Monitoring of reactive hyperemia using photoplethysmographic pulse amplitude and transit time

Background: Peripheral arterial tonometry and Ultrasound measurement of flow mediated dilation have been the widely reported noninvasive techniques to assess vasodilation during reactive hyperemia (RH). Objective: Simultaneous monitoring of dilatation and tone of the vasculature during RH induced by venous occlusion (VO) and arterial occlusion (AO) has been presently attempted using simple noninvasive measures of photoplethysmography (PPG). Methods: Finger-PPG characteristics that include pulse timings, amplitude, upstroke-slope and pulse transit time (PTT) were studied before (1 min), post-VO (5 min) and post-AO (5 min) in 11 healthy volunteers. Results: PPG amplitude was significantly increased to maximum at 2nd min of post-AO (1.28±0.11 vs. 1.0 nu, P<0.05) as compared to the baseline; meanwhile, no significant changes (P>0.05) in PPG amplitude was observed during post-VO. Tremendous increase in PTT was evident at 1st min of post-AO (196.6±3.3 vs. 185.3±3.6 ms, P<0.0001) and was maintained significantly longer through 1–5 min of post-AO. Relatively small but significant increase in PTT was noticed only at 1st min of post-VO (193.9±6.8 vs. 189.6±6.2 ms, P<0.0001), followed by an immediate recovery to baseline by 2nd min of post-VO. The increase in PTT (i.e. ΔPTT) was higher at 1st min of post-AO (11.4±1.3 vs. 4.3±1.1 ms) as compared to post-VO. Conclusion: Results suggests that PTT response reflects the myogenic components in the early part of RH and PPG amplitude response reflects the metabolic component reinforcing the later course of RH. PPG amplitude and PTT can be used to quantify the changes in diameter and tone of the vessel wall, respectively during RH. The collective responses of PPG amplitude and PTT can be more appropriate to facilitate PPG technique for monitoring of vasodilation caused by RH.