Whole-body cooling effectiveness of cold intravenous saline following exercise hyperthermia: a randomized trial

In some athletic, occupational, military and emergency settings, cold intravenous (IV) fluids are used to facilitate whole-body cooling in an effort to treat heat illness. This treatment has anecdotal support, but currently lacks evidence supporting it as a whole-body cooling modality. Other modalities may offer superior cooling rates, and thus, patient outcomes following treatment. We sought to evaluate cooling rates of cold-IV normal saline immediately following exercise-induced hyperthermia.

Eight healthy participants (3 females; 25 ± 2y; 72.9 ± 10.9 kg) completed 2 trials in random order. Prior to exercise, participants provided a small urine sample to confirm hydration status via urine specific gravity. Wet bulb globe temperature (WBGT) was assessed throughout trials. In both trials, participants exercised outdoors until rectal temperature (T_re) reached ∼38.9 °C, or volitional exhaustion, and then were cooled. In cooling, participants received either cold-IV (∼5 °C 0.9% NaCl fluids) or no treatment (sat in the shade; passive). Throughout exercise and treatment, T_re and heart rate (HR) were monitored. During exercise and every 10 min throughout cooling, participants were asked to assess thermal sensation.

Hydration status (P = .847) was not significantly different prior to exercise between trials. WBGT throughout was not different between trials (P = .426). Maximum T_re reached was not different between cold-IV (38.88 ± 0.30 °C) and passive cooling (38.76 ± 0.28 °C) trials (P = .184). Mean cooling rate for cold-IV (0.039 ± 0.005 °C·min^-1) was significantly greater than for passive cooling (0.028 ± 0.005 °C·min^-1; P = .002). T_re throughout cooling was not different between trials (P = .707), but did decrease throughout (P = .008), regardless of trial. HR was decreased over time (P < .001), but cold-IV and passive cooling were not different throughout HR recovery (P = .141). Thermal sensation decreased throughout cooling (P < .001), but was not different between trials (p = .278).

Emergency medical personnel should adopt treatment protocols that employ documented effective treatments for exertional heat stroke. In isolation, our data casts significant doubt for the use of cold-IV saline infusion for whole-body cooling of hyperthermic individuals.