Effect of Age on the Solubility of Volatile Anesthetics in Human Tissues

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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.degree. 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.