Heat transfer from the human body in hyperthermic, low gas-velocity environments.

We studied the physics of heat transfer from the human body under conditions of low air velocity (LAV) (less than 0.1 m.s-1). For 1 ATA, we calculated values of the evaporative heat transfer coefficient (he, kcal.m-2.h-1.mm Hg-1) of 5.24 and 5.37 where ambient temperature/relative humidities were 38 degrees C/70% and 35 degrees C/80%, respectively, and ambient velocities of 0.06 m.s-1 in both cases. This is near our newly conceived "minimum he value" of 4.94, significantly greater than the 2.8 and 3.4 values that are calculated using the Lewis relation. We further described an "iso-vapor pressure temperature difference," delta Tp, which transcribes a water vapor pressure difference into a temperature difference, thus allowing use of classical nondimensional analysis to account for the increasing importance of heat loss through evaporation in LAV. Water vapor pressure differences in LAV hypobaric environments will account for increased heat losses since, for example, the minimum value of he doubles from 4.94 to 9.88 at 0.5 ATA.

Distortion of calculated whole-body hematocrit during lower-body immersion in water.

We found a difference between the venous hematocrits of immersed and nonimmersed arms during immersion of the lower body in cold water but not during a comparable exposure to warm water. Fourteen healthy men were exposed to three different experimental conditions: arm immersion, body immersion, and control. The men always sat upright while both upper extremities hung vertically at their sides. During arm immersion, one forearm was completely immersed for 30 min in either cold water (28 degrees C, n = 7) or warm water (38 degrees C, n = 7). This cold-warm water protocol was repeated on separate days for exposure to the remaining conditions of body immersion (immersion of 1 forearm and all tissues below the xiphoid process) and control (no immersion). Blood samples were simultaneously drawn from cannulated veins in both antecubital fossae. Hematocrit difference (Hct diff) was measured by subtracting the nonimmersed forearm's hematocrit (Hct dry) from the immersed forearm's hematocrit (Hct wet). Hct diff was approximately zero when the men were exposed to the control condition and body immersion in warm water. In the remaining conditions, Hct wet dropped below Hct dry (P less than 0.01, 3-way analysis of variance). The decrements of Hct diff showed there were differences between venous hematocrits in immersed and nonimmersed regions of the body, indicating that changes of the whole-body hematocrit cannot be calculated from a large-vessel hematocrit soon after immersing the lower body in cold water.

Direct ascent from shallow air saturation exposures.

Thirty-four healthy human subjects were exposed to shallow air saturation for 48 h [1.77 ATA (25.5 fsw) n = 19, 1.89 ATA (29.5 fsw) n = 15] and then decompressed to 1 ATA (0 fsw) in about 2 min. Symptoms included fatigue, limb and joint pain, headache, myalgias, and pruritus. No subject of 19 was diagnosed as having decompression sickness (DCS) after the shallower exposure, but 4 of 15 were diagnosed and treated for DCS subsequent to the deeper exposure. Almost all subjects in both groups had Doppler-detectable venous gas emboli (VGE) lasting up to 12 h postdecompression. Treated subjects had a recurrence of VGE several hours after the hyperbaric oxygen treatment. Only the duration of VGE, and not the VGE score, correlated with symptoms; and only the subjects body weight and age correlated with the VGE variables. This study indicates that hyperbaric air exposures of this magnitude are not as benign as previously thought.

Cardiorespiratory fitness and cognitive performance before and after confinement in a nuclear submarine.

The aim of this study was to further document physiological deconditioning from occupational exposure to submarines as described in a small number of reports and determine whether cognitive performance parallels the physiological changes associated with physical training and deconditioning. We examined cardiorespiratory fitness and cognitive performance in 14 male subjects during 70 d of confinement in a nuclear submarine. Maximal oxygen consumption (VO2max) and anaerobic threshold (AT) were assessed before and after confinement. Six exercising subjects (ES) cycled 4-7 times per week for 20 min at 75% max heart rate for 8 weeks. Eight control subjects (CS) did no exercise. Every 14 d of the patrol, cognitive performance was evaluated in both groups by administering a mental arithmetic and choice reaction time tests before cycling, during cycling, and post cycling. The cycle bout consisted of exercising at 75% VO2max for 15 min. After confinement, VO2max remained constant for ES but declined 7% statistically nonsignificant for the CS. AT expressed as a percentage of VO2max increased 15% (p less than 0.05) in the ES and decreased 20% (p less than 0.05) in the CS. The only significant effect in the cognitive tests was that both groups responded faster in the choice reaction time test during the exercise session. In conclusion, these data demonstrate a training effect for AT in the ES and a deconditioning response for AT and a statistically nonsignificant reduction in VO2max in the CS. Under the conditions of this experiment we could find no effects of physical training and deconditioning on the cognitive performance test employed here, although some trends suggest that exercisers out-performed the control group.

