Vascular Unloading Method – Continuous Blood Pressure Monitoring Methods
The vascular unloading method, initially described by Penaz (1973), involves obtaining estimates of blood pressure from a small pressurized cuff positioned over a finger in conjunction with a photoplethysmograph. One such apparatus, called the Finger Arterial Pressure System or FINAPRES™, monitors blood flow into the finger and provides continuous information to a mechanism that automatically adjusts air pressure in the cuff to maintain a stable partial blood flow through the artery in the finger.
Blood flow oscillations are sensed by the encircling finger cuff and translated into beat-by-beat estimates of blood pressure. Although this device may become uncomfortable during extended measurement periods, it can be used to measure blood pressure continuously for a few hours, and the ambulatory version, which alternates blood pressure determinations between two integrated finger cuffs, has been used for periods as long as 24hours (Imholz et al., 1993).
Naturally, whenever a new method for assessing blood pressure is developed, it is important to validate it with established measurement strategies. In some studies of this type, blood pressure values obtained from the devices using the vascular unloading principle have been shown to compare favorably with intra-arterial measures of blood pressure (Imholz et al., 1990; Parati et al., 1989) as well as intermittent noninvasive measures (Dorlas et al., 1985; Pace and East, 1991).
However, other studies have reported that indices of blood pressure from these devices either overestimated (Epstein et al., 1989; Kurki et al., 1989) or underestimated blood pressure (Imholz et al., 1988; van Egmond, Hasenbos, and Crul, 1985). It appears that some of this lack of correspondence between methods of blood pressure determination is unique to the vascular structure of individuals. With the device positioned at the recommended heart level, some individuals display estimates of blood pressure that are over 20mm Hg higher than their corresponding oscillometric or auscultatory values, while others display estimates that are over 20mm Hg lower (Larkin et al., 1995).
Calibration of recordings by adjusting arm position above or below heart level improves correspondence between measures, but this effect appears to be only temporary (Larkin et al., 1995).
Regardless of whether the absolute measures of blood pressure obtained from devices employing the vascular unloading principle accurately portray an individual’s resting blood pressure level, the continuous finger arterial recordings provide a reliable index of change in blood pressure in response to acute environmental stimuli (Parati et al.,1989).
Therefore, perhaps the true utility of this instrument is to assist in providing accurate measures of acute blood pressure responses to stress, particularly responses to short-term stressors that may be missed if an intermittent noninvasive measurement is employed. In this usage, Gerin, Pieper, and Pickering (1993), indeed, demonstrated that the FINAPRES™-derived measures of blood pressure were more reliable than blood pressures obtained using an intermittent blood pressure measurement device.