Effects of exercise modality on patterns of ventilation and respiratory timing.

Ventilatory patterns and respiratory timing were measured in 14 subjects during cycling (CYC) and treadmill exercise (TM) at similar leg frequencies (fLEG) to determine if mode of exercise affects patterns of ventilation and respiratory timing. Measurements of breathing frequency (fR), tidal volume (VT), expired ventilation (VE), and inspiratory (TI) and expiratory (TE) time were obtained at fLEG of 50, 70, and 90 rev.min-1 (rpm) for CYC and at similar incremental fLEG (strides.min-1; spm) during TM achieved by increasing belt speed at 0% grade. CYC exercise intensity was approximately 50% VO2,max at all fLEG, whereas VO2 increased progressively with TM. fR increased significantly (P < 0.001) with increasing fLEG of TM (20.5 +/- 4.6, 25.4 +/- 5.8, and 36.3 +/- 7.6 breaths.min-1; mean +/- SD), but during CYC fR changed significantly (P < 0.05) only between fLEG of 70 and 90 rpm (25.0 +/- 5.9 vs 28.5 +/- 6.9 breaths.min-1). Both average breath TI and TE obtained by grouping into incremental ranges of fR decreased significantly (P < 0.05) with increasing fR up to 36 breaths.min-1 and the relationships of TI and TE to fR, TI to TE, and central inspiratory drive (VT/TI) to VE were the same for CYC and TM. Group average fR and fLEG were synchronized during TM, but individual subjects did not exhibit a high degree of entrainment. This study shows respiratory timing patterns to be independent of mode of exercise over the range of fR observed when describing patterns by grouping into incremental ranges of fR.

Fluid-electrolyte losses in uniforms during prolonged exercise at 30 degrees C.

The effects of several uniform configurations on fluid and electrolyte losses in a hot environment (30 degrees C db, 18 degrees C wb) were studied in 15 healthy males, during 6 h of intermittent treadmill exercise (1.56 m.s-1, 0% grade, 50 min.h-1, 28 km total distance). The uniforms were: the temperate battle dress uniform (BDU), two variants (C and F) of full military oriented protective posture (MOPP IV), and MOPP IV with no mask or hood (M). Sweat rate, urine volume, electrolyte losses (Na+, Cl-, K+, Mg++, Ca++) in sweat and urine, rectal temperature, mean skin temperature, heart rate, and temperature/humidity inside uniforms were measured. Observations indicated that both MOPP IV configurations (C and F) resulted in significantly greater (p. less than 0.05) fluid losses and physiological strain than BDU and M. However, there were no between-uniform differences in the total loss of any electrolyte. Total body fluid losses (kg.6h-1) and electrolyte losses in sweat (mEq.6h-1) were consistently greater than in urine. It was concluded that 6 h of exercise in uniforms at 30 degrees C can result in losses of Na+ and Ca++ which exceed the normal daily intake of these electrolytes. Supplementation of fluid, Na+, and Ca++ appears to be warranted when scenarios require 6 h or more of uniformed exposure to heat.

Acceptability and effect of carbohydrate-electrolyte solutions on electrolyte homeostasis during field training.

Two lemon-lime flavored 2.5% carbohydrate-electrolyte solutions (CE1 supplemented with Na+, K+, and Mg+; and CE2 supplemented with NA+) were compared to plain water (water) and lemon-lime flavored water placebo (placebo) to evaluate their acceptability and consumption during 8 days of field training in hot weather. Acceptability ratings and consumption of CE2 and the flavored water placebo were similar and greater, respectively, than those for CE1. Average Na+ and K+ intakes, and serum electrolytes levels, were not affected during the 8-day trial. If food intake is adequate, consumption of carbohydrate-electrolyte solutions is apparently unnecessary to maintain electrolyte homeostasis.

Incidence of hypohydration when consuming carbohydrate-electrolyte solutions during field training.

Carbohydrate-electrolyte solutions (CE1, CE2) were evaluated for their ability to reduce the incidence of hypohydration during field training in hot weather (max Tamb = 88 degrees-100 degrees F). Hydration status was monitored twice daily in Army reservists who consumed ad libitum CE1, or CE2, or water, or a flavored water placebo. The water group had the highest percentage incidence of urine specific gravity greater than or equal to 1.030 (22%), whereas CE2 and flavored water placebo groups had the lowest (6% and 8%, respectively). Average total fluid intake was greatest in CE2 and lowest in CE1. CE2 and flavored water placebo were more effective than CE1 and water in reducing the overall incidence of hypohydration during this field exercise.

