Figure 1. Effect of intrathecal bupivacaine block on thiopental anesthetic requirements. The intravenous dose of thiopental (given at 4 mg [center dot] kg sup -1 [center dot] min sup -1) required to block the described responses is displayed on the vertical axis. Saline (10 micro liter) or bupivacaine (10 micro liter, 0.75%) was administered intrathecally in a crossover design. The dose of thiopental required to block the described responses was decreased with intrathecally administered bupivacaine by 39% for the eyelid reflex, 43% for front limb withdrawal, and 31% for the corneal reflex. Corneal reflex = blockade of the blink to corneal stimulation by a 4.56-mN von Frey filament; eyelid reflex = ablation of the eyelid closing to tactile stimuli; front limb withdrawal = blockade of the movement to clamping of a digit;*P < 0.001;**P < 0.005;***P < 0.01.