Nitrogen elimination from the tissues during oxygen breathing and its relationship to the fat: muscle ratio and the localization of bends

Decompression strain in parachute jumpmasters during simulated high-altitude missions: a special reference to preoxygenation strategies

PURPOSE

Military parachute operations are often executed at high altitude, from an unpressurized aircraft compartment. Parachute jumpmasters (JM) are thus regularly exposed to 29,500 ft for 60 min. The aim was to investigate the decompression strain during a simulated JM mission at high altitude and to compare two strategies of preoxygenation, conducted either at sea-level or below 10,000 ft, during ascent to mission altitude.

METHODS

Ten JM completed, on separate occasions, a 45-min preoxygenation either at sea-level (normobaric: N) or 8200ft (hypobaric: H), followed by exposure to 28,000 ft for 60 min, whilst laying supine and breathing 100% oxygen. At min 45 of the exposure to 28,000 ft, the JM performed 10 weighted squats. Decompression strain was determined from ultrasound assessment of venous gas emboli (VGE) during supine rest (5-min intervals), after three unloaded knee-bends (15-min intervals) and immediately following the weighted squats. The VGE were scored using a six-graded scale (0-5).

RESULTS

In condition H, two JM experienced decompression sickness (DCS), whereas no DCS incidents were reported in condition N. The prevalence of VGE was higher in the H than the N condition, at rest [median(range), 3(0-4) vs 0(0-3); p = 0.017], after unloaded knee-bends [3(0-4) vs 0(0-3); p = 0.014] and after the 10 weighted squats [3(0-4) vs 0(0-3); p = 0.014]. VGE were detected earlier in the H (28 ± 20 min, p = 0.018) than the N condition (50 ± 19 min).

CONCLUSIONS

A preoxygenation/altitude procedure commonly used by JM, with a 60-min exposure to 28,000 ft after pre-oxygenation for 45 min at 8200 ft is associated with high risk of DCS. The decompression strain can be reduced by preoxygenating at sea level.

Contemporary N2 and SF6 multiple breath washout in infants and toddlers with cystic fibrosis

INTRODUCTION

Multiple breath washout (MBW) is used for early detection of cystic fibrosis (CF) lung disease, with SF₆ MBW commonly viewed as the reference method. The use of N₂ MBW in infants and toddlers has been questioned for technical and physiological reasons, but a new correction of the N₂  signal has minimized the technical part. The present study aimed to assess the remaining differences and the contributing mechanisms for the differences between SF₆ and N₂ MBW,corrected-such as tidal volume reduction during N₂ washout with pure O₂ .

METHOD

This was a longitudinal multicenter cohort study. SF₆ MBW and N₂ MBW were performed prospectively at three CF centers in the same visits on 154 test occasions across 62 children with CF (mean age: 22.7 months). Offline analysis using identical algorithms to the commercially available program provided outcomes of N_2,original and N_2,corrected for comparison with SF₆ MBW.

RESULTS

Mean functional residual capacity, FRC_N2,corrected was 14.3% lower than FRC_{N2, original} , and 1.0% different from FRC_SF6 . Lung clearance index, LCI_N2,corrected was 25.2% lower than LCI_N2,original , and 7.3% higher than LCI_SF6 . Mean (SD) tidal volume decreased significantly during N₂ MBW_corrected , compared to SF₆ MBW (-13.1 ml [-30.7; 4.6], p < 0.0001, equal to -12.0% [-25.7; 1.73]), but this tidal volume reduction did not correlate to the differences between LCI_N2,corrected and LCI_SF6 . The absolute differences in LCI increased significantly with higher LCI_SF6 (0.63/LCI_SF6 ) and (0.23/LCI_SF6 ), respectively, for N_2,original and N_2,corrected , but the relative differences were stable across disease severity for N_2,corrected , but not for N_2,original .

CONCLUSION

Only minor residual differences between FRC_N2,corrected and FRC_SF6 remained to show that the two methods measure gas volumes very similar in this age range. Small differences in LCI were found. Tidal volume reduction during N₂ MBW did not affect differences. The corrected N₂ MBW can now be used with confidence in young children with CF, although not interchangeably with SF₆ .

