[Noninvasive, continuous measurement of finger artery pressure with the servo-plethysmo-manometer Finapres]

Abstract

The servo-plethysmo-manometry, as described by Penaz, represents a method for noninvasive, continuous, true-phase measurement of the finger arterial pressure. PRINCIPLES OF MEASUREMENT The light transmission plethysmograph is integrated into a pneumatic cuff for the finger (Figure 1). The light intensity is a function of the instantaneous finger blood volume. It is assumed that the pulsatory volume fluctuations are a function of the instantaneous intra-arterial blood pressure. Since the elastic arterial walls counteract the volume fluctuations with resistance, for calibrated measurements, they must be in a fully relaxed state in the absence of transmural pressure. A criterium for assessment of the transmural pressure is the extent of the volume pulse amplitude which is relatively small in the presence of higher transmural pressure due to a lesser degree of distention of the arterial walls and which increases in association with higher extravascular pressure because of the lower transmural pressure. The registered photosignal is larger with continuous cuff pressure elevation and reduced when the intravascular pressure is exceeded. At maximal amplitude, that is, complete relaxation of the arterial wall and cessation of the transmural pressure, the servosystem is activated to regulate the cuff pressure such that the pressure in the finger cuff holds the finger blood volume constant during the pulse cycle with corresponding elevation during systole and reduction during diastole. Accordingly, the cuff pressure, with minimal delay, always can be equated with the intravascular pressure. MANOMETRY WITH THE FINAPRES Currently available Finapres models 4 and 5 are electronically similar, completely automatic and easy to handle. After positioning the finger cuff, the unit is adjusted automatically through comparison of the plethysmogram at each cuff pressure level with the predicted value stored as an algorhythm++.(ABSTRACT TRUNCATED AT 250 WORDS)