Revisiting the relationship between pulse transit time and arterial stiffness

Unlike secondary prevention, primary prevention of the major cardiovascular events has not yet been successful. One of the reasons is that current methodology has not succeeded in selecting high risk patients who are suitable candidates for primary prevention. Increased arterial stiffness, a known risk factor for cardiovascular events, has been commonly indexed by the pulse transit time (PTT), even though PTT is influenced by short-term blood pressure (BP) change. PTT and arterial BP of 93 human adults were analyzed. To determine the relationship between PTT and systolic blood pressure (SBP), the data was fitted to an exponential equation $(\mathrm{PTT}=\alpha {\times e}^{-\frac{\beta \times \mathrm{SBP}}{2}})$ which is derived from physiological theory. The index β in this equation, which reflects arterial stiffness, was found to be independent of SBP (R² = 0.006, p = 0.5088), while PTT had poor correlation with β (R² = 0.202, p < 0.0001). We propose a new index of arterial stiffness, $\frac{-2\times \frac{\Delta \mathrm{PTT}}{\Delta \mathrm{SBP}}}{\mathrm{PTTmean}}$ , which is independent of BP. This index is shown to approximate β and correlate well with it (R² = 0.935, p < 0.0001). PTT is affected by short-term BP changes, which makes it difficult to use for estimating arterial stiffness based on a single measurement in the hospital. Furthermore, it is difficult to estimate BP only from PTT, because BP itself is affected by arterial stiffness in addition to PTT. The proposed index characterizes the severity of atherosclerosis, independently of temporary BP change.