The quantitative aspect of photoplethysmography revised.

The overall transfer function of a photoplethysmograph (PPG) converting changes in intravascular volume to changes in recorder pen deflection was investigated. The specifications of the apparatus working as a red cell densitometer were assessed in vitro, and it was found that PPG output is a logarithmic function of red cell density roughly in accordance with the Lambert-Beer theorem. In an in vivo experiment it was found that PPG output is also a logarithmic function of digital intravascular volume as assessed by means of air plethysmography. It is suggested that the equation for this in vivo function is different because of a dual action of stasis, producing increases in both intravascular volume and haematocrit. Although the relative contributions of the two disparate phenomena are not assessed further, it is concluded that their joint action on PPG output can be described by a monologarithmic function. In the Appendix, sources of non-linearity in the electronics of PPG are pointed out and instructions for the use of an exponential amplifier which will linearize the overall transfer function are given.