INTRODUCTION: The frequency of long-duration, high-altitude missions with fighter aircraft is increasing, which may increase the incidence of decompression sickness (DCS).The aim of the present study was to compare decompression stress during simulated sustained high-altitude flying vs. high-altitude flying interrupted by periods of moderate or marked cabin pressure increase. METHODS: The level of venous gas emboli (VGE) was assessed from cardiac ultrasound images using the 5-degree Eftedal-Brubakk scale. Nitrogen washout/uptake was measured using a closed circuit rebreather. Eight men were investigated in three conditions: one 80-min continuous exposure to a simulated cabin altitude of A) 24,000 ft, or four 20-min exposures to 24,000 ft interspersed by three 20-min intervals at 8) 20,000 ft or C) 900 ft. RESULTS: A and B induced marked and persistent VGE, With peak bubble scores of [median (range)]: A 2.5 (1-3); B: 3.5 (2-4). Peak VGE score was less in C [1.0(1-2),P < 0.01]. Condition A exhibitedan initially high and exponentially decaying rate of nitrogen washout. In C the washout rate was similar in each period at 24,000 ft, and the nitrogen uptake rate was similar during each 900-ft exposure. B exhibited nitrogen washout during each period at 24,000 ft and the initial period at 20,000 ft, but on average no washout or uptake during the last period at 20,000 ft. DISCUSSION: Intermittent reductions of cabin altitude from 24,000 to 20,000 ft do not appear to alleviate the DCS risk, presumably because the pressure increase is not sufficient to eliminate VGE. The nitrogen washout/uptake rate did not reflect DCS risk in the present exposures.

INTRODUCTION: The evolution and preservation of venous gas emboli (VGE), as markers of decompression stress, were investigated during alternating high- and moderate altitude exposures, thus, simulating a fighter aircraft high-altitude flight, interrupted by refueling excursions to lower altitudes. METHODS: Eight men served as subjects during three normoxic simulated altitude exposures: High = 90 min at 24,000 ft; High-Low = three x 30 min at 24,000 ft, interspersed by two 30-min intervals at 15,000 ft; Low = 90 min at 15,000 ft. VGE scores were assessed by cardiac ultrasound, using a 5-grade scale. Respiratory nitrogen exchange was measured continuously using a modified closed-circuit electronic rebreather. RESULTS: Both High and High-Low induced persistent VGE, with no inter-condition difference either at rest [median (range): High: 1 (0-3), High-Low: 2 (0-3)] or during unloaded knee-bends [High: 3 (1-4), High-Low: 3 (0-4)], whereas VGE was considerably less in Low, both at rest [0 (0-1)] and during knee-bends [0 (0-2)]. In High-Low, VGE decreased temporarily during the 15,000-ft excursions, but resumed pre-excursion values upon return to 24,000 ft. During the final descent to ground level, VGE were more persistent following High-Low than High. In both High and Low, nitrogen was continuously washed out at altitude, whereas in High-Low, the washout at 24,000 ft was interrupted by nitrogen uptake at 15,000 ft. DISCUSSION: In normoxic conditions, long-duration flying at a cabin altitude of 24,000 ft is associated with substantial VGE occurrence, which is not abolished by intermittent excursions to a cabin altitude of 15,000 ft.

INTRODUCTION: In fighter aircraft, long-duration high-altitude sorties are typically interrupted by refueling excursions to lower altitude. In normoxia, excursions to moderate cabin altitude may increase the occurrence of venous gas emboli (VGE) at high cabin altitude. The aim was to investigate the effect of hyperoxia on VGE and decompression sickness (DCS) during alternating high and moderate altitude exposure.

METHODS: In an altitude chamber, 13 healthy men were exposed to three different conditions: A) 90 min at 24,000 ft (7315 m) breathing normoxic gas (54% O-2; H-NOR); B) 90 min at 24,000 ft breathing hyperoxic gas (90% O-2; H-HYP); and C) three 30-min exposures to 24,000 ft interspersed by two 30-min exposures to 18,000 ft (5486 m) breathing 90% O-2 (ALT-HYP). VGE occurrence was evaluated from cardiac ultrasound imaging. DCS symptoms were rated using a scale.