High-altitude decompression strain can be reduced by an early excursion to moderate altitude while breathing oxygen

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 24,000 ft (High; H) and once during 10 min at 24,000 ft, followed by 30 min at 15,000 ft and by 80 min at 24,000 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 24,000 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 15,000 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.

Improved agreement between N2 and SF6 multiple-breath washout in healthy infants and toddlers with improved EXHALYZER D sensor performance

Recent studies indicate limited utility of nitrogen multiple-breath washout (N₂MBW) in infancy and advocate for using sulfur hexafluoride (SF₆) MBW in this age-group. Modern N₂MBW systems, such as EXHALYZER D (ECO MEDICS AG, Duernten, Switzerland), use O₂ and CO₂ sensors to calculate N₂ concentrations (in principle, N₂% = 100 - CO₂% - O₂%). High O₂ and CO₂ concentrations have now been shown to significantly suppress signal output from the other sensor, raising apparent N₂ concentrations. We examined whether improved EXHALYZER D N₂ signal, accomplished after thorough examination of this CO₂ and O₂ interaction on gas sensors and its correction, leads to better agreement between N₂MBW and SF₆MBW in healthy infants and toddlers. Within the same session, 52 healthy children aged 1-36 mo [mean = 1.30 (SD = 0.72) yr] completed SF₆MBW and N₂MBW recordings (EXHALYZER D, SPIROWARE version 3.2.1) during supine quiet sleep. SF₆ and N₂ SPIROWARE files were reanalyzed offline with in-house software using identical algorithms as in SPIROWARE with or without application of the new correction factors for N₂MBW provided by ECO MEDICS AG. Applying the improved N₂ signal significantly reduced mean [95% confidence interval (CI)] differences between N₂MBW and SF₆MBW recorded functional residual capacity (FRC) and lung clearance index (LCI): for FRC, from 26.1 (21.0, 31.2) mL, P < 0.0001, to 1.18 (-2.3, 4.5) mL, P = 0.5, and for LCI, from 1.86 (1.68, 2.02), P < 0.001, to 0.44 (0.33, 0.55), P < 0.001. Correction of N₂ signal for CO₂ and O₂ interactions on gas sensors resulted in markedly closer agreement between N₂MBW and SF₆MBW outcomes in healthy infants and toddlers.NEW & NOTEWORTHY Modern nitrogen multiple-breath washout (N₂MBW) systems such as EXHALYZER D use O₂ and CO₂ sensors to calculate N₂ concentrations. New corrections for interactions between high O₂ and CO₂ concentrations on the gas sensors now provide accurate N₂ signals. The correct N₂ signal led to much improved agreement between N₂MBW and sulfur hexafluoride (SF₆) MBW functional residual capacity (FRC) and lung clearance index (LCI) in 52 sleeping healthy infants and toddlers, suggesting a role for N₂MBW in this age-group.

Hyperoxic Effects on Decompression Strain During Alternating High and Moderate Altitude Exposures

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% O₂; H-NOR); B) 90 min at 24,000 ft breathing hyperoxic gas (90% O₂; 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% O₂ (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.Ånell R, Grönkvist M, Gennser M, Eiken O. Hyperoxic effects on decompression strain during alternating high and moderate altitude exposures. Aerosp Med Hum Perform. 2021; 92(4):223230.

Preschool Multiple-Breath Washout Testing. An Official American Thoracic Society Technical Statement

BACKGROUND

Obstructive airway disease is nonuniformly distributed throughout the bronchial tree, although the extent to which this occurs can vary among conditions. The multiple-breath washout (MBW) test offers important insights into pediatric lung disease, not available through spirometry or resistance measurements. The European Respiratory Society/American Thoracic Society inert gas washout consensus statement led to the emergence of validated commercial equipment for the age group 6 years and above; specific recommendations for preschool children were beyond the scope of the document. Subsequently, the focus has shifted to MBW applications within preschool subjects (aged 2-6 yr), where a "window of opportunity" exists for early diagnosis of obstructive lung disease and intervention.

METHODS

This preschool-specific technical standards document was developed by an international group of experts, with expertise in both custom-built and commercial MBW equipment. A comprehensive review of published evidence was performed.

