Jung, S.; Thomson, S.; Pantelopoulos, A.; Sunden, L.; Richards, P.; Patel, S.; Sheng, S.

Impedance plethysmography (IPG) and photoplethysmography (PPG) are non-invasive techniques for measuring blood volume changes. This study investigated the differential responses of IPG and PPG to temperature-mediated vasoconstriction induced by localized cooling. Twenty-one participants underwent control and treatment conditions, with real or fake ice cubes applied to the forearm. PPG signal amplitude significantly decreased with cooling (p < 0.001), indicating sensitivity to capillary blood flow changes. In contrast, IPG signal amplitude remained stable, suggesting it primarily reflects blood flow in larger/deeper vessels. Blood pressure remained stable, while heart rate decreased. These findings suggest IPG is less sensitive to capillary-level changes than PPG and may be more suitable for monitoring deeper blood flow. This study provides insights into the distinct sensitivities of IPG and PPG, with implications for wearable device development and cardiovascular monitoring.