Mechanical Ventilation and Low Blood PaCO2 Attenuate Heart Rate Variability in the Anesthetized Horse

Mechanical ventilation decreases PaCO2, venous return, myocardial contractility, cardiac output and blood pressure in horses (Wagner et al. 1990, Am J Vet Res 51). The autonomic nervous system has a respiratory rhythm during spontaneous ventilation that may disappear with mechanical ventilation and low PaCO2. The ECG R‐R interval shows heart rate variability (HRV) with components of high frequency associated with parasympathetic and low frequency associated with parasympathetic and sympathetic tone. The coefficient of variation of RR intervals (CVRR) is a time domain index of total HRV without respiratory influence. Six sevoflurane anesthetized horses at 1.2 minimum alveolar concentration (MAC) either breathed spontaneously with PaCO2 of 67±10 mmHg or were mechanically ventilated to maintain PaCO2 of 42–46 mmHg. We used power spectrum analysis to calculate the CVRR from the HRV in each horse as an index for autonomic nervous system activity (Kuwahara et al. 1996, J Auton Nerv Syst 60). During spontaneous ventilation CVRR, heart rate (HR), blood pressure (SAP, MAP and DAP) were 1.6±0.6, 41.4±7 beat/min, 102±13 mmHg, 71.4±11.7 mmHg and 56.2±11.2 mmHg respectively. During mechanical ventilation HRV and cardiovascular parameters decreased to CVRR 0.4±0.1 (P<0.01), HR 37.5±3.3 beat/min, SAP 84.2±7.9 mmHg, MAP 59.6±4.8 mmHg and DAP 47.3±3.5 mmHg respectively (all P<0.01 except HR P=0.2). The results indicate that autonomic nervous system activity decreases during mechanical ventilation in anesthetized horses. The autonomic nervous system reduction correlates with lower blood pressure and arterial compliance. Mechanical ventilation in hemodynamically‐compromised horses can impair cardiovascular function by decreasing the autonomic nervous system activity that is synchronized with spontaneous ventilation.