Moderate exercise after altitude exposure fails to induce decompression sickness

INTRODUCTION

The objective of this study was to determine the effect of exercise after altitude exposure (post-exposure exercise) on subsequent altitude decompression sickness (DCS) incidence. Existing USAF prohibition of exercise following altitude chamber training exposures and interest from operational personnel prompted our evaluation of post-exposure exercise as a DCS-inducing stressor.

METHODS

After a 1-h resting preoxygenation, 67 subjects were exposed to 30,000 ft for 2-h while performing mild, upper body exercise. The subjects were monitored for venous gas emboli (VGE) with an echo-imaging system and observed for signs and symptoms of DCS. Subjects without DCS (n = 31) or with DCS which resolved during recompression (n = 29) were randomly assigned to post-exposure rest (control, n = 29) or moderate exercise (50% of peak oxygen uptake, dual-cycle ergometry; n = 31) and both groups were monitored for delayed or recurring DCS.

RESULTS

The altitude exposure resulted in 48.3% DCS in the 60 volunteers serving as test or control subjects. Of 31 subjects assigned to the post-exposure exercise group, 15 had developed DCS which resolved during descent. No cases of DCS were observed or reported during or following post-exposure exercise.

CONCLUSION

The results show that moderate exercise after exposure did not result in either delayed-onset or recurring DCS.

The effect of simulated weightlessness on hypobaric decompression sickness

BACKGROUND

A discrepancy exists between the incidence of ground-based decompression sickness (DCS) during simulated extravehicular activity (EVA) at hypobaric space suit pressure (20-40%) and crewmember reports during actual EVA (zero reports). This could be due to the effect of gravity during ground-based DCS studies.

HYPOTHESIS

At EVA suit pressures of 29.6 kPa (4.3 psia), there is no difference in the incidence of hypobaric DCS between a control group and group exposed to simulated weightlessness (supine body position).

METHODS

Male subjects were exposed to a hypobaric pressure of 29.6 kPa (4.3 psi) for up to 4 h. The control group (n = 26) pre-oxygenated for 60 min (first 10 min exercising) before hypobaric exposure and walking around in the altitude chamber. The test group (n = 39) remained supine for a 3 h prior to and during the 60-min pre-oxygenation (also including exercise) and at hypobaric pressure. DCS symptoms and venous gas emboli (VGE) at hypobaric pressure were registered.

RESULTS

DCS occurred in 42% in the control and in 44% in simulated weightlessness group (n.s.). The mean time for DCS to develop was 112 min (SD +/- 61) and 123 min (+/- 67), respectively. VGE occurred in 81% of the control group subjects and in 51% of the simulated weightlessness subjects (p = 0.02), while severe VGE occurred in 58% and 33%, respectively (p = 0.08). VGE started after 113 min (+/- 43) in the control and after 76 min (+/- 64) in the simulated weightlessness group.

CONCLUSIONS

No difference in incidence of DCS was shown between control and simulated weightlessness conditions. VGE occurred more frequently during the control condition with bubble-releasing arm and leg movements.

The effect of repeated altitude exposures on the incidence of decompression sickness

INTRODUCTION

Repeated altitude exposures in a single day occur during special operations parachute training, hypobaric chamber training, unpressurized flight, and extravehicular space activity. Inconsistent and contradictory information exists regarding the risk of decompression sickness (DCS) during such hypobaric exposures.

HYPOTHESIS

We hypothesized that four short exposures to altitude with and without ground intervals would result in a lower incidence of DCS than a single exposure of equal duration.

METHODS

The 32 subjects were exposed to 3 different hypobaric exposures--condition A: 2 h continuous exposure (control); condition B: four 30-min exposures with descent/ascent but no ground interval between the exposures; condition C: four 30-min exposures with descent/ascent and 60 min of ground interval breathing air between exposures. All exposures were to 25,000 ft with 100% oxygen breathing. Subjects were observed for symptoms of DCS, and precordial monitoring of venous gas emboli (VGE) was accomplished with a SONOS 1000 echo-imaging system.

