Peripheral assessment of phenylephrine-induced vasoconstriction by laser Doppler flowmetry and its potential relevance to homeostatic mechanisms

Background: Cutaneous laser Doppler flowmetry enables monitoring of changes in skin perfusion by quantifying the phase shift of laser light induced by moving red blood cells under a fiberoptic probe. It thus can identify the presence of and response to a vasoconstrictive stimulus. However, aspects of the technique must be defined before it can be used with maximum effectiveness. We evaluated the responses of two different laser Doppler outputs, the concentration of moving blood cells (CMBC) and red cell flux (CMBC times cell velocity), and the responses at two sites of probe application, the finger and forearm, during systemic infusions of phenylephrine. Methods and Results: Eight healthy volunteers were monitored with a brachial blood pressure cuff, ECG, and laser Doppler flowmeter probes applied to the palmar surface of the fourth finger and volar forearm of the arm opposite the pressure cuff. After baseline readings were obtained, the subjects received three 10-minute intravenous infusions of phenylephrine at rates of 0.04, 0.8, and 1.6 μg · kg^-1 · min^-1. The two parameters, flux and CMBC, trended similarly. Flux and CMBC at the finger declined significantly in response to each infusion (P<.05 using repeated-measures ANOVA with Duncan's multiple range test). In contrast, flux and CMBC of the forearm had highly variable responses, with an overall increase during each infusion (P<.05 for %Δ of forearm versus %Δ of finger readings during the 0.4 μg · kg^-1 · min^-1 infusion). Heart rate declined significantly during each infusion, consistent with a baroreceptor-mediated response, even though systolic and diastolic blood pressures each increased by less than 2 mm Hg during the 0.4 μg · kg^-1 · min^-1 infusion. Conclusions: As expected, laser Doppler readings at the finger decreased during infusion of an α₁-agonist. Although, like the digital vessels, forearm vessels have the potential to constrict, the increases in forearm readings suggest that these vessels are highly susceptible to homeostatic responses. The increase in CMBC (a parameter that is sensitive primarily to local changes in vascular caliber) suggested vasodilation of the underlying vessels. The forearm vasodilation and the concomitant decline in heart rate most likely represented vagally mediated baroreceptor activity, which was altered even though blood pressure changed minimally during the 0.4 μg · kg^-1 · min^{- 1} infusion. Thus, integrated assessment of skin perfusion at the finger and forearm may provide valuable information about the direct and indirect effects of a vasoactive stimulus. The present application of laser Doppler flowmetry suggests activation of vasodilatory reflexes despite minimal changes in blood pressure.