Obesity Modestly Affects Inhaled Anesthetic Kinetics in Humans

BACKGOUND: Few studies have determined the effect of obesity on inhaled anestheticpharmacokinetics. We hypothesized that the solubility of potent inhaled anesthetics infat and increased body mass index (BMI) in obese patients interact to increaseanesthetic uptake and decrease the rate at which the delivered (FD) and inspired (FI)concentrations of an inhaled anesthetic approach a constantly maintained alveolarconcentration (end-tidal or FA). This hypothesis implies that the effect of obesitywould be greater with a more soluble anesthetic such as isoflurane versus desflurane.METHODS: In 107 ASA physical status I–III patients, anesthesia was induced withpropofol, tracheal intubation facilitated with neuromuscular blockade, and ventilationcontrolled with 50% nitrous oxide in oxygen to maintain end-tidal carbon dioxideconcentrations between 35 and 45 mm Hg. Isoflurane or desflurane was administeredina1L/mininflowrateatFDconcentrationssufficienttomaintainFAat0.6minimumalveolar anesthetic concentration (0.7% or 3.7%, respectively). FD, FI, and FA weremeasured 5, 10, 20, 40, 60, 90, 120,150, and 180 min after starting potent inhaledanesthetic delivery.RESULTS: Fifty-nine patients received isoflurane and 48 received desflurane. BMIranged between 18 and 63 kg/m