RESULTS

DCS occurred in 19 subjects during A (mean onset 70+/-29 min), 7 subjects in B (60+/-34 min), and 2 subjects in C (40+/-18 min). There was a significant difference in DCS incidence between B and A (p = 0.0015) and C and A (p = 0.0002), but no significant difference between B and C. There were 28 cases of VGE in A (mean onset 30+/-23 min), 21 in B (41+/-35 min), and 21 in C (41+/-32 min) with a significant onset curve difference between B and A and between C and A, but not between B and C. Exposure A resulted in four cases of serious respiratory/neurological symptoms, while B had one and C had none. All symptoms resolved during recompression to ground level.

CONCLUSION

Data indicate that repeated simulated altitude exposures to 25,000 ft significantly reduce DCS and VGE incidence compared with a single continuous altitude exposure.

Preoxygenation time versus decompression sickness incidence

Preoxygenation, breathing 100% oxygen prior to decompression, has been used for well over half of this century to reduce decompression sickness (DCS) incidence. Duration of preoxygenation has been reported to be inversely related to subsequent DCS incidence. A direct comparison of DCS incidence at 30,000 ft versus preoxygenation time is needed to allow better-informed decisions regarding the cost vs. benefit of increasing preoxygenation time to prevent DCS. To obtain such a comparison, we accomplished a retrospective study of exposures to 30,000 ft (226 mm Hg; 4.37 psia) while performing mild exercise. The 86 male exposures were preceded by preoxygenation times of one to four hours. Venous gas emboli (VGE) and DCS symptom development were monitored and recorded. Although more protection was demonstrated with increasing preoxygenation time, the cost-to-benefit ratio also increases with each additional increment of preoxygenation time. The diminishing return of increasing preoxygenation to reduce DCS would eventually impact mission planning and crew duty limitations. Alteration in the physiology of denitrogenation, such as inclusion of exercise during preoxygenation, may provide better and more cost-effective DCS protection than simply increasing preoxygenation time.

The effect of exposure to 35,000 ft on incidence of altitude decompression sickness

INTRODUCTION

Exposure to 35,000 ft without preoxygenation (breathing 100% oxygen prior to decompression) can result in severe decompression sickness (DCS). Exercise while decompressed increases the incidence and severity of symptoms. Clarification of the level of activity vs. time to symptom onset is needed to refine recommendations for current operations requiring 35,000-ft exposures. Currently, the U.S. Air Force limits these operations to 30 min following 75 min of preoxygenation. The objective of this study was to determine the effect of exercise intensity on DCS incidence and severity at 35,000 ft.

METHODS

Following 75 or 90 min of ground-level preoxygenation, 54 male and 38 female subjects were exposed to 35,000 ft for 3 h while performing strenuous exercise, mild exercise, or seated rest. The subjects were monitored for venous gas emboli (VGE) with an echo-imaging system and observed for signs and symptoms of DCS.

RESULTS

Exposures involving strenuous and mild exercise resulted in higher incidence (p < 0.05) and earlier onset of symptoms (p < 0.05) of DCS than exposure at rest. Mild and strenuous exercise during exposure did not differ in incidence or rate of onset. Incidence at 30 min of exposure was 8% at rest and 23% while exercising.

CONCLUSION

The results showed that current guidelines for 35,000-ft exposures keep DCS risk below 10% at rest. Exercise, even at mild levels, greatly increases the incidence and rate of onset of DCS.

Test and evaluation of exercise-enhanced preoxygenation in U-2 operations

BACKGROUND

Preoxygenation to prevent decompression sickness (DCS) during U-2 reconnaissance flights requires considerable time and occasionally does not provide adequate protection. Increasing preoxygenation within a practical period of time provides marginally increased protection and is not always operationally feasible. Including exercise during preoxygenation to increase muscle tissue perfusion, cardiac output, and ventilation can improve the quality of the denitrogenation.

METHODS

A pilot, who reported two cases of DCS during his first 25 U-2 high flights involving cabin altitudes of 29,000-30,000 ft, volunteered to test exercise-enhanced preoxygenation. He performed 10 min of strenuous upper and lower body exercise at the beginning of preoxygenation prior to subsequent high flights without increasing total preoxygenation time.