Prediction of the exercise-heat tolerance of soldiers wearing protective overgarments.

The purpose of this investigation was to see whether subject characteristics and physiologic measurements predicted exercise-heat tolerance (EHT) and voluntary tolerance time in young soldiers. A total of 18 unacclimatized males attempted six 50-min periods of treadmill walking (4.0 km.h-1, 0% grade, 33 degrees C db, 20% rh) while wearing protective overgarments. Two post hoc groups of soldiers were defined: high EHT (H) and low EHT (L), having mean (+/- S.E.) tolerance times of 360 +/- 0 and 222 +/- 12 min, respectively. Significant (p less than 0.05) H vs. L differences were observed in pretrial body mass, percent fat, and mass-to-surface area ratio (M/SA), as well as 170 min HR, Tsk and heat storage. The first three of these factors indicated that preexercise anthropomorphic characteristics may be used to distinguish H from L. The HR and Tsk differences were interpreted to mean that L experienced greater cardiovascular strain in protective overgarments because of a higher Tsk, which resulted in increased pooling of blood in cutaneous vessels, decreased cardiac filling pressure, and increased fatigue. Because HR variables were the strongest correlates of exercise tolerance time (r2 = 0.60 for HR at 170 min, r2 = 0.83 for time to reach HR of 160 beats.min-1) a novel HR monitoring technique was proposed which uses a wrist-mounted cardiotachometer to predict tolerance time.

Carbohydrate-electrolyte solutions during field training: an overview.

The rationale and methods of evaluating two carbohydrate-electrolyte solutions at a field training exercise in hot weather are presented in this overview. The carbohydrate-electrolyte solutions were formulated to produce a multipurpose replacement beverage which could satisfy diverse requirements of troops working in hot climates. Several uses of these beverages include replacement of electrolytes and fluid while in Mission Oriented Protective Posture IV, fluid replacement for mild heat casualties, and oral rehydration therapy for treatment of diarrheal losses.

Patterns of human drinking: effects of exercise, water temperature, and food consumption.

The effects of exercise, water temperature, and food consumption on patterns of ad libitum drinking were studied in 33 men during 6 consecutive cycles of 30 min walking (4.8 km.h-1, 5% grade) and 30 min rest in a climatic chamber (40 degrees C, 40% relative humidity). On two nonconsecutive days, subjects consumed 15 degrees C (cool) water during one trial and 40 degrees C (warm) water during the other. We previously reported that two groups of drinkers can be identified during work in the heat by the criterion of body weight (BW) loss during the trial. Thus, avid drinkers (D) drank sufficiently such that they lost less than 2% of their initial BW when consuming cool water and libitum, while reluctant drinkers (RD) lost more than 2% of their BW. When warm water was provided, fluid consumption was reduced by 29% and 54% in D and RD, respectively and BW deficits were comparably increased. Intake of cool water elicited cyclic drinking patterns with higher rates during walking than during rest periods in both D and RD, whereas consumption of warm water produced this cyclic pattern only after food ingestion during the third rest. Food consumption stimulated fluid intake and reduced BW losses in both trials. Compared to pre-prandial (hours 1-2.5) rates, average post-lunch drinking rates during the last 3 h increased 14% in D and 19% in RD when consuming cool water, and by 46% and 74%, respectively, with warm water. While food consumption has been encouraged to replace electrolytes lost in sweat when working in hot climates, our results indicate that food ingestion also enhances fluid consumption.

Plasma renin activity and aldosterone: correlations with moderate hypohydration.

