Cold-induced vasodilation: A meta-analysis

Nearly a century ago, cold-induced vasodilation (CIVD) was first described as repeated episodes of warm blood flow to the fingers during cold-water immersion. Since then, hundreds of studies have examined this phenomenon, yet no comprehensive synthesis exists. To address this gap, we conducted a meta-analysis of studies in which the hand, or parts thereof, were immersed for 30 minutes in water below 20°C. A total of 80 studies met the inclusion criteria. Across studies, the weighted onset time of CIVD averaged 7.9 minutes [7.4-8.3], and the mean finger temperature averaged 10.0°C [9.5-10.6]. Onset time was weakly related to finger temperature during immersion (r = -0.21 to -0.27), supporting the theory that the onset of CIVD is triggered by low local tissue temperatures, while the magnitude is dependent on sympathetic activity. Onset time was longer for hand versus finger-only immersion, for individuals with a larger surface area, and for males compared to females. Onset time was shorter with higher ambient temperatures, in cold-indigenous populations, and with increasing age. To enrich the meta-analysis, we conducted a narrative literature review of the individual factors and previously proposed mechanisms of CIVD. Current evidence suggests that CIVD is mediated by 1) impaired transfer of noradrenaline from sympathetic nerves to the smooth muscle of the arterio-venous anastomoses or 2) nitric oxide release from these nerves, however, further research is needed to confirm these mechanisms. Future investigations should prioritize including more females and older adults, as these populations remain underrepresented in the literature.