[A possible circadian rhythm of susceptibility to thiamylal]

A circadian rhythm of the susceptibility to thiamylal was evaluated in 30 patients. They were divided into two groups; 15 patients were induced around 9 AM (Morning-group) and the rest were in the early afternoon (Afternoon-group). The induction was performed by intravenous administration of thiamylal sodium. Parameters measured were the time and doses of thiamylal sodium to obtain the loss of consciousness and the loss of eyelash reflex. The results were as follows; the doses of thiamylal sodium to obtain the loss of consciousness were 3.5 +/- 0.6 mg.kg-1 (mean +/- SD) in Morning-group, and 4.1 +/- 0.4 mg.kg-1 in Afternoon-group, respectively. Thus, larger doses of thiamylal were necessary to obtain the loss of consciousness in Afternoon-group than in Morning-group (P less than 0.01). The doses of thiamylal sodium to obtain the loss of eyelash reflex were 4.4 +/- 0.8 mg.kg-1 in Morning-group and 5.0 +/- 0.6 mg.kg-1 in Afternoon-group, respectively. Also, larger doses of thiamylal were necessary to obtain the loss of eyelash reflex in Afternoon-group than in Morning-group (P less than 0.05). This study suggests that a circadian rhythm of the susceptibility to thiamylal exists in humans.