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Pulmonary vascular resistance (PVR) has been used to assess right ventricular afterload; however, this is considered meaningless due to several physiologic shortcomings. Echocardiography has often been used to assess right ventricular (RV) afterload, but this has never been validated as an index of RV afterload. The purpose of this study is to evaluate echocardiographic indices of RV afterload in an experimental, open-pericardium pig model with induced, variable main pulmonary artery stenosis.
Eleven anesthesized pigs were instrumented for the measurement of arterial blood pressure, central venous pressure, RV and pulmonary pressure. An ultrasonic flowprobe (MA14PAX; Transonic) was positioned on the main pulmonary artery to obtain pulmonary flow. Distal to the flowprobe, a balloon-occluder was positioned facilitating gradual constriction of the pulmonary artery. To obtain a stepwise pressure difference increment over the banding of 10 mmHg at each measurement, we gradually inflated the balloonoccluder. After 10 minutes, all invasive hemodynamic data were registered and an epicardial echocardiography was performed to obtain tricuspid flow velocities, isovolumetric and isovolumetric relaxation time. To calculate the TEI index during one heartbeat, echocardiographic measurements were synchronized with flowprobe measurements to obtain ejection time. The E/E' ratio was obtained with tissue Doppler echocardiography of the lateral tricuspid annulus. All echocardiographic measurements were performed in triple and averaged. The ejection period and mean pulmonary acceleration and cardiac output were calculated from the pulmonary flow curve derived from the ultrasonic flowprobe. Resistance over the pulmonary banding was calculated by pressure gradient divided by cardiac output.
Of the 11 pigs, two pigs died prematurely, one inferior caval vein rupture, and one due to asystole while inserting the pulmonary artery catheter. After pulmonary banding, central venous pressure did not increase significantly. Mean acceleration correlated with the resistance over the banding (r = 0.59, P < 0.0001). The TEI index and E/E' neither correlated with the pressure gradient over the banding nor flow.
Mean acceleration correlated with RV afterload in this model.