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Effect of age on the solubility of volatile anesthetics in human tissues

Abstract:

To determine the effect of age on the solubility of volatile anesthetics in human tissues, the authors measured the solubilities of isoflurane, enflurane, halothane, and methoxyflurane in vitro at 37°C in 35 postmortem human tissue specimens. Specimens were taken from neonates, and young (20-50 yr), middle-aged (50-70 yr), and elderly adults (>70 yr). Brain/gas, heart/gas, and liver/gas partition coefficients for all four anesthetics increased significantly (P<0.05) between birth and adulthood, although brain/gas partition coefficients in young adults tended to be higher than those in middle-aged and elderly adults. Heart/gas and liver/gas partition coefficients tended to increase with aging. Muscle/gas partition coefficients for the four anesthetics increased linearly with age. Fat/gas partition coefficients did not change significantly with age. Tissue/blood solubilities for the four anesthetics were of the same order of magnitude for a given tissue and age group. Tissue/blood solubilities for enflurane were 30% lower than those for isoflurane in the same tissue and age group. In summary: 1) the solubility of volatile anesthetics in human tissues increases with age; 2) the lower solubility of anesthetics in neonates partially explains the more rapid increase of alveolar and tissue anesthetic partial pressures in neonates; 3) despite the higher blood solubility of enflurane, its lower tissue solubility may explain a rate of recovery comparable with that of isoflurane.