Ventilatory responses to hypercapnia during bupivacaine spinal anesthesia.

The effect of spinal anesthesia with isobaric 0.5% bupivacaine on ventilatory responsiveness to CO2 rebreathing was studied in ten unpremedicated patients. Minute ventilation (VE) at end-tidal PCO2 = 55 mm Hg increased from 18.7 +/- 6.7 L/min (mean +/- SD) to 22.3 +/- 10.1 L/min after induction of spinal anesthesia (P less than 0.05). Occlusion pressure (P0.1) at PCO2 = 55 mm Hg also increased, from 3.8 +/- 1.5 to 5.0 +/- 1.7 cm H2O (P less than 0.05). Spinal anesthesia was not associated with significant changes in vital capacity, maximal inspiratory pressure, resting end-tidal PCO2, or the slopes or intercepts of the lines relating VE or P0.1 to PCO2. These results show an increased ventilatory responsiveness to CO2 with bupivacaine spinal anesthesia.