TNO Finger Pressure Reference Guide - Integral Process

TNO Finger Pressure Reference Guide 20Finger blood pressure (mmHg)2001501005000 60 120 180 240 300Time (s)Figure 2.11 Physiocal frequency2.12 Note on blue fingertipsDuring a normal finger blood pressure measurement some coloring of the finger tipdistal from the finger cuff is observed. The also occurring whiteness of the fingerphalanx below the finger cuff is usually invisible.The arteries below the finger cuff are unloaded, that is, the arterial walls are held atzero transmural pressure and they are therefore at their unstressed diameter, cf.2.5). Unstressed arteries still have about 60% of their original distended diameteror approximately 1/3 or 1/2 their original cross-sectional area and volume. Theother blood vessels below the cuff are collapsed. Therefore, all the vessels behindthe cuff, in the finger tip, are at instantaneous arterial pressure including the microcirculationand the veins. Thus blood entering the finger tip during systole can onlyleave during diastole, when the pressure in the vessels of the finger tip is higherthan the pressure more proximal from the cuff. This causes some congestion. Sinceduring a blood pressure measurement the blood in the finger tip is less efficientlyrefreshed the reduced wash out of metabolites and increased de-oxygenation of thevenous blood, causes blue finger tips. Note, however, that the Physiocal algorithmalways checks that the artery is sufficiently open during a measurement. Inflow intoand outflow from the venous compartments is facilitated by the enlarged capillarydiameters at the arterial pressures their walls sustain. It is also facilitated by relaxedarterioles (including the arterio-venous anasthomoses). Relaxed arteries are present