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 cha nges in skin perfusion by quantifying the phase shift of laser light i nduced 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 us ed with maximum effectiveness. We evaluated the responses of two diffe rent laser Doppler outputs, the concentration of moving blood cells (C MBC) and red cell flux (CMBC times cell velocity), and the responses a t two sites of probe application, the finger and forearm, during syste mic infusions of phenylephrine. Methods and Results Eight healthy volu nteers were monitored with a brachial blood pressure cuff, ECG, and la ser Doppler flowmeter probes applied to the palmar surface of the four th finger and volar forearm of the arm opposite the pressure cuff. Aft er baseline readings were obtained, the subjects received three 10-min ute intravenous infusions of phenyelphrine at rates of 0.4, 0.8, and 1 .6 mu g.kg(-1).min(-1). The two parameters, flux and CMBC, trended sim ilarly. Flux and CMBC at the finger declined significantly in response to each infusion (P<.05 using repeated-measures ANOVA with Duncan's m ultiple range test). In contrast, flux and CMBC of the forearm had hig hly variable responses, with an overall increase during each infusion (P<.05 for %Delta of forearm versus %Delta of finger readings during t he 0.4 mu g.kg(-1).min(-1) infusion). Heart rate declined significantl y during each infusion, consistent with a baroreceptor-mediated respon se, even though systolic and diastolic blood pressures each increased by less than 2 mm Hg during the 0.4 mu g.kg(-1).min(-1) infusion. Conc lusions As expected, laser Doppler readings at the finger decreased du ring infusion of an alpha(1)-agonist. Although, like the digital vesse ls, 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 sensi tive primarily to local changes in vascular caliber) suggested vasodil ation of the underlying vessels. The forearm vasodilation and the conc omitant decline in heart rate most likely represented vagally mediated baroreceptor activity, which was altered even though blood pressure c hanged minimally during the 0.4 mu g.kg(-1).min(-1) infusion. Thus, in tegrated assessment of skin perfusion at the finger and forearm may pr ovide valuable information about the direct and indirect effects of a vasoactive stimulus. The present application of laser Doppler flowmetr y suggests activation of vasodilatory reflexes despite minimal changes in blood pressure.