Horses Auto-Recruit Their Lungs by Inspiratory Breath Holding Following Recovery from General Anaesthesia

Abstract

This study evaluated the breathing pattern and distribution of ventilation in horses prior to and following recovery from general anaesthesia using electrical impedance tomography (EIT). Six horses were anaesthetised for 6 hours in dorsal recumbency. Arterial blood gas and EIT measurements were performed 24 hours before (baseline) and 1, 2, 3, 4, 5 and 6 hours after horses stood following anaesthesia. At each time point 4 representative spontaneous breaths were analysed. The percentage of the total breath length during which impedance remained greater than 50% of the maximum inspiratory impedance change (breath holding), the fraction of total tidal ventilation within each of four stacked regions of interest (ROI) (distribution of ventilation) and the filling time and inflation period of seven ROI evenly distributed over the dorso-ventral height of the lungs were calculated. Mixed effects multi-linear regression and linear regression were used and significance was set at p<0.05. All horses demonstrated inspiratory breath holding until 5 hours after standing. No change from baseline was seen for the distribution of ventilation during inspiration. Filling time and inflation period were more rapid and shorter in ventral and slower and longer in most dorsal ROI compared to baseline, respectively. In a mixed effects multi-linear regression, breath holding was significantly correlated with PaCO2 in both the univariate and multivariate regression. Following recovery from anaesthesia, horses showed inspiratory breath holding during which gas redistributed from ventral into dorsal regions of the lungs. This suggests auto-recruitment of lung tissue which would have been dependent and likely atelectic during anaesthesia.

Restoration of arterial oxygen tension in horses recovering from general anaesthesia

“…Although overall tidal volume during BRCs is significantly increased, inspiratory tidal volume is significantly larger than the corresponding expiratory volume, resulting in increased end expiratory lung volume. ‘Deep sighing’ and ‘breath holding’ have been described in horses recovering from general anaesthesia, the latter study demonstrating, by use of electrical impedance tomography, that during these extended breath cycles gas redistributes within the lung, consistent with restoring ventilation to atelectic regions. It is possible, therefore, that the supra‐normal respiratory efforts observed following the apnoeic period resulted in functional restoration of atelectic lung regions, counteracting any negative effect.…”

Monitoring of tidal ventilation by electrical impedance tomography in anaesthetised horses

“…Such a factor would then have to be prospectively tested in healthy horses and horses with diseased lungs as well as under conditions of controlled and spontaneous breathing. EIT‐based monitoring of tidal volume would add clinical value to the scientifically well‐described assessment of the distribution of such tidal volumes in horses .…”

“…Such a factor would then have to be prospectively tested in healthy horses and horses with diseased lungs as well as under conditions of controlled and spontaneous breathing. EIT-based monitoring of tidal volume would add clinical value to the scientifically well-described assessment of the distribution of such tidal volumes in horses [2,3,21,22]. VT 4 9 NICO , tidal volume measured by spirometry; EIT ROI , impedance change measured within the two lung regions of interest; EIT thorax , impedance change measured within the whole EIT image; RR, respiratory rate; PIP, peak inspiratory pressure.…”

Maintenance of equine anaesthesia over the last 50 years: Controlled inhalation of volatile anaesthetics and pulmonary ventilation

“…Gas exchange returns to physiologic levels very quickly after the horses recover from anaesthesia . This might be because they can auto‐recruit their lungs after anaesthesia by inspiratory breath holding .…”