Cardiovascular, respiratory and metabolic responses to apnea induced by atlanto-occipital intrathecal lidocaine injection in anesthetized horses

OBJECTIVE: To determine physiologic responses to apnea-induced severe hypoxemia in anesthetized horses.

STUDY DESIGN: Prospective experimental study.

ANIMALS: Six university-owned horses with a median (range) body weight of 500 (220-510) kg and aged 13.5 (0.8-24.0) years scheduled for euthanasia.

METHODS: Xylazine-midazolam-ketamine-anesthetized horses breathing room air spontaneously were instrumented with a facial artery catheter for pressure measurement and blood sampling, and were made apneic with atlanto-occipital intrathecal lidocaine (4 mg kg-1 ). Cardiopulmonary, biochemical and hematologic variables were recorded before (baseline) and every minute for 10 minutes after lidocaine injection.

RESULTS: PaO2 values were: baseline, 55 mmHg (7.3 kPa); 1 minute, 28 mmHg (3.8 kPa); 2 minutes, 18 mmHg (2.4 kPa); 3 minutes, 15 mmHg (2.0 kPa), and 4-10 minutes, (8-11 mmHg (1.1-1.5 kPa). PaCO2 values were: baseline, 50 mmHg (6.7 kPa); 1 minute, 61 mmHg (8.1 kPa), and 2-10 minutes, 64-66 mmHg (8.5-8.8 kPa). Base excess values at baseline, 1 minute and 2-10 minutes were 5.3 mmol L-1 , 6.5 mmol L-1 and 7.0-8.1 mmol L-1 , respectively. Pulse rates at baseline, 1 minute and 2-7 minutes were 36, 53 and 54-85 beats minute-1 , respectively. Asystole occurred at 8 minutes. Pulse pressures were 50 mmHg at baseline and 1 minute, and 39 mmHg, 31 mmHg, 22 mmHg, 17 mmHg and 1-9 mmHg at 2, 3, 4, 5 and 6-10 minutes, respectively. Lactate was 0.9 mmol L-1 at baseline, progressively increasing to 1.7-2.4 mmol L-1 at 7-10 minutes. Packed cell volume increased after 7 minutes of apnea. There were no other major changes.

CONCLUSIONS AND CLINICAL RELEVANCE: Apnea immediately exacerbated hypoxemia and hypercapnia and rapidly caused hemodynamic instability. Apnea in hypoxemic anesthetized horses is associated with a serious risk for progress to cardiovascular collapse.