RESULTS

Recommendations were devised across areas that place specific age-related demands on MBW systems. Citing evidence where available in the literature, recommendations are made regarding procedures that should be used to achieve robust MBW results in the preschool age range. The present work also highlights the important unanswered questions that need to be addressed in future work.

CONCLUSIONS

Consensus recommendations are outlined to direct interested groups of manufacturers, researchers, and clinicians in preschool device design, test performance, and data analysis for the MBW technique.

A physiologically based model for denitrogenation kinetics

Under normal conditions we continuously breathe 78% nitrogen (N₂) such that the body tissues and fluids are saturated with dissolved N₂. For normobaric medical gas administration at high concentrations, the N₂ concentration must be less than that in the ambient atmosphere; therefore, nitrogen will begin to be released by the body tissues. There is a need to estimate the time needed for denitrogenation in the planning of surgical procedures. In this paper we will describe the application of a physiologically based pharmacokinetic model to denitrogenation kinetics. The results are compared to the data resulting from experiments in the literature that measured the end tidal N₂ concentration while breathing 100% oxygen in the form of moderately rapid and slow compartment time constants. It is shown that the model is in general agreement with published experimental data. Correlations for denitrogenation as a function of subject weight are provided.

Developments in multiple breath washout testing in children with cystic fibrosis

BACKGROUND

Lung clearance index (LCI) is becoming recognized as an important addition in the monitoring of pediatric cystic fibrosis (CF). The non-invasive technique is easy to perform in all ages, reproducible and increasingly being used in clinical trials. There is interest in utilizing it within the clinic setting but its current use is mostly as a research tool. The procedure is highly dependent on skilled operators and a relaxed testing environment is key to obtaining good quality measurements.

CONCLUSIONS

Standardization of LCI is part of an ongoing collaborative, multicenter process. This review describes the background to LCI, discusses technical issues and limitations and provides examples of its utility in clinical and research contexts.

Investigation of the potential for vascular bubble formation in a repetitively diving dolphin

The production of venous gas emboli (VGE) resulting from altered dive behavior is postulated as contributing to the stranding of beaked whales exposed to mid-frequency active sonar. To test whether nitrogen gas uptake during repetitive breath-hold diving is sufficient for asymptomatic VGE formation in odontocetes, a bottlenose dolphin (Tursiops truncatus Montagu) was trained to perform 10-12 serial dives with 60 s surface intervals to depths of 30, 50, 70 or 100 m. The dolphin remained at the bottom depth for 90 s on each dive. Doppler and/or two-dimensional imaging ultrasound did not detect VGE in the portal and brachiocephalic veins following a dive series. Van Slyke analyses of serial, post-dive blood samples drawn from the fluke yielded blood nitrogen partial pressure (P(N(2))) values that were negligibly different from control samples. Mean heart rate (HR; +/-1 s.d.) recorded during diving was 50+/-3 beats min(-1) and was not significantly different between the 50, 70 and 100 m dive sessions. The absence of VGE and elevated blood P(N(2)) during post-dive periods do not support the hypothesis that N(2) supersaturation during repetitive dives contributes to VGE formation in the dolphin. The diving HR pattern and the presumed rapid N(2) washout during the surface-interval tachycardia probably minimized N(2) accumulation in the blood during dive sessions.

Decompression to altitude: assumptions, experimental evidence, and future directions

Although differences exist, hypobaric and hyperbaric exposures share common physiological, biochemical, and clinical features, and their comparison may provide further insight into the mechanisms of decompression stress. Although altitude decompression illness (DCI) has been experienced by high-altitude Air Force pilots and is common in ground-based experiments simulating decompression profiles of extravehicular activities (EVAs) or astronauts' space walks, no case has been reported during actual EVAs in the non-weight-bearing microgravity environment of orbital space missions. We are uncertain whether gravity influences decompression outcomes via nitrogen tissue washout or via alterations related to skeletal muscle activity. However, robust experimental evidence demonstrated the role of skeletal muscle exercise, activities, and/or movement in bubble formation and DCI occurrence. Dualism of effects of exercise, positive or negative, on bubble formation and DCI is a striking feature in hypobaric exposure. Therefore, the discussion and the structure of this review are centered on those highlighted unresolved topics about the relationship between muscle activity, decompression, and microgravity. This article also provides, in the context of altitude decompression, an overview of the role of denitrogenation, metabolic gases, gas micronuclei, stabilization of bubbles, biochemical pathways activated by bubbles, nitric oxide, oxygen, anthropometric or physiological variables, Doppler-detectable bubbles, and potential arterialization of bubbles. These findings and uncertainties will produce further physiological challenges to solve in order to line up for the programmed human return to the Moon, the preparation for human exploration of Mars, and the EVAs implementation in a non-zero gravity environment.

