The effect of local cold exposure on pulse transit time.

Pulse transit time (PTT) has attracted great interest for developing continuous and noninvasive blood pressure (BP) monitoring devices. This study investigated the effect of finger cold exposure on PTT. 13 healthy and normotensive subjects, including 6 males and 7 females took part in the experiment. Cold exposure was performed by asking subjects to immerse one finger into cold water (9±2°C) for 2 minutes. The photoplethysmographic (PPG) signals were recorded from the cooled and adjacent non-cooled finger simultaneously. PTT was measured between the R-wave of the electrocardiogram (ECG) and a fiducial point in finger PPG pulse for each heartbeat. Three definitions of PTT were investigated and the difference of PTT between two fingers was calculated for each subject. By comparing the difference before and after cold exposure, it was found that local cold exposure had little effect on PTT1, which terminates at the foot of the PPG pulse; however, it affected the PTT3, which terminates at the peak of the PPG pulse. The results suggest that the measurement of PTT which terminates at the foot of the PPG pulse is reliable, while those measurements which terminate at the peak of the PPG pulse or other fiducial points nearby are subject to the effect of local cold exposure and can possibly affect the accuracy of PTT-based applications.