Ipsilateral and contralateral microvascular kinetics during unilateral forearm cold-water immersion: a within-subject laser Doppler flowmetry and heart rate variability study

OBJECTIVES: To quantify bilateral cutaneous microvascular responses to unilateral forearm cold-water immersion (CWI) and describe concurrent autonomic cardiac responses. METHODS: Thirty healthy adults completed a within-subject bilateral experiment with randomized unilateral 5 °C forearm immersion. Bilateral fingertip perfusion was recorded by laser Doppler flowmetry (LDF), and heart rate variability was recorded continuously. primary endpoint was the within-participant ipsilateral-contralateral difference in relative perfusion decrease from baseline to nadir during cold exposure. Secondary endpoints were LDF kinetic and recovery outcomes, and heart rate (HR) and heart rate variability (HRV) outcomes were treated as secondary autonomic outcomes. HRV-contralateral perfusion associations were exploratory. RESULTS: Resting perfusion was similar between limbs (mean difference, -0.07 perfusion units; 95% CI, -0.22 to 0.08; p = 0.370). For the primary endpoint, the immersed limb showed a greater relative perfusion decrease than the contralateral limb (60.2 ± 2.8% vs. 10.3 ± 1.2%; mean difference, 50.0% points; 95% CI, 48.7 to 51.2; dz = 15.28; adjusted p < 0.001). Secondary LDF endpoints showed earlier vasoconstriction, delayed post-removal peak perfusion, higher peak perfusion, and longer recovery in the immersed limb. Heart rate increased by 9.21 bpm, whereas RMSSD and SDNN decreased by 9.28 and 7.44 ms, respectively (all adjusted p < 0.001). Exploratory HRV-contralateral perfusion correlations were not significant. CONCLUSIONS: Unilateral 5 °C forearm immersion elicited a dominant local vasoconstrictive-hyperemic response, smaller contralateral changes, and concurrent autonomic cardiac activation.