Fig. 1. (A ) Example of recordings of arterial pressure (AP) and plethysmographic signals (Pleth.) in a patient at three different levels of AP during an anesthetic procedure. B  represents the relationships within a cardiac cycle between arterial and plethysmographic signals at the three different levels of AP. The arrow  indicates the increase in AP. It is to be noted that the slope of the loop decreased, whereas its area increased when the AP increased. This is indicative of a change of the recorded compliance and of the phase shift between the two signals, whatever the mechanism (see discussion). AU = arbitrary unit.