Regional ventilation distribution and dead space in anaesthetized horses treated with and without continuous positive airway pressure: novel insights by electrical impedance tomography and volumetric capnography.

Objective: 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 anaesthetized horses.

Study Design: Randomized, experimental, crossover study.

Animals: A total of eight healthy adult horses.

Methods: Horses were anaesthetized twice with isoflurane in 50% oxygen and medetomidine as continuous infusion in dorsal recumbency, and administered in random order either CPAP (8 cmHO) or NO CPAP for 3 hours. Electrical impedance tomography (and volumetric capnography (VCap) measurements were performed every 30 minutes. Lung regions with little ventilation [dependent silent spaces (DSSs) and nondependent silent spaces (NSSs)], centre of ventilation (CoV) and dead space variables, as well as venous admixture were calculated. Statistical analysis was performed using multivariate analysis of variance 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 DSSs were significantly smaller (p < 0.001), while no difference was seen in NSSs. 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 anaesthetized horses, CPAP of 8 cmHO 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.