ABSTRACT
Near Infrared Photoplethysmography (NIRP) is a non-invasive technique that has the potential to be used to assess the state of the peripheral nerves and microcirculation. However, the parameters that reflect the state of the microvascular system have not been established. Therefore, we tested the hypothesis that the resistance of the blood vessels in the microcirculation can be determined by analyzing the temporal components of the photoplethysmographic trace. NIRP incorporates two light-emitting diodes and a photodetector. The emergent light enters the skin and penetrates to the level of the dermis, is backscattered, collected by the photodetector and converted to a recordable trace. This trace consists of a series of pulses synchronous with the heart period, Th. It is possible to divide each pulse into a rise time from baseline to peak, Tg, and fall time from peak to baseline, Ta. All NIRP recordings were taken from the right index finger with the person in the supine position. In a first study involving five healthy male volunteers, the subjects right arm was cuffed approximately three minutes during which he exercised the fingers to increase a buildup of tissue metabolites; this was done in order to decrease microvascular resistance. In a second study, the above parameters (Th, Tg and Ta) were measured in 31 patients with chronic diabetes mellitus (diabetic group, DG) and compared to those of 29 healthy normal persons (healthy group, HG). In the first study, the ratio of the rise time to the heart period (Tg/Th) or of the rise time of the fall time (Tg/Ta) was increased, whereas that to the fall time of the heart period (Ta/Th) was decreased. In the second study, the mean for the Tg/Th was 0.235 ñ 0.008 (mean ñ sem) for the HG and 0.292 ñ 0.007 for the DG; the increase was highly significant, p< 0.001 (one-tail unpaired t-test). The mean for the Tg/Ta was 0.312 ñ 0.013 for the HG and 0.413 ñ 0.012 for the DG; this increase was also highly significant, p< 0.001. The mean for the Ta/Th was 0.765 ñ 0.008 for the HG and 0.711 ñ 0.006 for the DG; the decrease was highly significant, p< 0.001. In conclusion, an increase in microvascular resistance was reflected in an increase in the rise time and a decrease in the fall time. Thus this system appears to be a useful one in quantifying the increase in microvascvular resistance in diabetics. (AU)