RESULTS

The exercise was performed at 75% of maximal oxygen uptake based on the estimated maximal oxygen uptake determined during an Air Force aerobic fitness test and heart rate. The pilot's next 36 high flights, using exercise-enhanced preoxygenation, were completed with no reports of DCS.

CONCLUSIONS

This statistically significant operational test reinforced the laboratory studies. Implementation of this procedure for reducing DCS in susceptible U-2 pilots and collecting additional data from the U-2 pilot population is recommended.

The effect of staged decompression while breathing 100% oxygen on altitude decompression sickness

INTRODUCTION

Space Shuttle extravehicular activity (EVA) requires decompression from sea level pressure (14.7 psia) to a 4.3 psia (30,300 ft) pressure suit. The transition currently involves altering the shuttle atmosphere to allow shirt-sleeve denitrogenation to occur during a 12 to 36-h staged decompression (SD) at 10.2 psia (9,800 ft) with an oxygen-enriched breathing gas (26.5% oxygen, 73.5% nitrogen). The denitrogenation provides protection from decompression sickness (DCS) during EVA in a 4.3 psia pressure suit. Our goal was to determine the highest altitude at which SD while breathing 100% oxygen (SD100) could provide effective protection from development of DCS symptoms after further decompression to 29,500 ft (4.5 psia).

METHODS

There were 30 male subjects exposed to at least 6 of 11 conditions in random order on successive months to 29,500 ft for 4 h while performing mild exercise and being monitored for venous gas emboli (VGE) with an echo-imaging system. The subjects received 15 min of ground-level (GL) preoxygenation and an additional 60 or 120 min of SD100 at one of four altitudes between 8,000 ft (10.9 psia) and 18,000 ft (7.3 psia). Control exposures followed a 75- or 135-min ground-level preoxygenation.

RESULTS

During SD100, one case of DCS occurred at 18,000 ft, but not at lower staging altitudes. Higher levels of VGE were observed during SD100 at 18,000 ft than during SD100 at any lower altitude.

CONCLUSION

Staged decompression at 16,000 ft and below results in decompression risk during subsequent decompression to 29,500 ft similar to that following equivalent periods of ground-level preoxygenation.

Exercise-induced altitude decompression sickness

BACKGROUND

It has been known since World War II that exercise at altitude increases incidence of decompression sickness (DCS). However, data on the effects of specific exercise types at altitude are lacking. This research focused on the relative hazards of exercise without motion (isometric, straining) vs. dynamic exercise involving motion. The study also compared arm vs. leg exercise.

METHODS

There were 32 healthy male subjects exposed, while resting, to 29,500 ft (8992 m) for 4 h or until DCS occurred, at which time they were brought to ground level. If the subject developed DCS on this exposure, he was exposed in successive months to lower altitudes, using the same procedure, until the subject was free of symptoms for the 4-h exposure. At this symptom-free altitude, as low as 20,000 ft (6096 m), the subject performed isometric arm, isometric leg, dynamic arm and dynamic leg exercises at less than 10% of maximal oxygen consumption, each during separate exposure months. Precordial venous gas emboli (VGE) were monitored every 20 min during each exposure with a Hewlett-Packard SONOS 1000 Echo Imaging System.

RESULTS

Dynamic arm, dynamic leg, isometric arm, and isometric leg exercise induced DCS in 50%, 38%, 41% and 31% of the subjects, respectively. VGE incidence varied from 47-66%. No significant differences in DCS or VGE were found.

CONCLUSIONS

Under our test conditions, there was no difference between dynamic and isometric exercise in eliciting DCS. Exercise during exposure to the symptom-free altitude for 4 h produced a 40% incidence DCS.

An abrupt zero-preoxygenation altitude threshold for decompression sickness symptoms

INTRODUCTION

The altitude threshold for decompression sickness (DCS) symptoms has been variously described as being 18,000 ft (5,487 m) to above 25,000 ft (7,620 m). Safety and efficiency of aerospace operations require more precise determination of the DCS threshold.

