[Ventilation under high spinal anesthesia--the effect of hypotension]

The ventilatory changes during the course of high spinal anesthesia and the effect of hypotension on ventilation during high spinal anesthesia were studied. Spinal anesthesia with hyperbaric tetracaine was applied to 30 patients scheduled for elective surgery. Patients breathed by mask for ten minutes at rest before and after receiving spinal anesthesia. Respiratory parameters were measured in supine position during (1) pre-anesthetic period under resting condition, (2) anesthetic period when analgesia with pin prick extended to T4 level and (3) anesthetic period when analgesia extended to T1 level. The patients were divided into two groups; those with and without hypotension. In hypotension group, tidal volume and minute ventilation decreased significantly for 30% compared with the control values after spinal anesthesia. PaO2 decreased and PaCO2 increased. In non-hypotension group, tidal volume and minute ventilation after spinal anesthesia increased for 10% compared with the control values. In conclusion, hyperventilation tended to occur in patients with high spinal anesthesia unless hypotension was severe enough. Once severe hypotension had occurred, obvious hypoventilation and respiratory irregularity were observed. Decrease of tidal volume and minute ventilation, hypoxia, hypercarbia and increase in VD/VT were significant during hypotension. The results suggest that during high spinal anesthesia severe hypotension causes hypoventilation and if not treated respiratory arrest ensues.