RESULTS: DCS occurred in all conditions and altogether in 6 of the 39 exposures. The prevalence of VGE was similar in H-NOR and H-HYP throughout the exposures. During the initial 30 min at 24,000 ft, the prevalence of VGE was similar in ALT-HYP as in the other two conditions, whereas, after the first excursion to 18,000 ft, the VGE score was lower in ALT-HYP than in H-NOR and H-HYP.

DISCUSSION: Hyperoxic excursions from 24,000 to 18,000 ft reduces VGE occurrence, presumably by facilitating diffusive gas exchange across the bubble surfaces, increasing the share of bubble content contributed by oxygen. Still, the excursions did not abolish the DCS risk.

Recent observations suggest that development of venous gas emboli (VGE) during high-altitude flying whilst breathing hyperoxic gas will be reduced by intermittent excursions to moderate altitude. The present study aimed to investigate if an early, single excursion from high to moderate altitude can be used as an in-flight means to reduce high-altitude decompression strain. Ten healthy men were investigated whilst breathing oxygen in a hypobaric chamber under two conditions, once during a 90-min continuous exposure to a simulated cabin altitude of 24000 ft (High; H) and once during 10 min at 24000 ft, followed by 30 min at 15000 ft and by 80 min at 24000 ft (High-Low-High; H-L-H). VGE scores were assessed by cardiac ultrasound, using a 6-graded scale. In H, VGE increased throughout the course of the sojourn at 24000 ft to attain peak value (median (range)) of 3 (2-4) at min 90, just prior to descent. In H-L-H, median VGE scores were 0 throughout the trial, except for at min 10, just prior to the excursion to 15000 ft, whence the VGE score was 1.5 (0-3). Thus, an early, single excursion from high to moderate cabin altitude holds promise as an in-flight means to reduce the risk of altitude decompression sickness during long-duration high-altitude flying in aircraft with limited cabin pressurization. Presumably, such excursion acts by facilitating the gas exchange in decompression bubbles from a predomination of nitrogen to that of oxygen.

Key words: altitude decompression sickness, decompression sickness risk, fighter aircraft, gas bubble formation, repeated altitude decompression, VGE.

INTRODUCTION: Hypobaric decompression sickness remains a problem during high-altitude aviation. The prevalence of venous gas emboli (VGE) serves as a marker of decompression stress and has been used as a method in evaluating the safety/risk associated with aviation profiles and/or gas mixtures. However, information is lacking concerning the variability of VGE formation when exposed to the same hypobaric profile at different occasions. In this paper intra-individual test-retest variation regarding bubble formation during repeated hypobaric exposures is presented. The data can be used to determine the sample size needed for statistical power. 

METHOD: Nineteen male, non-smoking subjects volunteered for altitude exposures to 24,000 ft. VGE was measured using ultrasound scanning and scored according to the Eftedal-Brubakk (EB) scale. Intra-individual test-retest variation in bubble formation (maximum VGE) was evaluated in subjects exposed more than once to hypobaric pressure. The statistical reliability was examined between paired exposures using the Intraclass Correlation test. G*Power, version 3.1.9.6 was used for power calculations.

RESULTS: During repeated 20-30 and 70 min exposures to 24,000 ft 42% (n = 19, CI 23%-67%) respectively 29% (n = 7, CI 5%-70%) of the subjects varied between maximum EB scores <3 and ≥3. The sample size needed to detect 1 EB step between two paired exposures varied between 29-51 subjects.

CONCLUSION. The large intra-individual test-retest variations in bubble grades during repeated hypobaric exposures, highlight the need for relatively large numbers of subjects to reach statistical power for when there are no or small differences in decompression stress between the exposures.