Noninvasive pulse transit time measurement for arterial stiffness monitoring in microgravity.

The use of a noninvasive hemodynamic monitor to estimate arterial stiffness, by measurement of pulse transit time (PTT), was demonstrated in microgravity. The monitor's utility for space applications was shown by establishing the correlation between ground-based and microgravity-based measurements. The system consists of a scale-based ballistocardiogram (BCG) and a toe-mounted photoplethysmogram (PPG). PTT was measured from the BCG I-wave to the intersecting tangents of the first trough and maximum first derivative of the PPG waveforms of each subject. The system was tested on a recent series of parabolic flights in which the PTT of nine subjects was measured on the ground and in microgravity. An average of 60.2 ms PTT increase from ground to microgravity environments was shown, and was consistent across all test subjects (standard deviation = 32.9 ms). This increase in PTT could be explained by a number of factors associated with microgravity and reported in previous research, including elimination of hydrostatic pressure, reduction of intrathoracic pressure, and reduction of mean arterial pressure induced by vasodilation.