Adult male test subjects (n = 16) were assigned to one of three clothing configurations (Army fatigues, fatigues with impermeable chemical protective garments, and fatigues with protective garments plus protective masks) prior to exercise (level treadmill, 1.11 m/s, 50 min/h, 6 h) in a moderate (wet bulb globe temperature, WBGT = 23 degrees C) environment with ad lib water consumption. When protective masks were worn, two through-mask drinking systems were evaluated: the current gravity-fed system for fluid delivery and a new system utilizing a small hydraulic pump (Fist-Flex). Antecubital blood samples were taken prior to the start of and subsequent to the completion of exercise and analyzed for fluid-electrolyte regulatory hormones. During all trials with chemical protective garments, plasma renin activity (PRA) and aldosterone levels (PA) were significantly (p less than 0.05) elevated following the exercise protocol while neither was affected during exercise in fatigues only. Individual hypohydration levels during all trials ranged from low (0.84%) to moderate (4.04%). Levels of PRA were closely correlated (r = 0.635, t = 4.35, p less than 0.001) with hypohydration as measured by percentage of body weight lost during the 6 h trial. Likewise, PA was also correlated (r = 0.47, t = 2.81, p less than 0.01) with body weight loss. We concluded from this study that PRA and PA responses were exacerbated in moderate environments by the additional heat stress, sweat rate, and dehydration caused by the impermeable garments. Further, the logistical difficulty inherent in delivering fluid through the chemical protective mask reduced voluntary consumption, increased hypohydration, and elicited the greatest elevations in PRA and PA.(ABSTRACT TRUNCATED AT 250 WORDS)

Variability in intake and dehydration in young men during a simulated desert walk.

Voluntary dehydration was examined in young unacclimatized men walking under simulated desert conditions. Thirty-three subjects (20-33 years) walked on a treadmill (4.82 km.h-1, 5% grade) for 30 min.h-1 for 6 h in a hot environment (40 degrees C db/26 degrees C wb, 4.02 km.h-1 windspeed). Cool (15 degrees C) water was provided ad libitum in canteens. Because thirst is stimulated and drinking should occur at about 2% body weight loss as body water, we used this criterion to identify two groups of individuals. Individuals who maintained body weight (BW) loss at less than 2% (0.44-1.88%) were defined as avid drinkers (D, n = 20) and those who exceeded the 2% BW loss (2.07-3.51%) despite the continual availability of cool (15 degrees C) water were called reluctant drinkers (RD, n = 13). RD consumed 31% less water (2.05 +/- 0.14 L) than D (2.98 +/- 0.12 L), and this resulted in a significantly greater BW loss in RD (2.65 +/- 0.11%) than D (1.16 +/- 0.11%). However, the only statistically significant differences in plasma indices of dehydration were the higher final plasma Na+ and protein levels in RD. Rectal temperature was higher in the RD, whereas final heart rates were unaffected. In the current study, about 40% of the young adult male subjects were reluctant to drink, and thus voluntarily dehydrated even when given cool water ad libitum during intermittent exercise in the heat. The reduced intake of these reluctant drinkers may be critical in predisposing them to increased risk of dehydration and heat injury.

Effects of water temperature and flavoring on voluntary dehydration in men.

Effects of water temperature and flavoring on fluid consumption and body weight losses were studied in fourteen unacclimatized men (21-33 years) during 6 hr of treadmill exercise (4.8 km.hr-1, 5% grade for 30 min.hr-1) in a hot environment. Subjects consumed each of four beverages (15 degrees C water, 40 degrees C water, 15 degrees C flavored water, and 40 degrees C flavored water) on four nonconsecutive days. We identified two groups of individuals by body weight (BW) loss during the cool water trial: drinkers (D) who lost less than 2% initial BW (0.80 +/- 0.15%) and reluctant drinkers (RD) who lost more than 2% (2.53 +/- 0.12%). Although sweat losses were not different between the two groups, D consumed 31% more cool water than RD and experienced 68% less BW loss. Compared to the warm water trial, 6 hr consumption of cool water was significantly increased in both D (59%) and RD (141%) and BW loss was dramatically reduced in both groups. Flavoring significantly enhanced warm water consumption and reduced BW loss in RD only. Reduced consumption of warm water increased rectal temperature, heart rate and plasma osmolality in both groups. The results of this study indicate that either flavoring or cooling warm water will enhance fluid intake and reduce body weight deficits in men reluctant to drink.

Validation of a modified one-step rebreathing technique for measuring exercise cardiac output.

