PTT method

Blood pressure was measured by the PTT method. Specifically, electrocardiography monitoring was applied to monitor changes in cardiac electricity; a fingertip pulse oximeter was used to monitor pulse wave changes at the fingertip. The PTT is defined as the time required for the arterial pulse pressure wave to travel from the left ventricle (peak of the electrocardiography R wave) to a predetermined peripheral site (peak point of the pulse wave). The distance from the left ventricle to the fingertip is positively correlated with the body height. Therefore, pulse‐related blood pressure can be calculated based on the PTT and body height; blood pressure data can be obtained for each beat.⁹ In the present study, blood pressure data were recorded every 30 seconds to monitor changes with respect to individual sleep stages.