The purpose of this study was to examine the effects of controlled ventilation on various indices of ventricular function as determined with transesophageal Doppler echocardiography. Eighteen adult horses weighing 545 ± 59 kg were studied. Horses were premedicated with 0.1 mg/kg romifidine intravenously and anesthesia was induced with 0.05 mg/kg of diazepam and 2.2 mg/kg of ketamine administered intravenously. Anesthesia was maintained with halothane in oxygen. Ten horses were positioned in lateral recumbency (group L) and eight horses in dorsal recumbency (group D). Controlled ventilation was delivered using a Dräger Control Center set at a frequency of 6 and an I:E ratio of 1:3.5 to 1:4. Tidal volume was varied to maintain PaCO2 at 45-55 mmHg and peak inspiratory pressure of 24-28 cm H2O. Heart rate, maximal aortic blood flow velocity (Vmax), velocity time integral (VTI), pre-ejection period (PEP), and systolic ejection time (ET) were measured from the velocity spectra recorded from the aorta using transesophageal Doppler echocardiography. Aortic diameter was measured from a 2-D ultrasound image. Stroke volume was calculated. Aortic velocity spectra were recorded 20 minutes after initiation of halothane and at subsequent 10 minute intervals to 70 minutes. For each time period, the velocity spectra immediately preceding ventilation (PV) and the spectra after the ventilatory cycle was completed (AV) were analyzed. PV and AV measurements at each time period were compared for horses within group L and within group D. The differences between PV and AV data were calculated at each time period and group L horses compared to group D. Data within groups were analyzed using a Wilcoxon signed rank test and between groups using a Mann-Whitney test with P < 0.05 considered significant.

For horses in lateral recumbency, VTI was higher PV vs AV at 30, 40, 50, and 60 minutes. Stroke volume was higher PV vs AV at 30 and 50 minutes, and Vmax was higher PV at 20 minutes. For horses in dorsal recumbency, Vmax, VTI and SV were higher PV vs AV at 30, 40, 50, and 60 minutes. The PEP was longer after ventilation and ejection time was shorter. Differences in VTI, SV, PEP, and ET between PV and AV at each time period were greater for horses in dorsal recumbency vs lateral recumbency. This study suggests that the effects of controlled ventilation on indices of ventricular function are greater in dorsally recumbent horses than in horses in lateral recumbency.

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