A modification of the Farhi one-step rebreathing technique (1) is described for determining submaximal exercise cardiac output (Q). Factors critical in the estimation of Q are initial rebreathing bag volume and constant bag volume during the maneuver. By substituting a high flow rate analyzer (500 ml.min-1) for the recommended low flow rate mass spectrometer (60 ml.min-1), adding a recirculation circuit from the outlet of the analyzer to an inlet at the base of the rebreathing bag, and reducing the length of sample tubing to the analyzer, we were able to recirculate the subject's expired gas and achieve no loss of bag volume. No statistically significant differences in estimate of cardiac output were noted between the mass spectrometer and LB-2 analyzer with recirculation circuit during submaximal cycling. Heart rate and oxygen uptake were highly correlated with cardiac output and agreed well with the literature, irrespective of the CO2 analyzer system used. A unique feature of our method is that the subject's tidal volume is measured prior to the maneuver and then used as the initial rebreathing bag volume. Varying the bag volume by +/- 0.2 L from the tidal volume had no significant effect on the estimate of cardiac output during exercise. Now quick, reliable, and noninvasive measurements of cardiac output are feasible in subjects--not only in the laboratory but also in the field where a mass spectrometer is not readily portable.

Carotid bifurcation sympathectomy and deafferentation: respiration during hypercapnia.

The respiratory effects of hypercapnia were studied in six awake cats 1) after bilateral sympathectomy of the carotid bifurcations and 2) after bilateral section of the carotid sinus nerves. When cats breathed either 2 or 4% CO2 in air, neither denervation affected the absolute level of ventilation, the percent change in ventilation, or the range of breath-to-breath variability in ventilation (V). However, in all six cats tidal volume (VT) increased for some levels of breath V after sympathectomy of the carotid bifurcations during inhalation of 4% CO2 in air. Moreover, after the subsequent carotid deafferentation, increased VT during fractional concentration of inspired CO2 (FICO2) of 4% persisted in four of six cats. Thus increased VT after sympathectomy could not be attributed to increased carotid chemoreceptor afferent activity but may have been due to reduced baroreceptor activity. On the other hand, sympathectomy-induced differences in breath timing, present during inhalation of 2% CO2, were reversed to intact values after sinus nerve section. In contrast to 2% CO2, changes in respiratory timing in intact cats associated with 4% CO2 were not altered significantly by sympathectomy or deafferentation of the carotid bifurcations. The latter indicates that above a critical FICO2 central mechanisms, unrelated to the carotid bifurcation, dominated respiratory timing in the hypercapnic awake cats.

Heat intolerance, heat exhaustion monitored: a case report.

A 32-year-old male (Mr. A.), monitored during an 8-d heat acclimation (HA) investigation, unexpectedly exhibited heat intolerance and heat exhaustion. Thirteen other males completed HA without indications of either heat intolerance or heat exhaustion. Because Mr. A. responded normally to HA on days 1-4, the intervention of an unknown host factor on days 5-8 was suggested. Mr. A.'s heat exhaustion episode (day 8) was apparently forewarned by loss of body weight and increased delta HR, delta Tsk (days 5-8) and delta Tre (days 7-8) during daily 90-min trials. His symptoms indicated classical salt depletion heat exhaustion, but the calculated salt deficit (less than 0.1 g NaCl.kg-1 body weight) was mild. Post-heat exhaustion serum enzyme levels were either normal (ALT, AST) or acutely elevated (CPK). Blood beta-endorphin and cortisol levels were six times and two times greater than control values, respectively. This case report is unique because clinical/physiological measurements and blood analyses were performed before, during, and after heat intolerance and heat exhaustion.

Urinary and hematologic indexes of hypohydration.

As part of a large-scale field feeding system test we were able to collect and study hundreds of aliquots of overnight urine samples obtained immediately prior to a fasting blood sample on days 1, 20, and 44 of the field test. The large number of experimental samples (greater than 650) and concomitant collection of blood and urine aliquots along with data on body weights gave us the opportunity to assess and quantitate the sensitivity of commonly used criteria of hypohydration. Urine aliquots for all test days were initially categorized by specific gravity (SG) greater than or equal to 1.03 (n = 124) or less than 1.03 (n = 540). Creatinine levels were elevated (P less than 0.001) in the concentrated urine samples, but a decreased trend in sodium-to-potassium ratios in these samples failed to achieve statistical significance (P greater than 0.05). However, when individuals with high SG urine were subclassified by a criterion of weight loss greater than 3% from original body weight, then creatinine concentrations were elevated (P = 0.05), whereas sodium-to-potassium ratios were decreased (P = 0.05) when subjects also with high SG but weight loss less than 3% were compared. Because of the moderate altitude (2,000 m) of the field site and the time of sojourn (44 days), there occurred a slight, but significant (P less than 0.001), erythropoietic response. Hematocrit and serum osmolality were not significantly different when examined by the criteria of high or low SG urine and weight loss greater than or less than 3% original body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Respiration in awake cats: sympathectomy and deafferentation of carotid bifurcations.

