Unfortunately, both auscultatory and oscillometric methods of blood pressure assessment are intermittent measures in that a single blood pressure determination can take almost an entire minute to obtain.
Additionally, a brief rest period is recommended between measures of blood pressure that require use of an occluding cuff to allow circulation in the limb to return to normal. Therefore, if an investigator is interested in measuring immediate and short-lived alterations in blood pressure, intermittent blood pressure measures would not be a good choice. Two noninvasive approaches for measuring blood pressure continuously have been developed, pulse transit time (or pulse wave velocity) and the vascular unloading method.
Pulse Transit Time
Pulse transit time reflects the time it takes the pulse wave to travel from the heart to a site in the peripheral circulation, typically the finger or earlobe. It is commonly assessed by measuring the duration of time (in ms) between the initiation of the cardiac contraction from the electrocardiogram (ECG) and the arrival of the pulse wave at the peripheral site, typically measured using photoplethysmography.
Presumably, as arterial pressure increases, the pulse wave travels more quickly to the peripheral site (lower pulse transit time); conversely, as arterial pressure declines, pulse transit time lengthens (Gribbin, Steptoe, and Sleight, 1976). Although studies comparing changes in pulse transit time with blood pressure change have yielded significant inverse correlations, these correlations have been more commonly observed between measures of pulse transit time and SBP than between pulse transit time and DBP (Newlin, 1981; Obrist et al., 1979).
Furthermore, researchers who employed measures of pulse transit time have disagreed as to whether the continuous temporal parameter actually represented an index of blood pressure, as there was considerable evidence suggesting it was more strongly linked to beta-adrenergic cardiac activity than to blood pressure (Newlin, 1981; Obrist et al., 1979). Because of these equivocal findings linking changes in pulse transit time to alterations in blood pressure, this method has not been recommended as a surrogate measure of blood pressure (Shapiro et al., 1996).