Non-invasive studies of peripheral vascular compliance using a non-occluding photoplethysmographic method.

A non-invasive technique is implemented to measure a peripheral vascular compliance index Cindex, using an infrared photoplethysmographic waveform as an indicator of intravascular volume change and a continuous blood pressure monitor to measure the blood pressure during each heart-beat. The non-linear behaviour of Cindex with pressure and the effect of age on Cindex are studied in 62 males (15-73 years). Repeatability tests and the effect of ice-water exposure of a portion of a limb are studied in 10 and 14 subjects, respectively. For each individual, Cindex measurements are taken at discrete values of local mean arterial pressure (Pmean), and a Cindex against Pmean plot is obtained. There is a statistically significant difference (p < 0.05) in Cindex for the lower values of Pmean (60-100 mmHg) between two age groups formed (15-52 and 58-73 years). The cold-pressor test (CPT) shows a 68% median decrease in Cindex, with an inter-quartile range of 60-77%, in a matter of seconds. The results suggest that Cindex may be a useful noninvasive indicator of peripheral vascular compliance in humans.