Respiratory effects of sympathectomy of the carotid bifurcations and, in a subsequent experiment, bilateral carotid sinus nerve section were examined in six awake resting cats. In each intact and denervated state, sequential breaths were analyzed at 10-min intervals up to 80 min. Individual breath frequency (f), tidal volume (VT), and ventilation (V = f X VT) were determined. In individual cats, sympathectomy or deafferentation could cause significant increases or decreases in ventilation or no change. Thus the range of spontaneous variability in breath V as well as minute ventilation (VE), averaged for the group, were not consistently altered in the same direction by either sympathectomy or deafferentation of the carotid bifurcations. Interestingly, in most cats after both sympathectomy (5 of 6) and deafferentation (4 of 6), VT increased and f decreased relative to V. Despite this, after sinus nerve section in two cats arterial PO2 decreased and arterial PCO2 tended to increase relative to VE, suggesting possible effects of deafferentation on ventilation-perfusion balance. Sympathectomy also affected timing such that inspiratory time began to exceed 0.5 of the breath duration at a lower breath f; this effect of sympathectomy was reversed to intact values by subsequent sinus deafferentation. Thus, in eupneic awake cats, sympathetics normally suppress reflex modulation of central timing from carotid chemoreceptors and/or baroreceptors.

Effects of hypercapnia on variability of normal respiratory behavior in awake cats.

Resting quiet awake cats breathing air in a steady state have a range of respiratory behavior, and this encompasses nonpurring and purring (D. B. Jennings and P. C. Szlyk, Can. J. Physiol. Pharmacol. 63: 148-154, 1985). On a given study day, individual cats usually breathed in a limited part of their potential respiratory range. Respiratory pattern, such as average breath frequency (f) and average tidal volume (VT) utilized for a given level of ventilation (V), could be predicted when cats breathed air; as well, inspiratory (TI) and expiratory (TE) times were specific for a given breath f. Inhalation of 2% and 4% CO2 in air caused an average increase in ventilation of 16 and 100%, respectively but breath-to-breath variability of V, f, and VT persisted at each fractional concentration of inspired CO2 (FICO2). The range of different V utilized breath to breath when breathing 2% CO2 overlapped with V during air control studies. Substantial overlap with control V also occurred in three of six cats when breathing 4% CO2. The most consistent effect of progressive hypercapnia was to increase VT and decrease f at a given level of V; increase in V during hypercapnia was accounted for by an increase in mean inspiratory flow (VT/TI). Hypercapnia also caused the fraction of breathing cycle devoted to inspiration (TI/TT) to increase at low f but not at high f.

An atropinized heat-stressed rat model: dose response effects and pharmacokinetics.

Atropine and other anticholinergic drugs are widely used in common medications and in the treatment of organophosphate poisoning. Man dissipates heat by the evaporation of sweat. Analogously, rats spread saliva over their bodies for evaporative cooling. Atropine inhibits both sweating and salivation. Therefore, we sought to quantitate the effects of atropine in our rat heatstroke model. While heat-stressing adult male rats of 500 or 250 g at 41.5 degrees C, we measured the effects of i.v. atropine (10-4000 micrograms X kg-1) on the following: heating rate (HR), % wt loss (saliva production), and fecal loss (intestinal motility). HR (degree C X min-1) was the most sensitive index of drug activity with a 200 micrograms X kg-1 dose (equivalent to 2 mg in man for organophosphate poisoning) eliciting an increased HR from 0.022 degrees C (saline) to 0.087 degrees C X min-1 (atropine). Atropine (200 micrograms X kg-1) increased HR even if administered 3 h prior to heat exposure. Large (500 g) rats showed an increase in HR with 25 micrograms X kg-1 of atropine, but 250 g rats required 50 micrograms X kg-1. This model could be used to assess the relative effects of other anticholinergic drugs and as a non-dehydrated heatstroke model.