The kinetics of distribution of the fat-soluble inert gas cyclopropane in the body

A kinetic analysis is made of the experimentally measured time course of respiratory uptake of the highly fat-soluble, inert gas cyclopropane by normal human subjects. The analysis is based on the well-known perfusion-limited model in which a number of body compartments are arranged in parallel with the lungs via the circulating blood. Three distinct body compartments are derived from the data. These are tentatively identified as: (a) adipose tissue (b) fat-poor tissue of low perfusion such as resting muscle, skin, and connective tissue (c) fat-poor tissue of high perfusion such as brain, heart, gut, liver, and kidney. Blood flow rates to the several compartments are also derived from the data. The rates to compartments (a) and (b) are each approximately 10 per cent of the estimated total cardiac output. The derived perfusion (blood flow rate/compartment weight) of the three compartments are in the range, respectively, (a) 2 to 4, (b) 1 to 2.5, (c) 25 to 75 ml/min/100 gm. Uncertainties arising from the experimental data and from simplifications of the model (neglect of lung fill-up phase of uptake and gross diffusion of cyclopropane from one tissue into another) are discussed. The present type of uptake experiment is significant for the problems of total body fat determination, of gross body composition in relation to weight change, of gross shunting of blood flow from one compartment to another, of anesthesia by fat-soluble substances, and of decompression sickness.

Comparison of two methods to assess the tissue/blood partition coefficient for xenon in subcutaneous adipose tissue in man

A new method to calculate the tissue/blood partition coefficient (lambda) for xenon in studies on the subcutaneous adipose tissue blood flow was compared with a previously reported method based on local skinfold thickness (lambda LST). The former method included needle biopsies from the abdominal and femoral subcutaneous adipose tissue, and the mean fat cell diameter was measured (lambda ECT). The extracellular tissue fraction in subcutaneous tissue was then estimated from a diagram. The tissue lipid content was approximated to equal the relative intracellular volume and Ostwald's solubility coefficients for 133Xe, based on the distribution of xenon in lipid, albumin and 0.9% saline were applied. Estimated lambda-values based on needle biopsies from the abdominal site were: 8.6 +/- 0.1 versus 9.9 +/- 0.4 ml g-1 (mean +/- SE) (P < 0.05) and from the femoral site: 9.1 +/- 0.1 versus 9.6 +/- 0.2 in lean (n = 10) and obese subjects (n = 10), respectively. The corresponding lambda-values obtained from skinfold measurements were: 6.2 +/- 0.5 versus 11.0 +/- 0.4 (P < 0.001) and 6.9 +/- 0.3 versus 11.4 +/- 0.4 (P < 0.001) in lean and obese subjects, respectively. Pooled lambda LST-values correlated positively with estimated adipose tissue blood flow (ATBF) (r: 0.34, P < 0.05, n = 40) whereas no such correlation was found for lambda ECT-values. In conclusion, a new method is presented which may allow an accurate determination of, and which may lead to reliable data on, subcutaneous ATBF in both lean and obese subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Reference values for resting blood flow to organs of man

The lack of a reliable quantitative description of blood flow in man has hampered the development of accurate biokinetic models of essential elements, drugs, imaging agents, and carcinogens. In this paper we review and analyse data on blood flow and identify representative percentages of cardiac output and absolute blood flow rates to organs and tissues of man for use as reference values for biokinetic models. To keep the review and analysis to a manageable size we have limited attention to the resting state and have suggested reference values for absolute and relative flow rates only for adult males and females.