Isoflurane anesthesia and circadian temperature cycles in humans

Cognitive performance in postsurgical patients may be impaired by disturbances of normal circadian rhythm analogous to those produced by rapid transmeridian travel ("jet lag"). We therefore tested the hypothesis that isoflurane anesthesia alone produces a phase-delay in the human circadian temperature rhythm. We monitored central body temperature rhythms (using an ingested sensor) in five young, healthy, male volunteers at 3-min intervals for a total of 5 days. On the 3rd day, 3 h of 1.0% isoflurane anesthesia was administered beginning at approximately 10:00 AM. Thiopental, opioids, and other medications were not administered; volunteers were kept normothermic during anesthesia. Visual inspection of the data confirmed that periodicity of the temperature cycles remained near 24 h and that the curve was sinusoidal. Data were fit using a two-step sine and cosine regression for each 24-h period. Before anesthesia, volunteers demonstrated a consistent 24-h cycle, with a mean temperature (mesor) of 36.8 +/- 0.2 degrees C, amplitude of 0.8 +/- 0.2 degrees C, and time of maximum temperature (acrophase) of 3:06 PM +/- 2.4 h. Isoflurane anesthesia did not produce significant changes in the central temperature mesor. Peak-to-trough range (amplitude) of the temperature cycle was significantly reduced on the day of anesthesia (0.5 +/- 0.2 degrees C) but returned to normal on the subsequent day. Compared with the 2 days preceding isoflurane administration, there was no statistically significant change in acrophase on the day following anesthesia. These data do not support our hypothesis and suggest that the internal timer controlling circadian temperature cycles is resistant to clinical concentrations of isoflurane.