Fig. 1. Representative recordings of aortic pressure (AP), central venous pressure (CVP), and electrocardiogram (ECG) with the heart beating and during circulatory arrest by evoked ventricular fibrillation in a patient undergoing implantation of an automatic cardioverter/defibrillator, and explanation of curve fitting techniques. ( A ) Slow recording over 45 s of variables showing transition from circulation intact to circulatory arrest. CVP increases immediately, but remains similar after 10 s of circulatory arrest. ( B ) Same recording on an extended time scale, showing initial decay of arterial pressure with circulatory arrest. ( C ) Curve fitting technique of the arterial pressure decay over 30 s. Interval used for curve fitting is indicated by markers. Data sampled at 10 ms intervals were transferred to a mathematical software package, and curve fittings were performed for calculation of pressure asymptotes (Pcrit) using three different models. ( D ) Calculation of arterial downstream pressure from the diastolic aortic pressure decay with the circulation intact. Points bracketing data used for curve fitting are indicated by markers.