and high rate of laryngospasm and breath-holding — sevoflurane is the go-to. Once intubated, isoflurane is usable for maintenance. MAC is higher in infants and young children.

Obstetric Implications - Crosses the placenta readily and causes dose-dependent uterine relaxation, which can increase blood loss at delivery. At sub-MAC concentrations (0.5 MAC) used for cesarean section, generally well tolerated for both mother and neonate. Neonatal depression is possible at higher doses.

Absolute:

Known or suspected MH susceptibility

Known hypersensitivity to halogenated agents

Prior halothane hepatitis (cross-reactivity possible)

Relative:

Severe coronary artery disease with high steal risk

Raised intracranial pressure

Hypovolemia (exaggerated hypotension)

Caution:

Elderly patients (MAC significantly reduced)

Patients requiring uterine tone (postpartum hemorrhage risk)

Prolonged exposure at low fresh gas flows

Potentiates GABA-A receptor activity and inhibits NMDA receptors, reducing neuronal excitability. Also modulates sodium and potassium channels. Exact mechanism of consciousness suppression remains incompletely understood.

Hepatic (minimal, ~0.2%)

Pulmonary (>99% exhaled unchanged)

Minimum alveolar concentration (MAC) of 1.2% is the highest of the commonly used modern volatiles — factor this into vaporizer settings

Blood:gas partition coefficient of 1.4 means slower onset and offset compared to sevoflurane (0.65) and desflurane (0.42)

Stored at room temperature; no preservatives required

Older agent — largely replaced by sevoflurane and desflurane in high-resource settings, but still widely used globally due to lower cost