Effects of CO2 on the occurrence of decompression sickness: review of the literature.

Introduction: Inhalation of high concentrations of carbon dioxide (CO2) at atmospheric pressure can be toxic with dose-dependent effects on the cardiorespiratory system or the central nervous system. Exposure to both hyperbaric and hypobaric environments can result in decompression sickness (DCS). The effects of CO2 on DCS are not well documented with conflicting results. The objective was to review the literature to clarify the effects of CO2 inhalation on DCS in the context of hypobaric or hyperbaric exposure. Methods: The systematic review included experimental animal and human studies in hyper- and hypobaric conditions evaluating the effects of CO2 on bubble formation, denitrogenation or the occurrence of DCS. The search was based on MEDLINE and PubMed articles with no language or date restrictions and also included articles from the underwater and aviation medicine literature. Results: Out of 43 articles, only 11 articles were retained and classified according to the criteria of hypo- or hyperbaric exposure, taking into account the duration of CO2 inhalation in relation to exposure and distinguishing experimental work from studies conducted in humans. Conclusions: Before or during a stay in hypobaric conditions, exposure to high concentrations of CO2 favors bubble formation and the occurrence of DCS. In hyperbaric conditions, high CO2 concentrations increase the occurrence of DCS when exposure occurs during the bottom phase at maximum pressure, whereas beneficial effects are observed when exposure occurs during decompression. These opposite effects depending on the timing of exposure could be related to 1) the physical properties of CO2, a highly diffusible gas that can influence bubble formation, 2) vasomotor effects (vasodilation), and 3) anti-inflammatory effects (kinase-nuclear factor and heme oxygenase-1 pathways). The use of O2-CO2 breathing mixtures on the surface after diving may be an avenue worth exploring to prevent DCS.

Static Immersion and Negative Static Lung Load-Induced Right Ventricle Systolic Function Adaptation: A Risk Factor for Immersion Pulmonary Edema.

BACKGROUND: Immersion pulmonary edema (IPE) is a form of hemodynamic edema likely involving individual susceptibility. RESEARCH QUESTION: Can assessing right ventricle (RV) systolic adaptation during immersion be a marker for IPE susceptibility? STUDY DESIGN AND METHODS: Twenty-eight divers participated: 15 study participants with a history of IPE (IPE group; mean ± SD age, 40.2 ± 8.2 years; two women) and 13 control participants (no IPE group; mean ± SD age, 43.1 ± 8.5 years; two women) underwent three transthoracic echocardiography studies under three different conditions: dry (participants were in the supine position on an examination table without immersion), surface immersion (participants were floating prone on the water's surface and breathing through a snorkel), and immersion and negative static lung load (divers were submerged 20 cm below the water's surface in the prone position using a specific snorkel connected to the surface for breathing). Echocardiographic measurements included tricuspid annular plane systolic excursion (TAPSE), tissue S' wave, and right ventricle global strain (RVGLS). RESULTS: For all divers, immersion increased RV preload. In the no IPE group, the increase in RV preload induced by immersion was accompanied by an improvement in the contractility of the RV, as evidenced by increases in TAPSE (17.08 ± 1.15 mm vs 20.89 ± 1.32 mm), S' wave (14.58 ± 2.91 cm/s vs. 16.26 ± 2.77 cm/s), and RVGLS (25.37 ± 2.79 % vs. 27.09 ± 2.89 %). Negative SLL amplified these RV adaptations. In contrast, among divers with IPE, the increase in RV preload did not coincide with an improvement in RV contractility, indicating altered adaptive responses. In the IPE group, the TAPSE values changed from 17.19 ± 1.28 mm to 21.69 ± 1.67 mm and then to 23.55 ± 0.78 mm, respectively, in the dry, surface immersion, and immersion and negative SLL conditions. The S' wave values changed from 13.42 ± 2.94 cm/s to 13.26 ± 2.96 cm/s and then to 12.49 ± 0.77 cm/s, respectively, and the RVGLS values changed from -24.09% ± 2.91% to -23.99% ± 3.38% and then to -21.96% ± 0.55%, respectively. INTERPRETATION: Changes in RV systolic function induced by immersion (especially with the addition of negative static lung load) vary among divers based on the history of IPE. Analyzing ventricular contractility during immersion, particularly RVGLS, could help to identify individual susceptibility in divers. These findings provide insights for the development of preventive strategies. TRIAL REGISTRY: Comité de Protection des Personnes; No.: 21.05.05.35821; Recherche Impliquant la Personne Humaine de type 1 (RIPH1) HPS; No.: 2021-A01225-36.

