Objective: Cold pressor test (CPT) assesses sympathetic reactivity by the rise in diastolic blood pressure secondary to the vasoconstriction during immersion of hand in cold water. Presently, monitoring of vascular reactivity in health and diabetes during CPT has been attempted by objective measures of Photoplethysmogram (PPG) that include amplitude, upstroke-slope, pulse timings and pulse transit time (PTT).
Methods: Finger-PPG characteristics were studied before and during CPT (1 min) in 11 healthy volunteers and 10 diagnosed Type 2 Diabetes Mellitus (DM) patients. In controls, the recordings were continued for 5 min after CPT.
Results: The amplitude of PPG significantly decreased due to cold stress in both control and DM groups (P < 0.0001 and P < 0.003, respectively). However, the decrease in amplitude was significantly lesser (0.42 +/- 0.08 nu vs. 0.25 +/- 0.03 nu, P = 0.04) in DM group than controls. The slope response of PPG resembled the amplitude. PTT was significantly shortened in control and DM groups (180.0 +/- 3.8 ms vs. 187.1 +/- 3.9 ms, P < 0.006, 177.7 +/- 7.0 ms vs. 192.9 +/- 5.6 ms, P = 0.002, respectively) during CPT as compared to baseline. However, the decrease in PTT was significantly higher (-15.2 +/- 3.4 ms vs. -6.0 +/- 1.9 ms, P = 0.03) in DM patients than controls. No significant differences were noticed in Delta changes of peak-to-peak interval, crest time and decay time of PPG between the two groups.
Conclusion: This preliminary study suggests that the collective responses of PPG amplitude and PTT can be used to objectively quantify the sympathetic reactivity to cold stress in health as well as to detect the deficits of vascular reactivity in diabetes. Further studies would substantiate the simple PPG technique in quantifying the neuronal and vascular dysfunction.