Ventilatory responses to hypercapnia during tetracaine spinal anesthesia

The effect of spinal anesthesia with hyperbaric tetracaine with epinephrine on resting ventilation and on ventilatory responsiveness to CO2 rebreathing was studied in 10 unpremedicated patients. Resting end-tidal PCO2 (PETCO2) decreased from 37 +/- 3 mmHg (mean +/- SD) to 34 +/- 2 mmHg after induction of spinal anesthesia (p less than 0.05). Minute ventilation (VE) and occlusion pressure (P0.1) at PETCO2 = 55 mmHg increased during spinal anesthesia from 32.0 +/- 12.9 to 40.2 +/- 17.0 l/min and from 5.0 +/- 1.8 to 8.6 +/- 4.7 cmH2O, respectively. The magnitude of the increase in VE during spinal anesthesia correlated inversely with age. Spinal anesthesia was not associated with significant changes in vital capacity, maximal inspiratory pressure, or the slopes of the lines relating VE or P0.1 to PCO2. These results show increased ventilatory responsiveness to CO2 (a parallel leftward shift of the CO2 response curve) with tetracaine spinal anesthesia.