Case report: Reassessing guidelines for safe resumption of diving after spinal decompression sickness: insights from a challenging case.

Background: Recreational divers who have experienced Spinal Decompression Sickness (DCS) often aspire to return to their diving activities. Traditionally, it is recommended to observe a waiting period of several months before contemplating a return to unrestricted diving, particularly when clinical symptoms are absent, spinal cord Magnetic Resonance Imaging shows no anomalies, and the evaluation for Patent Foramen Ovale (PFO) returns negative results. Methods: This article presents a compelling case study involving a 51-year-old recreational scuba diver who encountered two episodes of spinal decompression illness within a two-year timeframe. Notably, the search for a PFO produced negative results. The primary objective of this article is to underscore the critical importance of a meticulously planned approach to resuming diving after DCS incidents, emphasizing the potential for recurrence and the essential preventive measures. Conclusion: We delve into the intricate decision-making process for returning to diving, emphasizing the significance of clinical evaluations, PFO assessments, spinal cord Magnetic Resonance Imaging, and the absence of clinical symptoms. By recognizing the risk of recurrence and the need for proactive prevention measures, we provide recommendations for both medical professionals and divers, with the ultimate goal of enhancing safety and informed decision-making within the diving community.

Temporal adaptation of the postural control following a prolonged fin swimming.

PURPOSE: Postural control deteriorates following a transition between two environments, highlighting a sensory conflict when returning to natural conditions. Aquatic immersion offers new perspectives for studying postural control adaptation in transitional situations. Our aim is to study immediate and post-task static postural control adaptation on land after a prolonged fin swimming exercise in total immersion. METHODS: Standing static postural control was assessed in 14 professional or recreational SCUBA divers (11 men, 3 women; 33.21 ± 10.70 years), with eyes open and closed, before, immediately after, and in the following 20 min following a fully-immersed 45-min fin swimming exercise. Centre-of-pressure metrics (COP) including average position, amplitude, velocity, length and 95% ellipse were evaluated in medial-lateral (x-axis) and anterior-posterior (y-axis) directions with a force platform. The Romberg ratio was also assessed for each metric. RESULTS: A two-way repeated measures analysis of variance revealed a significant effect of the measurement period on COPx vel (p = 0.01), COPy vel (p < 0.01) and Length (p < 0.01), and of the visual condition on COPy vel (p < 0.01) and Length (p < 0.01). Eyes closed measures were systematically higher than eyes open measures despite there being no significant difference in the Romberg ratio in all periods. Post-immersion, the velocity and total trajectory of the centre of pressure remained systematically lower than baseline values in both visual conditions. CONCLUSION: Post-immersion, COP velocity and length significantly decreased, suggesting a sensory reweighting strategy potentially associated with ankle stiffening.

Characterizing Immersion Pulmonary Edema (IPE): A Comparative Study of Military and Recreational Divers.

