Quantifying skin microvascular function responses to distinct forms of heat stress in humans using optical coherence tomography

Optical coherence tomography (OCT) enables visualization and quantification of the cutaneous microvasculature, yet no study has compared responses to distinct forms of heating in humans. We hypothesized that local skin heating (LH) would evoke larger responses in microvascular diameter, velocity, flow and density than passive whole-body heating (PH) or heated exercise (HE), and that HE responses would exceed PH. Twelve healthy young adults completed four interventions: baseline (33°C; BL), LH, PH (seated) and HE (ergometer cycling) in a climatic chamber (50 min, 40°C, 50% relative humidity). OCT was used to quantify microvascular variables immediately after each intervention. Microvascular responses differed across conditions (P < 0.001). LH induced the largest responses in all OCT indices (all P < 0.001): diameter (67 µm), velocity (195 µm s⁻¹), flow (687 picolitres s⁻¹) and density (56.0%), compared with BL (42 µm, 106 µm s⁻¹,154 picolitres s⁻¹ and 26.6%, respectively), PH (45 µm, 99 µm s⁻¹, 165 picolitres s⁻¹ and 34.4%, respectively) and HE (49 µm, 105 µm s⁻¹, 208 picolitres s⁻¹ and 34.5%, respectively). Although the diameter response was higher after HE (P = 0.046), no differences were documented for PH and HE relative to 33°C BL for other OCT measures (all P > 0.05). Comparable responses were observed between PH and HE across all variables (all P > 0.05). Local heating elicited substantially greater increases in all OCT-derived microvascular metrics compared with PH and HE. Although both PH and HE activate the cutaneous microvasculature, neither stimulus approaches the magnitude of response achieved with local heating. These findings demonstrate that OCT provides quantifiable insights into the distinct ways in which the skin microvasculature responds to different heat exposures.