METHODS

Subjects were 124 males who were exposed to simulated altitudes (11 at 11,500 ft; 10 at 15,000 ft; 8 at 16,500 ft; 10 at 18,100 ft; 10 at 19,800 ft; 20 at 21,200 ft; 20 at 22,500 ft; 10 at 23,800 ft, and 25 at 25,000 ft) for 4 to 8 h. All breathed 100% oxygen beginning with ascent. Subjects were monitored for precordial venous gas emboli (VGE) and DCS symptoms. Probit curves representing altitude vs. incidence of DCS symptoms and VGE allowed estimation of respective risk.

RESULTS

VGE were first observed at 15,000 ft with increasing incidence at higher altitudes; over 50% at 21,200 ft and 70% or higher at 22,500 ft and above. The lowest altitude occurrence of DCS was a 5% incidence at 21,200 ft. At 22,500 ft, the DCS incidence abruptly climbed to 55%.

CONCLUSION

A 5% threshold for DCS symptoms was concluded to be 20,500 ft under the conditions of this study. The abrupt increase in DCS symptoms, with zero-preoxygenation exposure above 21,200 ft implies a need for reconsideration of current USAF and FAA altitude exposure guidance.

A new preoxygenation procedure for extravehicular activity (EVA)

A 10.2 psi staged-decompression schedule or a 4-hour preoxygenation at 14.7 psi is required prior to extravehicular activity (EVA) to reduce decompression sickness (DCS) risk. Results of recent research at the Air Force Research Laboratory (AFRL) showed that a 1-hour resting preoxygenation followed by a 4-hour, 4.3 psi exposure resulted in 77% DCS risk (N=26), while the same profile beginning with 10 min of exercise at 75% of VO2peak during preoxygenation reduced the DCS risk to 42% (P<.03; N=26). A 4-hour preoxygenation without exercise followed by the 4.3 psi exposure resulted in 47% DCS risk (N=30). The 1-hour preoxygenation with exercise and the 4-hour preoxygenation without exercise results were not significantly different. Elimination of either 3 hours of preoxygenation or 12 hours of staged-decompression are compelling reasons to consider incorporation of exercise-enhanced preoxygenation.

Exercise-enhanced preoxygenation increases protection from decompression sickness

INTRODUCTION

Prevention of decompression sickness (DCS) during exposure to altitude equivalents of 30,000 ft (9144 m) requires extensive denitrogenation. In preparation for extravehicular activity (EVA), present NASA policy is to denitrogenate using a 10.2 psia staged decompression of the entire shuttle for at least 12 h, including 100 min of preoxygenation (breathing 100% oxygen at 14.7 psia prior to decompression), before decompression to the 4.3 psia (30,000 ft; 9144 m) suit pressure. This staged decompression provides the same or better protection from DCS as a 3.5- or 4-h preoxygenation used on earlier Shuttle EVA's. For high altitude reconnaissance flights at similar cockpit altitudes, a 1-h preoxygenation is currently required.

METHODS

We have investigated the use of a 1-h and a 15-min preoxygenation period, each beginning with 10 min of dual-cycle ergometry performed at 75% of each subject's peak oxygen consumption (VO2peak) to enhance preoxygenation efficiency by increasing perfusion and ventilation. Male subjects accomplished a 1-h preoxygenation with exercise, a 15-min preoxygenation with exercise, or a 1-h resting preoxygenation before exposure to 4.3 psia for 4 h while performing light to moderate exercise.

RESULTS

Incidence of DCS following the 1-h preoxygenation with exercise (42%; n = 26) was significantly less than that following the 1-h resting preoxygenation (77%; n = 26). Incidence and onset of DCS following the 15-min preoxygenation with exercise (64%; n = 22) was not significantly different from the incidence following the 1-h resting control.

CONCLUSION

Preoxygenation with exercise has been shown to provide significantly improved DCS protection when compared with resting preoxygenation.

