Anne van den Heuvel

To Cool, But Not Too Cool : That Is the Question-Immersion Cooling for Hyperthermia

INTRODUCTION Patient cooling time can impact upon the prognosis of heat illness. Although ice-cold-water immersion will rapidly extract heat, access to ice or cold water may be limited in hot climates. Indeed, some have concerns regarding the sudden cold-water immersion of hyperthermic individuals, whereas others believe that cutaneous vasoconstriction may reduce convective heat transfer from the core. It was hypothesized that warmer immersion temperatures, which induce less powerful vasoconstriction, may still facilitate rapid cooling in hyperthermic individuals. METHODS Eight males participated in three trials and were heated to an esophageal temperature of 39.5 degrees C by exercising in the heat (36 degrees C, 50% relative humidity) while wearing a water-perfusion garment (40 degrees C). Subjects were cooled using each of the following methods: air (20-22 degrees C), cold-water immersion (14 degrees C), and temperate-water immersion (26 degrees C). RESULTS The time to reach an esophageal temperature of 37.5 degrees C averaged 22.81 min (air), 2.16 min (cold), and 2.91 min (temperate). Whereas each of the between-trial comparisons was statistically significant (P < 0.05), cooling in temperate water took only marginally longer than that in cold water, and one cannot imagine that the 45-s cooling time difference would have any meaningful physiological or clinical implications. CONCLUSION It is assumed that this rapid heat loss was due to a less powerful peripheral vasoconstrictor response, with central heat being more rapidly transported to the skin surface for dissipation. Although the core-to-water thermal gradient was much smaller with temperate-water cooling, greater skin and deeper tissue blood flows would support a superior convective heat delivery. Thus, a sustained physiological mechanism (blood flow) appears to have countered a less powerful thermal gradient, resulting in clinically insignificant differences in heat extraction between the cold and temperate cooling trials.

Torso Undergarments: Their Merit for Clothed and Armored Individuals in Hot-Dry Conditions

INTRODUCTION The aim of this study was to evaluate how the textile composition of torso undergarment fabrics may impact upon thermal strain, moisture transfer, and the thermal and clothing comfort of fully clothed, armored individuals working in a hot-dry environment (41.2 degrees C and 29.8% relative humidity). METHODS Five undergarment configurations were assessed using eight men who walked for 120 min (4 km x h(-1)), then alternated running (2 min at 10 km x h(-1)) and walking (2 min at 4 km x h(-1)) for 20 min. Trials differed only in the torso undergarments worn: no t-shirt (Ensemble A); 100% cotton t-shirt (Ensemble B); 100% woolen t-shirt (Ensemble C); synthetic t-shirt (Ensemble D: nylon, polyethylene, elastane); hybrid shirt (Ensemble E). RESULTS Thermal and cardiovascular strain progressively increased throughout each trial, with the average terminal core temperature being 38.5 degrees C and heart rate peaking at 170 bpm across all trials. However, no significant between-trial separations were evident for core or mean skin temperatures, or for heart rate, sweat production, evaporation, the within-ensemble water vapor pressures, or for thermal or clothing discomfort. CONCLUSION Thus, under these conditions, neither the t-shirt textile compositions, nor the presence or absence of an undergarment, offered any significant thermal, central cardiac, or comfort advantages. Furthermore, there was no evidence that any of these fabrics created a significantly drier microclimate next to the skin.

Balancing ballistic protection against physiological strain: evidence from laboratory and field trials

This project was based on the premise that decisions concerning the ballistic protection provided to defence personnel should derive from an evaluation of the balance between protection level and its impact on physiological function, mobility, and operational capability. Civilians and soldiers participated in laboratory- and field-based studies in which ensembles providing five levels of ballistic protection were evaluated, each with progressive increases in protection, mass (3.4-11.0 kg), and surface-area coverage (0.25-0.52 m(2)). Physiological trials were conducted on volunteers (N = 8) in a laboratory, under hot-dry conditions simulating an urban patrol: walking at 4 km·h(-1) (90 min) and 6 km·h(-1) (30 min or to fatigue). Field-based trials were used to evaluate tactical battlefield movements (mobility) of soldiers (N = 31) under tropical conditions, and across functional tests of power, speed, agility, endurance, and balance. Finally, trials were conducted at a jungle training centre, with soldiers (N = 32) patrolling under tropical conditions (averaging 5 h). In the laboratory, work tolerance was reduced as protection increased, with deep-body temperature climbing relentlessly. However, the protective ensembles could be grouped into two equally stressful categories, each providing a different level of ballistic protection. This outcome was supported during the mobility trials, with the greatest performance decrement evident during fire and movement simulations, as the ensemble mass was increased (-2.12%·kg(-1)). The jungle patrol trials similarly supported this outcome. Therefore, although ballistic protection does increase physiological strain, this research has provided a basis on which to determine how that strain can be balanced against the mission-specific level of required personal protection.

A vascular mechanism to explain thermally mediated variations in deep‐body cooling rates during the immersion of profoundly hyperthermic individuals

What is the central question of this study? Does the cold‐water immersion (14°C) of profoundly hyperthermic individuals induce reductions in cutaneous and limb blood flow of sufficient magnitude to impair heat loss relative to the size of the thermal gradient? What is the main finding and its importance? The temperate‐water cooling (26°C) of profoundly hyperthermic individuals was found to be rapid and reproducible. A vascular mechanism accounted for that outcome, with temperature‐dependent differences in cutaneous and limb blood flows observed during cooling. Decisions relating to cooling strategies must be based upon deep‐body temperature measurements that have response dynamics consistent with the urgency for cooling.