Effect of breathing a heliox gas mixture on cardiopulmonary function in horses-anesthetized immediately following exercise

Abstract

The purpose of this study was to determine the cardiopulmonary effects of breathing a heliox (60% He: 40% O2) carrier gas mixture in horses anesthetized immediately following treadmill exercise. Using a randomized crossover design, six healthy, Thoroughbred horses (five males, one female) weighing 517 ± 36.9 kg were instrumented to enable measurement of heart rate (HR), systolic (SAP), mean (MAP) and diastolic (DAP) carotid arterial pressures, mean pulmonary arterial pressure (PAP), central venous pressure (CVP), pulmonary arterial temperature (TEMP), and cardiac output. Horses were warmed up for 3 minutes at 4 m second−1, then exercised to the point of fatigue at 115% VO2max and immediately anesthetized using detomidine (0.04 mg kg−1 IV) followed by tiletamine-zolazepam (1.25 mg kg−1 IV). Anesthesia was maintained using 1.3 MAC isoflurane (ISO) vaporized in either 100% O2 (O), 60% N2 : 40% O2 (N), or 60% He : 40% O2 (H). End-tidal ISO concentration was measured using a RASCAL II anesthetic gas analyzer. Minute ventilation (MV) was measured during ISO anesthesia using a heated, calibrated pneumotachometer and differential pressure transducer. Ventilation during ISO anesthesia was controlled to maintain constant, similar minute ventilation between the O, N and H treatment groups. Cardiovascular data and samples for arterial and mixed venous blood gas analysis were recorded while the horse stood quietly prior to exercise, 30 seconds after the point of fatigue, immediately prior to isoflurane anesthesia and 15, 30, 45 and 60 minutes after initiating isoflurane anesthesia. Data were analyzed using repeated measures anova with posthoc differences between means identified using Bonferroni's method (p < 0.05). Exercise was associated with significant increases in HR, SAP, MAP, DAP, PAP, CVP, TEMP and significant decreases in pH, HCO3– and BE. Anesthesia was associated with a return of HR, SAP, MAP, DAP, PAP and CVP to pre-exercise values. MV measured during ISO was not different between groups. Horses in the O group had significantly higher values for PaO2 (156.7 mm Hg, 20.9 kPa) compared to the N group (93.6 mm Hg, 12.5 kPa) or the H group (90.4 mm Hg, 12.1 kPa). No other differences between treatments were identified. We conclude that use of H as a carrier gas does not improve gas exchange in horses anesthetized immediately following exercise.