Effect of a 68-day submarine patrol on serum 25-hydroxyvitamin D levels in healthy men.

Serum 25-OH-D levels were determined for 30 submariners prior to, and near the end (day 63), of a patrol lasting 68 d. A significant reduction occurred in serum 25-OH-D levels during the patrol compared to levels obtained before the patrol. Nearly one-third of the sample has serum 25-OH-D levels that were below the minimum normal levels. The late-patrol depletion of serum 25-OH-D was not associated with lower levels of calcium. On the contrary, serum calcium levels were found to be significantly elevated during the late-patrol period compared to pre-patrol levels, even among those submariners who had serum 25-OH-D levels in the low normal or below normal range. This calcium effect may be mediated by the high levels of carbon dioxide maintained in the submarine breathing atmosphere.

Carboxyhemoglobin levels during a submarine patrol.

One of the major contaminants from tobacco smoking aboard a nuclear submarine is carbon monoxide (CO). While this gas is controlled to 15 parts per million or lower by catalytic burners, there still remains a residual low level in the atmosphere. This study has shown that, on one submarine, the average ambient concentration was 7 ppm, which produced an average carboxyhemoglobin level in 15 nonsmokers of 2.1%, 1.7%, and 1.7% at the start, middle, and end of a 40-d patrol. Because submariners are generally healthy and young, it is concluded that CO exposures at these ambient levels do not constitute a major risk factor to the physiological well-being of these submariners, nor are they expected to cause any decrement in performance.

Joint crepitation after chamber air dives to 188 fsw.

Joint crepitation after chamber air dives to 188 fsw. Undersea Biomed. Res. 4(1):89-94.-Joint crepitation was observed in all subjects (8) who were decompressed from 188 fsw (gauge) dry chamber air dives with a bottom time of 45 min. Crepitation was elicited by bending a joint as far as comfortably possible, and intensity of the sound was rated on a scale of 0 to +4. Fingers, wrists, and shoulders of both arms were examined immediately postdive, 1-4, 6- 10, and 20-24 hours postdive. Typically, the intensity and/or the number of joints affected increased during the 1-4 hour postdive period and then disappeared over the next 6-24 hours. The typical "squishy" sound heard in these joints may be due to bubble formation, although radiographic evidence was inconclusive. Closer examination for crepitance in future dives and the reporting of its occurrence may help to elucidate the mechanisms responsible for these joint sounds.

Closing volumes in man immersed to the neck in water.

Closing volumes (CV), along with residual volume (RV), vital capacity (VC), along with residual volume (RV), vital capacity (VC), and expiratory reserve volume (ERV) were determined in 10 subjects in the dry and while immersed to the neck in water. Closing volumes during immersion increased 41.3% (P less than 0.001) over dry values while RV decreased 9.35% (P less than 0.001) and VC decreased 9.94% (P less than 0.001). The large decrease of 71.3% (P less than 0.001) in ERV resulted in the impingement of closing capacity (CV + RV) on the tidal volume in 9 out of 10 subjects. We interpret this to mean that airway closure occurs during tidal ventilation in immersed subjects and may result in impaired gas exchange. When tourniquets were applied to all four limbs during immersion closing volumes increased only 32.1%, but increased to 64.3% when they were removed. If engorgement of peribronchial vessels predisposes airways to collapse, a reduction of plasma volume during an extended period of immersion might lessen this possibility. In a series of long term (2.5-h) immersion experiments where moderate reductions (-10 to -7%y in plasma volume were observed, we found, however, no correlative changes in closing volume.