BACKGROUND: Immersion Pulmonary Edema (IPE) is a common and potentially serious diving accident that can have significant respiratory and cardiac consequences and, in some cases, be fatal. Our objective was to characterize cases of IPE among military trainees and recreational divers and to associate their occurrence with exposure and individual background factors such as age and comorbidity. We conducted a retrospective analysis on the medical records and diving parameters of all patients who were treated for IPE at the Hyperbaric Medicine Department of Sainte-Anne Military Hospital in Toulon, France, between January 2017 and August 2019. In total, 57 subjects were included in this study, with ages ranging from 20 to 62 years. These subjects were divided into two distinct groups based on exposure categories: (1) underwater/surface military training and (2) recreational scuba diving. The first group consisted of 14 individuals (25%) with a mean age of 26.5 ± 2.6 years; while, the second group comprised 43 individuals (75%) with a mean age of 51.2 ± 7.5 years. All divers under the age of 40 were military divers. RESULTS: In 40% of cases, IPE occurred following intense physical exercise. However, this association was observed in only 26% of recreational divers, compared to 86% of military divers. Among civilian recreational divers, no cases of IPE were observed in subjects under the age of 40. The intensity of symptoms was similar between the two groups, but the duration of hospitalization was significantly longer for the recreational subjects. CONCLUSION: It seems that the occurrence of IPE in young and healthy individuals requires their engagement in vigorous physical activity. Additionally, exposure to significant ventilatory constraints is a contributing factor, with the intensity of these conditions seemingly exclusive to military diving environments. In contrast, among civilian recreational divers, IPE tends to occur in subjects with an average age twice that of military divers. Moreover, these individuals exhibit more prominent comorbidity factors, and the average level of environmental stressors is comparatively lower.

Individual Changes in Respiratory Compliance Upon Immersion May Predict Susceptibility to Immersion Pulmonary Edema.

BACKGROUND: Immersion pulmonary edema (IPE) is a frequent diving accident, and it is the primary cause of hospitalization for young military divers during training. The objective of this study was to identify immersion-induced parameters predicting individual susceptibility to IPE. METHODS: Eighteen experienced male divers having completed at least 100 dives were recruited. Eight divers had previously been hospitalized for IPE (IPE), and the other ten had never developed IPE (non-IPE). The two groups were matched for age, BMI, and number of dives performed. Ventilatory function and overall compliance of the respiratory system (Crs) were measured on land and during head-out-of-water immersion. Subjects also performed 30 min of fin swimming in a channel at 33 m min-1. Following this exercise, the presence of extravascular lung water, revealed by ultrasound lung comets (ULC), was assessed. RESULTS: In the whole group, the decrease in Crs upon immersion correlated with the immersion-induced alterations to expiratory reserve volume, ERV (r2 = 0.91; p < 0.001), inspiratory reserve volume, IRV (r2 = 0.94; p < 0.001), and tidal volume, Vt, changes (r2 = 0.43; p < 0.003). The number of ULC correlated strongly with immersion-induced changes in ventilatory function (r2 = 0.818; p < 0.001 for ERV, r2 = 0.849; p < 0.001 for IRV, r2 = 0.304; p = 0.0164 for Vt) and reduced Crs (r2 = 0.19; p < 0.001). The variations of ERV, IRV, and Crs at rest induced by head-out-of-water immersion and the number of ULC measured after swimming for 30 min were significantly greater in IPE subjects. CONCLUSION: In the face of similar immersion stresses, the extent of alterations to ventilatory function and the number of ULCs were very different between individuals but remained statistically correlated. These parameters were significantly greater in divers with a history of IPE. Alterations to pulmonary function and, in particular, to pulmonary compliance induced by head-out-of-water immersion, through their effects on work of breathing appear to allow the identification of divers with a greater susceptibility to developing IPE. Measurement of these parameters could therefore be proposed as a predictive test for the risk of developing IPE.

Oxygen uptake ( V ˙ O2) and pulmonary ventilation ( V ˙ E) during military surface fin swimming in a swimming flume: Effects of surface immersion.

