Pulsed delivery of inhaled nitric oxide counteracts hypoxaemia during 2.5 hours of inhalation anaesthesia in dorsally recumbent horses

Abstract

ObjectiveThe study aimed to investigate the effect of varying pulse lengths of inhaled nitric oxide (iNO), and 2.5 hours of continuous pulse-delivered iNO on pulmonary gas exchange in anaesthetized horses. Study DesignExperimental study. AnimalsSix Standardbred horses. MethodsHorses received acepromazine, detomidine, guaifenesin, thiopentone and isoflurane in oxygen, were positioned in dorsal recumbency and were breathing spontaneously. iNO was on average pulsed during the first 20, 30, 43 or 73% of the inspiration in 15 minute steps. The pulse length that corresponded to the highest (peak) partial pressure of arterial oxygen (PaO2) in the individual horses was determined and delivered for a further 1.5 hours. Data measured or calculated included arterial and mixed venous partial pressures of O2 and CO2, heart rate, respiratory rate, expired minute ventilation, pulmonary and systemic arterial mean pressures, cardiac output and venous admixture. Data (mean ± SD) was analysed using anova with p < 0.05 considered significant. ResultsAlthough the pulse length of iNO that corresponded to peak PaO2 varied between horses, administration of all pulse lengths of iNO increased PaO2 compared to baseline. The shortest pulse lengths that resulted in the peak PaO2 were 30 and 43% of the inspiration. Administration of iNO increased PaO2 (12.6 ± 4.1 kPa [95 ± 31 mmHg] at baseline to a range of 23.0 ± 8.4 to 25.3 ± 9.0 kPa [173 to 190 mmHg]) and PaCO2 (8.5 ± 1.2 kPa [64 ± 9 mmHg] to 9.8 ± 1.5 kPa [73 ± 11 mmHg]) and decreased venous admixture from 32 ± 6% to 25 ± 6%. The increase in PaO2 and decrease in venous admixture was sustained for the entire 2.5 hours of iNO delivery. ConclusionsThe improvement in arterial oxygenation during pulsed delivery of iNO was significant and sustained throughout 2.5 hours of anaesthesia. Clinical relevancePulsed iNO potentially could be used clinically to counteract hypoxemia in anaesthetized horses.