Graying of the critical items: effects of aging on responding to MMPI-2 critical items

The relation between age and several critical item sets on the revised Minnesota Multiphasic Personality Inventory (MMPI-2; Butcher, Dahlstrom, Graham, Tellegen, & Kaemmer, 1989) was examined. MMPI-2 protocols from veterans entering a Veterans Affairs domiciliary were obtained, and the number of Grayson critical items, Koss-Butcher critical items. Lachar-Wrobel critical items, and Caldwell critical items were tabulated. Another critical item set consisting of all items of the previously mentioned sets was also tabulated. This composite set was divided into a set that has items scored on Scales 1, 2, and 3, and a second set of items that were not scored on those three scales. The effect of age on Scales L and K was also studied. A one-way analysis of variance confirmed that the number of endorsed critical items was significantly less in older age groups, and L and K increased. The implication is that endorsement of a critical item may have increased clinical significance as a person ages.

The effect of head and body position on +Gz acceleration tolerance

It has been suggested there is a relationship between acceleration-induced loss of consciousness (G-LOC) and head/body position. A two-part investigation was conducted to determine whether head and body position affects acceleration tolerance. A retrospective analysis of high-G training data (N = 1,914) compared G-LOC occurrence during straight-ahead exposure to a "check-6" exposure [10 s at +9 Gz; 6 G/s onset rate; G-suit inflated; anti-G straining maneuver (AGSM) performed]. A prospective study (N = 12) was conducted with acceleration exposures using light loss criteria with subjects in straight-ahead, above, over-the-right shoulder, or over-the-left shoulder positions. Profiles consisted of 0.1 G/s onset-rate runs (no G-suit inflation; relaxed) to a maximum of +9 Gz and 0.5 G/s onset-rate runs (G-suit inflated; AGSM performed) to +9 Gz for up to 26 s. In the retrospective study, no significant difference existed between G-LOC occurrence during straight-ahead (22/1914) and check-6 (32/1914) positions. During the prospective study with AGSM runs, there was no significant difference in the time at maximum G among any of the positions. During the relaxed runs, several comparisons yielded significant differences in peak G attained. These results indicate there may be an underlying physiologic effect of head and body position on acceleration tolerance; however, the AGSM and the G-suit overcame this effect. Although task saturation and distraction may compromise performance of the AGSM and subsequently predispose acceleration-related hazards, a proper AGSM, combined with effective protective systems, remains essential components of a protection strategy.

Prevention of decompression sickness in current and future fighter aircraft

United States Air Force oxygen regulators set to "NORMAL OXYGEN" deliver up to 60% nitrogen to the pilot at cockpit altitudes of 15,000 to 20,000 ft (4573-6096 m). Research chamber exposure to these altitudes while breathing 50% nitrogen has resulted in high grades of venous gas emboli. Expansion of existing gas emboli following an unplanned decompression to ambient aircraft altitude (e.g., loss of canopy) could result in rapid development of decompression sickness (DCS) symptoms. To reduce this potential problem, regulators in current fighters should be set to "100% OXYGEN" until descent from cruise to increase denitrogenation. The United States' Advanced Tactical Fighter and the European Fighter Aircraft may be designed to cruise above 50,000 ft (15,240 m), where cockpit altitudes exceed 20,000 ft with a 5-psi differential (psid) cockpit pressurization schedule. Increasing cockpit differential pressure to 7 psid while breathing 100% oxygen would greatly reduce the chance of significant emboli formation and the potential for DCS, but would slightly elevate the risks associated with pulmonary overpressure during rapid decompression.

Breathing 100% oxygen compared with 50% oxygen: 50% nitrogen reduces altitude-induced venous gas emboli

The risk of venous gas emboli (VGE) and decompression sickness (DCS) must be determined before selection of the lowest pressure for an extravehicular activity (EVA) pressure suit which eliminates the requirement for prebreathing. In earlier studies, use of a 50% oxygen:50% nitrogen breathing mixture (50:50 mix) during 139 zero-prebreathe decompressions of male subjects to 8.3-7.8 psia resulted in 51 instances of severe VGE and one case of DCS. Our current study investigated effects of 40 zero-prebreathe decompressions of male subjects to 8.3-6.8 psia for 6 h while breathing 100% oxygen and performing moderate exercise. No DCS symptoms were observed. Severe VGE were not detected at 8.3 psia, but were present during 10%, 20%, and 40% of the exposures at 7.8, 7.3, and 6.8 psia, respectively. Zero-prebreathe decompression while breathing 100% oxygen results in significantly lower VGE and DCS risk levels than while breathing a 50:50 mix. Our results show that 7.3 psia EVA pressure suits with 100% oxygen should be safer than 8.3 psia suits with a 50:50 mix.

