Calibration of the photoplethysmogram to arterial blood pressure: capabilities and limitations for continuous pressure monitoring.

It is well known that the photoplethysmogram (PPG) is related to the underlying arterial blood pressure (ABP). However, there is ambiguity regarding the precise relationship of these two circulatory signals: some investigators have described a static relationship, while others have used more complex dynamic characterizations. This paper attempts to reconcile these models, employing results obtained from controlled human subject tests. Our data indicate that there is no simple relationship between the continuous PPG and ABP. However, when several serial beats are aggregated, a static, reproducible relationship is observed. This suggests that, in a stable experimental environment, most of the factors that decouple the ABP and PPG operate over short time scales. When these factors are eliminated through beat averaging, calibrated plethysmography offers a near-continuous estimation of ABP. Our data suggest that this PPG/ABP relationship is not stationary, since it is less consistent over time scales greater than 20 minutes. Variable local ABP was generated by raising and lowering the subjects' hands; this method also offered the means for oscillometric BP measurement without any additional actuator such as a conventional pneumatic cuff.