SVR Volatile Agents — FRCA Primary MCQ

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HardPharmacology - Inhalational AgentsSVR Volatile AgentsFRCA Primary

During steady-state anaesthesia in a healthy, normocapnic adult without surgical stimulation, each agent is administered in oxygen at 1.5 age-adjusted MAC. The end-tidal concentration has been increased gradually. Which volatile anaesthetic produces the smallest reduction in systemic vascular resistance?

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Correct answer: E — Halothane

E. Halothane produces the smallest reduction in systemic vascular resistance. Its dose-dependent hypotension is caused predominantly by myocardial depression and a fall in cardiac output rather than by peripheral vasodilation. Isoflurane and desflurane preserve cardiac output relatively better but lower arterial pressure substantially through reduced SVR. Rapid increases in desflurane can cause transient sympathetic activation, but this is excluded by the gradual, steady-state conditions in the stem. Sevoflurane also lowers SVR and has haemodynamic effects broadly resembling isoflurane. Enflurane reduces both myocardial contractility and SVR more than halothane. Nitrous oxide is not an option here because it is an anaesthetic gas rather than a volatile liquid, and its MAC is approximately 104%, so 1 MAC cannot be delivered under ordinary normobaric conditions while maintaining an adequate oxygen concentration.

Reference: Shigematsu T et al. Effects of sevoflurane on hemodynamics during the induction of anesthesia compared with those of isoflurane, enflurane and halothane. Masui, 1993. https://pubmed.ncbi.nlm.nih.gov/8301820/