Hypoxia causes apnea during epidural anesthesia in rabbits.

Abstract

Although pulmonary function is minimally changed by neuraxial blockade in most cases, ventilatory arrest may ensue in rare cases. The authors examined the mechanism of apnea in a rabbit model of sudden ventilatory arrest during the combination of epidural anesthesia and hypoxia. Rabbits were studied during alpha-chloralose sedation and spontaneous ventilation through a tracheostomy tube. Heart rate and mean arterial pressure were monitored by intraarterial cannulation. Respiratory rate and tidal volume were measured by pneumotachograph. Responses were recorded during administration of oxygen at inspired oxygen concentrations of 11% for 2.5 min and 0% for 40 s, before and after either thoracolumbar epidural blockade (0.4 ml/kg lidocaine, 1.5%) or intramuscular lidocaine (15 mg/kg). In a third group of animals, epinephrine was given intravenously during epidural blockade to return mean arterial pressure to baseline values before hypoxia. In a fourth group of animals, which did not get lidocaine, sympathetic blockade and hypotension were produced with intravenously administered trimethaphan rather than epidural blockade. Thoracolumbar epidural anesthesia decreased mean arterial pressure from 76 +/- 4 mmHg (mean +/- SE) to 42 +/- 2 mmHg. Apnea during hypoxia occurred in 90% of these animals (nine of ten) but in only 11% of animals (one of nine) after intramuscularly administered lidocaine (P < 0.01). Treatment of epidural hypotension with epinephrine prevented apnea (zero of nine animals). Apnea during hypoxia occurred in 50% (three of six) of animals given trimethaphan. Apnea in all groups was sudden in onset, with no preceding decreases in respiratory rate or tidal volume. Epidural anesthesia results in a narrowed margin of safety for oxygen delivery to the brain and predisposes subjects to ventilatory arrest during hypoxia. This results from the combined effects of decreased blood oxygen content, which is due to decreased inspired oxygen concentration superimposed on circulatory depression due to neural blockade.