Carbon dioxide vs. warm-water hand baths in systemic sclerosis with secondary Raynaud's syndrome – A capillaroscopy centered randomized controlled trial

BACKGROUND: Raynaud's phenomenon (RP) affects nearly all patients with systemic sclerosis (SSc) and causes substantial morbidity and functional impairment. We evaluated the immediate, segment-specific microvascular effects of a single carbon dioxide (CO₂) versus warm water hand baths in SSc with secondary RP, including healthy controls as physiological reference. METHODS: bath. Nailfold capillaroscopy assessed vas afferens (VA), apex, and vas efferens (VE) diameters at baseline, immediately, and 10 min post-intervention. Primary outcome was immediate diameter change; secondary outcomes were persistence and between-cohort differences. RESULTS: induced dilation in healthy controls, but VA increase was smaller than after CO₂ in SSc (p = 0.010). At 10 min, VA remained higher in the CO₂ vs. water group (p = 0.006). No adverse events occurred. CONCLUSIONS: A single CO₂ hand bath induced greater and partly sustained nailfold capillary dilation of the arterial limb and apex than warm water in SSc-associated RP. The lack of response to warm water suggests impaired heat-induced vasodilation in SSc, whereas CO₂ may engage alternative vasodilatory mechanisms. These findings support CO₂ hand baths as a safe, low-cost, non-pharmacological adjunct. Larger trials with clinical endpoints are warranted.