Left ventricular afterload and systemic hydraulic power during in-series cardiac assistance: studies using intraaortic balloon pumping in dogs.

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The effects of in-series mechanical assistance on left ventricular (LV) afterload and aortic power dissipation were studied in four groups of open chest mongrel dogs: control, acute myocardial ischemia, cholinergic and beta-adrenergic blockade, and combined ischemia and blockade. Aortic root pressure, flow, power, and impedance and LV pressure and power were obtained. Assistance was provided by intraaortic balloon pumping. Times of inflation and deflation of the balloon were controlled to maintain a phase difference of 180° between the fundamental components of aortic root pressure and flow. Differences in hemodynamic parameters before and after 2–5 min of cardiac assistance were calculated. The results confirm other observations regarding effects of in-series assistance on LV and aortic pressure, cardiac output, and peripheral resistance. No consistent changes were obtained in the pulsatile components of aortic input impedance. A significant decrease (14–20%) was observed in the dc component of the impedance in all animals. The results reported here contradict previous reports regarding decreased LV power generation. It was found that LV power generation and aortic power dissipation increased significantly during assistance (LV, 3–17%; aortic, 4–19%). The results of this investigation also appear to conflict with previous reports regarding the role of cardiovascular control in “counteracting” the effects of assistance in the normal experimental preparation. They suggest that the short term effects are attributable directly to the device and are not mediated by the autonomic nervous system.