Fig. 3. A sigmoid-shaped curve, such as the one above, captures some of the underlying relation between arterial blood pressure (ABP) and the photoplethysmogram (PPG). Note the following: (1) The abscissa is the transmural arterial wall pressure (P^net), the net difference between arterial pressure (P^hem) and any externally applied cuff pressure (P^cuff). (2) The curve is most compliant (maximum slope) when transmural pressure is zero. (3) There are nonlinear portions at the extremes of the curve: It is increasingly difficult to further expand or collapse the vessel after it reaches certain limits. (4) The amplitude and shape of the photoplethysmogram can be changed by different external cuff or probe pressures, illustrated above: Given a relatively consistent arterial blood pressure waveform, the resultant photoplethysmogram (a) is collapsed, (b) is similar to the blood pressure waveform, or (c) has its peaks compressed, when the external probe/cuff pressure is (a) high, (b) medium, and (c) low, respectively. The data above were collected in our laboratory with institutional human use committee approval and subject consent. The photoplethysmogram was recorded overlying a digital artery.