Unpredictability of fighter pilot G tolerance using anthropometric and physiologic variables

Correlation and regression analyses were used to study relationships between centrifuge G tolerances of 1,434 fighter pilots during High-G Training (HGT) and anthropometric and physiologic variables. Multiple regression analyses yielded a four-variable model in which gradual onset run (GOR) relaxed-G tolerance was inversely correlated with height and directly correlated with age, weight, and diastolic blood pressure. Although the four-variable model was able to predict more of the variation in G tolerance than any single variable, neither method showed a correlation (r) of greater than 0.35 with GOR relaxed or straining G tolerance. No subject variable was significantly different between the pilot groups that did and did not experience G-induced loss of consciousness. We conclude that prediction of G tolerance during centrifuge HGT is unreliable using anthropometric and physiologic variables. The anti-G straining maneuver remains the major determinant of an individual's G tolerance.

Human tolerance to 100% oxygen at 9.5 psia during five daily simulated 8-hour EVA exposures

Extravehicular activity (EVA) currently involves decompression to 4.3 psia. This degree of decompression carries a significant potential for decompression sickness (DCS) which could be alleviated if a pressure of 9.5 psia could be maintained in the pressure suit. Previous studies have not evaluated the potential for oxygen toxicity at 9.5 psia. Twenty-one subjects were exposed to 100% oxygen at 9.5 psia for 5 consecutive days, 8 h.d-1 while performing moderate exercise to simulate a typical work-week in the proposed pressure suit environment. No DCS or venous gas bubbles were detected. Pulmonary function tests, physical exams, blood analyses, arterial oxygen saturation monitoring, and X-rays showed no evidence of oxygen toxicity under these conditions. These results suggest that a 100% oxygen, 9.5 psia pressure suit environment could avoid both DCS and oxygen toxicity during EVAs of comparable duration and physical activity.

Blood factors and venous gas emboli: surface to 429 mmHg (8.3 psi)

Analyses of 43 parameters were performed on blood obtained from 30 volunteer subjects before and after a 6-h chamber decompression from the surface to 429 mmHg. Eight subjects (5 male, 3 female) were bubble-prone (bubble grades 3 and 4), and 22 (15 male, 7 female) were resistant (bubble grade 0) to forming bubbles as detected with precordial Doppler. Significant (P less than 0.05) differences include the following: higher levels of cholesterol in the bubble-prone males and combined subjects (males and females) than in their resistant counterparts; higher magnesium in the bubble-prone males; shorter preexposure prothrombin time in bubble-prone males and combined subjects; increased partial thromboplastin time in bubble-prone females vs. the resistant females, who showed a decrease during exposure; higher preexposure hemoglobin, hematocrit, and red blood cell count in the bubble-prone females; and significant reduction in hemoglobin, red blood cell count, and serum osmolality in the bubble-prone females during the exposure relative to changes in the resistant females. In this study, high cholesterol and hemoconcentration seem to be characteristics of bubble-prone subjects.

Glutamine synthetase activity in subdivisions of brain of the shark, Squalus acanthias

Specific activity of glutamine synthetase in Squalus acanthias (spiny dogfish) central nervous system regions was highest in the cerebellum and lowest in the spinal cord. The levels of activity may relate to the excitability of each region by regulating the glutamate pool.

Glutamine synthetase: assimilatory role in liver as related to urea retention in marine chondrichthyes

The levels of gluatmine synthetase specific activity in hepatic and renal tissue are higher in fish that are ureosmoregulators than in those that are not. Enzyme activities in the liver and kidney of 18 species of fish correlated directly with the ureosmoregulatory adaptation of each species.