Pulse transit time (PTT) is the time delay for the energy wave to travel between two sites in the arteries. (The energy wave can be visualized as acute dilation of the arterial wall and usually moves much faster than blood.) PTT in the form of pulse wave velocity (PWV = D/PTT, where D is the distance between the two sites) has proven to be a marker of arterial stiffness and a major cardiovascular risk factor based on a large body of epidemiological data. The Bramwell-Hill (BH) equation relates arterial stiffness and radius to PWV. Arterial stiffness positively relates to blood pressure (BP); thus, many have pursued BP monitoring via PWV. We investigated PWV from theoretical, experimental, and application prospective.In the first theory study, we investigated the 100-year Bramwell-Hill equation, which relates PWV to BP and thus represents a basis for cuff-less BP monitoring. However, it has long been known that this equation underestimates PWV in a BP-dependent manner. We developed a new equation that accounts for spatial changes in arterial cross-sectional area. This new equation largely corrected this well-known underestimation and predicted PWV better based on experimental data in the literature.In the second experimental study, we examined the most popular PTT (finger PTT). We hypothesize that whole body PTT could be better than finger PTT due to less smooth muscle contraction. We collected data from 32 participants in a near supine position. We placed sensors including electrodes on the chest for an ECG waveform, clips on the ear, finger, and toe for photo-plethysmography (PPG) waveforms, and a cuff on the arm for auscultation BP. We recorded the waveforms and referenced BP before and after mental arithmetic, a cold pressor test, slow breathing, and nitroglycerin. Conventional PTTs were assessed as markers of BP in human subjects undergoing a battery of interventions to change BP. This experimental study concludes that PTTs through the whole body rather than the arm show the best BP change tracking ability.Thirdly, we repeated the previous study for seven subjects in three sessions one year apart to see how much PTTs and other PPG waveform feature models change after one year. While it is known that the PTTs calibration models must be updated periodically to account for aging effects, data on the time period required for these "cuff re-calibrations" are scant. Our experimental finding suggests that the PTTs model through the whole body could hold up after one year, and calibration may occur every year, which is reasonably practical.Finally, because of the inverse relation of PWV and arterial radius in the BH equation, we investigated innovative applications for PTT and other physiology-inspired features of carotid and femoral waveform for screening and surveillance of aortic abdominal aneurysm diameter. We hypothesized that arterial waveform features such as PTT constitute a non-imaging solution for the aneurysm size of the aorta. The features detected AAA with 72-80% accuracy. In conclusion, from a theoretical standpoint, we improved the 100-year PWV equation by relaxing its main assumption. From an experimental perspective, we showed the best body location for measuring PWV in the sense of BP prediction and robustness after one year. Lastly, we found that PWV could be used as markers of the abdominal aortic aneurysm from an application standpoint.
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