Regional ventilation distribution and dead space in anaesthetised horses treated with and without continuous positive airway pressure (CPAP)

AbstractObjective The aim of this study was to evaluate the effect of continuous positive airway pressure (CPAP) on regional distribution of ventilation and dead space in anaesthetised horses. Study design Randomized, experimental, crossover study. Animals Eight healthy adult horses. Methods Horses were anaesthetised twice with isoflurane in 50% oxygen and medetomidine as continuous infusion in dorsal recumbency, and administered in random order either CPAP (8 cmH2O) or NO CPAP for 3 hours. Electrical impedance tomography (EIT) and volumetric capnography (VCap) measurements were performed every 30 minutes. Lung regions with little ventilation [dependent (DSS) and non-dependent silent spaces (NSS)], centre of ventilation (CoV) and dead space variables as well as venous admixture were calculated. Statistical analysis was performed using MANOVA and Pearson correlation. Results Data from six horses were statistically analysed. In CPAP the CoV shifted to dependent parts of the lungs (P <0.001) and DSS were significantly smaller (P<0.001), while no difference was seen in NSS. Venous admixture was significantly correlated with DSS with the treatment time taken as covariate (P <0.0001; r=0.65). No differences were found for any VCap parameters. Conclusions and clinical relevance In dorsally recumbent anaesthetised horses, CPAP of 8 cmH2O results in redistribution of ventilation towards the dependent lung regions thereby improving ventilation-perfusion matching. This improvement was not associated with an increase in dead space indicative for a lack in distension of the airways or impairment of alveolar perfusion.