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.

Efficacy of field treatments to reduce body core temperature in hyperthermic subjects.

PURPOSE: To contrast the effects of three postcooling techniques in reducing body core temperature (Tc) in exercise-induced hyperthermic participants on the cessation of exercise. METHODS: Eleven healthy active male volunteers were cooled during a 40-min period using three different methods: ice packs to the neck, axillae, and groin (ICE); water spray and fan (FAN); and 2 L of chilled (20 degrees C) intravenous saline administered during a 20-min period (IV). Rate of decrease in Tc, cardiovascular responses, and any incidence of reported adverse effects were investigated. Trials were presented in a counterbalanced order with the volunteers' body core temperature being elevated to 40.0 degrees C on three occasions via an intermittent walk-run (2 min at 6 km x h and 4 min at 10 km x h) protocol conducted within a climate-controlled chamber (34.2 +/- 0.5 degrees C and 62.3 +/- 3.1% relative humidity). RESULTS: Rate of Tc reduction during the first 20 min of cooling was greater for FAN compared with ICE (0.09 +/- 0.02 degrees C.min vs 0.07 +/- 0.02 degrees C.min, P < 0.05), whereas IV did not differ with the other trials (0.08 +/- 0.01 degrees C.min, P > 0.05). Three participants complained of numbness or paresthesia in their arm or hand during administration of the chilled saline, although these symptoms resolved within 5 min of ceasing the infusion. CONCLUSIONS: All three cooling techniques reduced Tc and would be suitable for first aid application in a field setting during transportation to adequate medical facilities. Chilled IV saline did not produce any contraindications, providing a suitable alternative for Tc cooling.

Influence of postexercise cooling techniques on heart rate variability in men.

The reduction of core body temperature (T(C)) is vitally important in the treatment of hyperthermia; however, little is known regarding the impact of cooling treatments on the autonomic control of heart rate (HR). The aim of the present study was to examine the influence of three field-based hyperthermia treatments on the neural control of HR via heart rate variability (HRV). Following exercise-induced hyperthermia (T(C) approximately 40.0 degrees C) in a warm environment (34.2 +/- 0.5 degrees C), nine healthy, active men were treated during recovery, in a randomized order, with intravenous cold saline infusion (IV) or ice packs (ICE) or fan cooling with intermittent water spray (FAN) for 40 min. During each treatment, HR dynamics via power spectral (VLF, LF, HF), Poincare plot (SD1, SD2), approximate entropy (ApEn) and short- (alpha(1)) and long-term (alpha(2)) fractal scaling analyses were determined every 10 min. At recovery onset, HR and T(C) were similar between treatments and were significantly reduced over the 40 min recovery period. During recovery, HR and alpha(2) were significantly reduced from initial levels but were significantly greater for IV compared with ICE and FAN. In contrast, VLF, LF, HF, SD1, SD2 and ApEn increased during recovery, with all being significantly lower for IV compared with ICE and/or FAN. The present results demonstrated that IV, compared with ICE and FAN, resulted in significantly greater HR, reduced spectral and geometrical HRV, lower HR complexity and reduced long-term HR control, indicative of reduced vagal and/or increased sympathetic modulation. Specific treatments for exercise-induced hyperthermia may result in an altered sympathovagal balance that requires further examination.

Direct and indirect methods for determining plasma volume during thermoneutral and cold-water immersion.

Plasma volume (PV), determined indirectly from changes in haematocrit (Hct) and haemoglobin concentration ([Hb]), underestimates the absolute PV change (Evans blue dye) during thermoneutral immersion. Since PV changes during cold-water immersion have only been determined indirectly, we hypothesised that a similar underestimation may occur. Therefore, we compared the indirectly-measured PV with a direct-tracer dilution method (Evans blue dye column elution) in seven healthy males, during three, 60-min exposures: air (control; 21.2 degrees C), thermoneutral immersion (34.5 degrees C) and cold-water immersion (18.6 degrees C). During thermoneutral immersion, the directly-measured PV increased by 16.2 (1.4)% (P<0.05) and the indirectly-measured by 8.5 (0.8)% (P<0.05), with the latter underestimating the former by 43 (9.1)% (P<0.05). During cold immersion, the direct PV decreased by 17.9 (3.0)% (P<0.05) and the indirect by 8.0 (1.2)% (P<0.05), with the latter representing a 52 (6.8)% (P<0.05) underestimation of the direct PV change. Directionally-equivalent underestimations of PV change occur when using the indirect method during both thermoneutral and cold-water immersion. The assumptions inherent in the indirect method (constant F-cell ratio) appear to be violated during water immersion.