When blood pressure is measured in the finger using the volume clamp method the value at which the vascular volume is clamped is of crucial importance. Since the discovery of the method, several criteria of finding a correct set point have been elaborated: 1. The volume oscillations reach their maximum amplitude at cuff pressure equalling mean blood pressure. 2. The form of the diastolic portion of volume pulsations changes if the cuff pressure moves around the mean blood pressure. 3. The set point can be positioned at one third of the arterial volume. 4. The dynamic vascular compliance (DVC) may be continuously measured as the instantaneous amplitude of vascular volume oscillations is elicited by a relatively small and rapid vibration of the cuff pressure. The shape of the DVC pulse characteristically depends on the transmural pressure (TP): at negative TP (cuff pressure exceeding the blood pressure) it shows a distinct positive systolic peak, at positive TP the polarity of the DVC pulse is reversed. In contrast to the first three ways to find the set point, the last one may operate even in closed-loop performance, i.e. during the blood pressure measurement.
The influence of posture on the rhythms in blood pressure, heart rate and respiration was tested by means of spectral analysis in 14 healthy subjects. During squatting, standing and sitting, the finger blood pressure was recorded by the non- invasive Pefiaz technique together with cardiac intervals and respiratory movements. The power spectra obtained from five-minute samples showed that the respiratory components of cardiac interval and pulse pressure were reduced significantly in standing. Compared to squatting, a significant increase of total power in the medium frequency band (0.05-0.15 Hz) for cardiac interval, diastolic and mean pressure could be detected.