Continuous positive airway pressure (CPAP) decreases pulmonary shunt in anaesthetized horses

Abstract

Objective: To evaluate the effects of continuous positive airway pressure (CPAP) on intrapulmonary shunt, cardiac output and oxygen delivery in horses subjected to a 6 hour period of general anaesthesia. Study design: Randomized, experimental, crossover study. Animals: Ten healthy adult horses. Methods: Following medetomidine, diazepam and ketamine administration, orotracheal intubation was performed and horses positioned in dorsal recumbency. Anaesthesia was maintained with isoflurane carried in an oxygen and air mix (FiO2 0.5) combined with a medetomidine infusion. Horses were anaesthetized twice and either CPAP (8 cmH2O) or physiologic airway pressure (NO CPAP) was applied to the lungs for 6 hours; the order of treatments was randomly assigned. Following induction of anaesthesia, cardiovascular and respiratory variables (including arterial blood gas analysis) were recorded every 30 minutes, cardiac output was measured every 60 minutes using the lithium dilution technique and oxygen delivery calculated. If PaCO2 exceeded 100 mmHg (13.3 kPa), controlled ventilation was initiated and horses excluded from further data collection. Groups were compared using a general linear model. Results: Data from eight horses were analysed. PaO2 was 15–56 mmHg (2.00–7.45 kPa) higher (p < 0.001) and shunt fraction 6–14% lower (p < 0.001) in the CPAP group. No differences were seen for cardiac output and oxygen delivery. The lack of difference in oxygen delivery was attributed to lower haemoglobin levels in the CPAP group than in the NO CPAP group. Conclusions and clinical relevance: CPAP of 8 cmH2O can be used in dorsally recumbent horses to decrease pulmonary shunt fraction without causing a decrease in cardiac output during longterm anaesthesia.