Introduction: During military fin swimming, we suspected that oxygen uptake ( V ˙ O2) and pulmonary ventilation ( V ˙ E) might be much higher than expected. In this framework, we compared these variables in the responses of trained military divers during land cycling and snorkeling exercises. Methods: Eighteen male military divers (32.3 ± 4.2 years; 178.0 ± 5.0 cm; 76.4 ± 3.4 kg; 24.1 ± 2.1 kg m-2) participated in this study. They performed two test exercises on two separate days: a maximal incremental cycle test (land condition), and an incremental fin swimming (fin condition) in a motorized swimming flume. Results: The respective fin and land V ˙ O2max were 3,701 ± 39 mL min-1 and 4,029 ± 63 mL min-1 (p = 0.07), these values were strongly correlated (r 2 = 0.78 p < 0.01). Differences in V ˙ O2max between conditions increased relative to l; V ˙ O2max (r 2 = 0.4 p = 0.01). Fin V ˙ E max values were significantly lower than land V ˙ E max values (p = 0.01). This result was related to both the significantly lower fin Vt and f (p < 0.01 and <0.04, respectively). Consequently, the fin V ˙ E max / V ˙ O2max ratios were significantly lower than the corresponding ratios for land values (p < 0.01), and the fin and land V ˙ E max were not correlated. Other parameters measured at exhaustion-PaO2, PaCO2, and SO2 - were similar in fin and land conditions. Furthermore, no significant differences between land and fin conditions were observed for peak values for heart rate, blood lactate concentration, and respiratory exchange ratio R. Conclusion: Surface immersion did not significantly reduce the V ˙ O2max in trained divers relative to land conditions. As long as V ˙ O2 remained below V ˙ O2max , the V ˙ E values were identical in the two conditions. Only at V ˙ O2max was V ˙ E higher on land. Although reduced by immersion, V ˙ E max provided adequate pulmonary gas exchange during maximal fin swimming.

Chest CT scan for the screening of air anomalies at risk of pulmonary barotrauma for the initial medical assessment of fitness to dive in a military population.

Introduction: The presence of intra-pulmonary air lesions such as cysts, blebs and emphysema bullae, predisposes to pulmonary barotrauma during pressure variations, especially during underwater diving activities. These rare accidents can have dramatic consequences. Chest radiography has long been the baseline examination for the detection of respiratory pathologies in occupational medicine. It has been replaced since 2018 by the thoracic CT scan for military diving fitness in France. The objective of this work was to evaluate the prevalence of the pulmonary abnormalities of the thoracic CT scan, and to relate them to the characteristics of this population and the results of the spirometry. Methods: 330 records of military diving candidates who underwent an initial assessment between October 2018 and March 2021 were analyzed, in a single-center retrospective analysis. The following data were collected: sex, age, BMI, history of respiratory pathologies and smoking, treatments, allergies, diving practice, results of spirometry, reports of thoracic CT scans, as well as fitness decision. Results: The study included 307 candidates, mostly male, with a median age of 25 years. 19% of the subjects had abnormal spirometry. We identified 25% of divers with CT scan abnormalities. 76% of the abnormal scans were benign nodules, 26% of which measured 6 mm or more. Abnormalities with an aerial component accounted for 13% of the abnormal scans with six emphysema bullae, three bronchial dilatations and one cystic lesion. No association was found between the presence of nodules and the general characteristics of the population, whereas in six subjects emphysema bullae were found statistically associated with active smoking or abnormal spirometry results. Conclusion: The systematic performance of thoracic CT scan in a young population free of pulmonary pathology revealed a majority of benign nodules. Abnormalities with an aerial component are much less frequent, but their presence generally leads to a decision of unfitness. These results argue in favor of a systematic screening of aeric pleuro-pulmonary lesions during the initial assessment for professional divers.

A neoprene vest hastens dyspnoea and leg fatigue during exercise testing: entangled breathing and cardiac hindrance?

