Time-related changes of the cardiovascular system during maintenance anesthesia with sevoflurane and isoflurane in horses.

To clarify time-related changes in equine cardiovascular system during maintenance anesthesia (180 min, 1.2 minimum alveolar concentration) with sevoflurane (Sev-group) compared to isoflurane (Iso-group) as the basis for clinical use of Sev, horses were examined for the heart rate (HR), mean arterial pressure (MAP), cardiac index (CI), systemic vascular resistance (SVR) and pre-ejection period (PEP)/ejection time (ET) that is an index of the cardiac contractility. The HR was almost 30 beats/min in both groups without significant temporal change. MAP was significantly elevated with time but there was no significant difference between the groups. In the Sev-group, CI remained unchanged but the significant increase of CI with time was observed in the Iso-group. In the Sev-group SVR was significantly higher than that of the Iso-group and increased with time. No significant difference of PEP/ET was seen between the groups, but PEP/ET lowered with time in the Iso-group in association with prolonged ET. The results indicated that the time-dependent elevation of MAP in the Sev-group reflected increased SVR without increase of CI and that it reflected increased CI resulting from increased stroke volume in the Iso-group in association with lowered PEP/ET, that is, increased cardiac contractility.