Basic Sciences Foundations of Anesthesia Practice
Core principles
Anesthesia practice rests on applied physiology, pharmacology, and physics. For IV agents, think of a three-compartment model: a small central volume produces rapid onset, while redistribution and elimination terminate effect. Context-sensitive half-time describes the time for a 50% decrease in plasma concentration after an infusion stops and increases with infusion duration for lipophilic drugs; it is not the same as elimination half-life. Volatile potency is usually expressed as MAC, the minimum alveolar concentration at 1 atm that prevents movement in 50% of patients to a standardized noxious stimulus. MAC values are additive, and MAC does not measure sedation, amnesia, or hemodynamic stability.
Local anesthetics work by blocking voltage-gated sodium channels. Their pKa relative to tissue pH determines the fraction of nonionized drug available to cross membranes; inflammation lowers pH, reduces nonionized drug, and slows onset. In overdose, cardiotoxicity and seizures dominate. Oxygen delivery depends on cardiac output, hemoglobin, and saturation; the oxyhemoglobin dissociation curve shifts right with acidosis, hypercapnia, and fever, facilitating unloading but impairing loading. Thermoregulation, acid-base balance, and renal/hepatic clearance matter because they alter drug effect and recovery.
Traps and a rule
Common exam traps include equating MAC with awareness prevention, confusing context-sensitive half-time with elimination half-life, assuming all local anesthetic toxicity begins with seizures, and ignoring additive MAC. Another trap is applying adult clearances directly to neonates, who have immature hepatic enzymes and lower protein binding.
Rule of thumb: if 1 MAC sevoflurane is about 2% and 1 MAC nitrous oxide is about 105%, then 0.5 MAC sevoflurane (about 1%) plus 0.5 MAC nitrous oxide (about 52%) approximates 1 MAC. This additive model guides balanced volatile dosing, but it does not guarantee amnesia or immobility in every patient.