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Shear‐induced vascular adaptations: therapeutic insights and heterogeneity throughout the arterial tree

A recent study by Green et al. (2010) published in The Journal of Physiology examined the novel hypothesis that repeated skin heating improves cutaneous microvascular vasodilator function via a shear stress-dependent mechanism. Local heating of both forearms to 42°C (via water immersion) was applied to 10 healthy men for 30 min, 3 times per week for 8 weeks. During the immersion sessions, skin blood flow and, presumably shear stress, were cleverly manipulated in one arm by inflation of a proximal forearm cuff to 100 mmHg, while the other arm remained uncuffed. To assess microvascular adaptations a local heating protocol was used that elicits vasodilatation through largely nitric oxide-dependent mechanisms (Kellogg et al. 1999). After repeated bouts of limb heating, the cutaneous vascular response to acute local heating was increased in the uncuffed arm (i.e. the arm with the greatest chronic increase in shear), whereas it remained unchanged in the cuffed arm. The results of Green et al. (2010) represent the first data that support the hypothesis of shear-induced endothelial adaptations in the human skin circulation. 
 
Without diminishing the significance of the data presented by Green et al. (2010), it is important to recognize that assumptions about the microcirculatory shear profile on the basis of blood flow estimates have not been validated. At present it is unclear how changes in microvascular diameter and blood velocity during forearm heating translate to alterations in shear stress in the skin circulation, and laser Doppler flowmetry is incapable of yielding this information (particularly for microvascular diameter). Nevertheless, the findings of Green et al. provide insight into at least two exciting areas of cardiovascular physiology.