Small changes in hand height alter absorbance, but not pulsation, in the finger pulse plethysmograph

Objective Previous studies of finger pulse plethysmography have used large changes in finger height to assess how pressure influences light transmission. Light transmission is affected by pulsation transmitted from the arterial pressure, and also by the volume of blood held in the finger vasculature. Smaller changes of hand height, in relation to the neck, could elucidate more precisely the pressures and mechanisms associated with vascular volume changes. Approach We studied 18 (15 male, 3 female) healthy young volunteers in warm quiet surroundings, with one arm supported horizontally at the level of the shoulder. Comparisons were made with the hand placed 5 cm above and 5 cm below the horizontal, in random sequence. Red and infrared transmission were recorded from a standard pulse oximeter finger probe. Sections of recording were chosen where short periods of vasoconstriction were absent. Total absorbance values at peak and trough, and absorbance pulse amplitude, were calculated. We compared measures in the two hand positions, and before and after inflation of a cuff at the wrist, within each subject. Main results In the dependent position, infrared light absorbances were greater: peak, 0.054 (0.043, 0.070) and trough (0.053 (0.045, 0.069) (median, quartiles) (P < 0.0001). Pulse amplitude was unaffected. Cuff inflation had no effect on the peak and trough values, but pulse amplitude decreased: the difference between paired values for infrared was 0.0049 (0.0075, 0.0015) and red, 0.0030 (0.0043, 0.0010) (median paired difference, 95% confidence limits). (P < 0.005). Significance The minimal effect of cuff venous obstruction supports previous work reporting flow limitation in venous drainage of the arm. Our observations suggest that an increase in filling pressure caused by lowering the hand by 10 cm increases overall blood volume in the finger, suggesting that the plethysmograph waveform may result from pressure changes much less than arterial.