Alterations in Body Fluid Balance During Fin Swimming in 29 °C Water in a Population of Special Forces Divers.

Highly trained "combat swimmers" encounter physiological difficulties when performing missions in warm water. The aim of this study was to assess the respective roles of immersion and physical activity in perturbing fluid balance of military divers on duty in warm water. 12 trained divers performed 2 dives each (2 h, 3 m depth) in fresh water at 29 °C. Divers either remained Static or swam continuously (Fin) during the dive. In the Fin condition, oxygen consumption and heart rate were 2-fold greater than during the Static dive. Core and skin temperatures were also higher (Fin: 38.5±0.4 °C and 36.2±0.3 °C and Static: 37.2±0.3 °C and 34.3±0.3 °C; respectively p=0.0002 and p=0.0003). During the Fin dive, the average mass loss was 989 g (39% urine loss, 41% sweating and 20% insensible water loss and blood sampling); Static divers lost 720 g (84% urine loss, 2% sweating and 14% insensible water loss and blood sampling) (p=0.003). In the Fin condition, a greater decrease in total body mass and greater sweating occurred, without effects on circulating renin and aldosterone concentrations; diuresis was reduced, and plasma volume decreased more than in the Static condition.

Cardiac alterations induced by a fish-catching diving competition.

Cardiac changes induced by repeated breath-hold diving were investigated after a fish-catching diving competition. Eleven healthy subjects carried out repeated breath-hold dives at a mean maximal depth of 20 ± 2.7 msw (66 ± 9 fsw) during 5 h. One hour after the competition, the body mass loss was -1.7 ± 0.5 kg. Most of the breath-hold divers suffered from cold and although the core temperature remained normal, a decrease in cutaneous temperature was recorded in the extremities. Systolic blood pressure was reduced in both upper and lower limbs. Heart rate was unchanged, but left ventricular (LV) stroke volume was reduced leading to a decrease in cardiac output (-20%). Left atrial and LV diameters were significantly decreased. LV filling was assessed on a trans-mitral profile. An increase in the contribution of the atrial contraction to LV filling was observed. Right cavity diameters were increased. The cardiac autonomic alterations were in favor of sympathetic hyperactivity. After a fish-catching diving competition in cold water, alterations suggesting dehydration, contraction in plasma volume and sympathetic hyperactivity were observed. Furthermore, enlargements of right cavities were in favor of right ventricular strains. Repeated apnea and swimming in cold water may account for these alterations.

Thermoregulatory effects of three different types of head cooling in humans during a mild hyperthermia.

Seven healthy young men participated in six trials with three different types of local cooling [cool air breathing (CAB), face skin cooling (FaC), and combined cooling (CoC)] in a warm environment for 90 min while either resting (operative temperature: T(0) = 40 degrees C, dew point temperature: T(dp) = 15 degrees C, air velocity: v(a) = 0.3 m x s(-1)) or exercising on a cycle ergometer with an external work load of 90 W (T(0) = 36 degrees C, T(dp) = 15 degrees C, v(a) = 0.3 m x s(-1)). Cool air (10 degrees C) arrived at the entry point of the hood and/or the mask at a ventilation rate of 12 m x s(-1). Oesophageal temperature was not affected by any kind of cooling, while tympanic temperature was decreased at rest by both FaC and CoC [respectively -0.15 (0.06) and -0.09 (0.03) degrees C, P

Comparison of sweat rate measured by a pouch collector and a hygrometric technique during exercise.

Measurements of local sweat rate (back) determined with a closed-pouch collector made of polyethylene (110 cm2) were compared with those obtained from a ventilated capsule using an infrared photometric hygrometer technique. Eight young male subjects underwent three exercise sessions each for 60 min at 45% VO2max on a cycle ergometer at an ambient temperature of 35 degrees C and 35% relative humidity. When the onset and transient sweating periods (0-20 min) are excluded from calculations, the difference between the mean values obtained by the collector and the sweat capsule during the 20-60 time period is only 0.06 +/- 0.04 mg x cm-2 x min-1 (5%). Although a significant correlation (r = .74, p < .05) was obtained between methods, individual differences in sweat-rate measurements varied on average +/-0.22 mg x cm-2 x min-1 (+/-19%). Designed more specifically for sweat-content analysis, the pouch collector may serve as a satisfactory field method to approximate the local sweat rate and excretion induced by prolonged exercise in a hot environment.

Alcohol and its variable effect on human thermoregulatory response to exercise in a warm environment.

The aim of this study was to observe the effect of alcohol ingestion on body temperature and local sweat rate during endogenous and exogenous heat stress. After ingesting either alcohol (1.2 g alcohol/kg of body weight) or a placebo drink, 8 subjects exercised for 60 minutes at 45% VO2max in a warm environment (35 degrees C, 45% RH). Varying patterns of response were observed in these subjects, with no consistent effect on the thermoregulatory response seen. The absence of any significant change in skin and body temperature and in sweat rate suggests that the capacity of the body to struggle against exogenous and endogenous heat is not fundamentally altered by alcohol ingestion. The difference in individual response observed in our experiment is in accord with the previous lack of clearcut effect of alcohol reported in the literature.

Effects of local restriction of evaporation and moderate local ventilation on thermoregulatory responses in exercising humans.

Ten healthy young men participated in two series of three trials: series 1 (C1) with, or without, local restriction of evaporation (either on the trunk or on the legs) and series 2 (C2) with, or without, local moderate nitrogen ventilation (40 l.min-1) under an impermeable garment (trunk or leg ventilation). After 60-min rest in a thermoneutral environment, the subjects exercised in a warm environment [30 degrees C, 47% relative humidity (rh) during C1 and 29% rh during C2] on a cycle ergometer for 60 min at 70 W during C1 or at 60 W during C2. During C1, local covering with plastic foil did not increase internal temperature, but increased the mean skin temperature with a higher effect in the case of leg restriction. The trunk skin temperature was affected by the leg covering while the leg skin temperature was not changed by the trunk covering. Only the local sweat rate of the trunk was increased by the two restriction conditions. During C2, internal temperature was decreased by local ventilation while mean skin temperature was reduced only by trunk ventilation. The local ventilation affected only trunk skin temperature with a greater decrease during trunk ventilation. Trunk ventilation did not influence the skin temperature of the legs while ventilation of the legs decreased trunk skin temperature. In addition, leg ventilation decreased the sweat rate of the legs. The impermeable suit worn during C2 led to a greater physiological strain compared to the plastic film worn during C1 even with local ventilation under the impermeable garment. As expected, limiting sweat evaporation led to an increase in physiological strain. Microclimate ventilation at a rate of 40 l.min-1 was not sufficient to allow total heat dissipation but allowed 60-min exercise in a warm environment to be completed without excessive heat accumulation. It would appear that ventilation of the trunk locally was the best solution because of the smaller increase in skin temperature and higher sweating capacity of the trunk.