Symptoms and contributing factors of immersion pulmonary oedema (IPO) are not observed during non-immersed heart and lung function assessments. We report a case in which intense snorkelling led to IPO, which was subsequently investigated by duplicating cardiopulmonary exercise testing with (neoprene vest test - NVT) and without (standard test - ST) the wearing of a neoprene vest. The two trials utilised the same incremental cycling exercise protocol. The vest hastened the occurrence and intensity of dyspnoea and leg fatigue (Borg scales) and led to an earlier interruption of effort. Minute ventilation and breathing frequency rose faster in the NVT, while systolic blood pressure and pulse pressure were lower than in the ST. These observations suggest that restrictive loading of inspiratory work caused a faster rise of intensity and unpleasant sensations while possibly promoting pulmonary congestion, heart filling impairment and lowering blood flow to the exercising muscles. The subject reported sensations close to those of the immersed event in the NVT. These observations may indicate that increased external inspiratory loading imposed by a tight vest during immersion could contribute to pathophysiological events.

Broad individual immersion-scattering of respiratory compliance likely substantiates dissimilar breathing mechanics.

Head-out water immersion alters respiratory compliance which underpins defining pressure at a "Lung centroid" and the breathing "Static Lung Load". In diving medicine as in designing dive-breathing devices a single value of lung centroid pressure is presumed as everyone's standard. On the contrary, we considered that immersed respiratory compliance is disparate among a homogenous adult group (young, healthy, sporty). We wanted to substantiate this ample scattering for two reasons: (i) it may question the European standard used in designing dive-breathing devices; (ii) it may contribute to understand the diverse individual figures of immersed work of breathing. Resting spirometric measurements of lung volumes and the pressure-volume curve of the respiratory system were assessed for 18 subjects in two body positions (upright Up, and supine Sup). Measurements were taken in air (Air) and with subjects immersed up to the sternal notch (Imm). Compliance of the respiratory system (Crs) was calculated from pressure-volume curves for each condition. A median 60.45% reduction in Crs was recorded between Up-Air and Up-Imm (1.68 vs 0.66 L/kPa), with individual reductions ranging from 16.8 to 82.7%. We hypothesize that the previously disregarded scattering of immersion-reduced respiratory compliance might participate to substantial differences in immersed work of breathing.

Epidemiological, clinical, and echocardiographic features of twenty 'Takotsubo-like' reversible myocardial dysfunction cases with normal coronarography following immersion pulmonary oedema.

BACKGROUND: Pulmonary immersion oedema is a frequent diving accident. Although its outcome is generally favourable within 72 h, it can nonetheless lead to heart failure or sudden death. Cases of transient myocardial dysfunction have been reported in the literature. This phenomenon is similar to Takotsubo syndrome in many ways. It is characterised by transient myocardial hypokinesia, without associated coronary lesions. METHODS: We report on 20 cases of patients who showed transient alteration of left ventricular kinetics with normal coronary angiography over the course of an immersion pulmonary oedema. RESULTS: The echocardiographic localisation of the myocardial damage was generally focal and not centred on the apex with an average left ventricular ejection fraction of 45%. The main anomalies in the electrocardiographic repolarisation were T wave inversion with corrected QT interval prolongation. We also observed a moderate increase in troponin levels, with discordance between the enzymatic peak and the severity of the left ventricle segmental dysfunction. CONCLUSION: These cases suggest the incidence of a clinical entity strongly reminiscent of Takotsubo phenomenon of atypical topography as a consequence of diving accidents.

In COPD, Nocturnal Noninvasive Ventilation Reduces the FIO2 Delivered Compared With Long-Term Oxygen Therapy at the Same Flow.

BACKGROUND: Nocturnal noninvasive ventilation is recommended for patients with hypercapnic COPD. Long-term oxygen therapy improves survival in patients with hypoxemic disease. However, leaks during noninvasive ventilation are likely to reduce the fraction of inspired oxygen. OBJECTIVES: To compare nocturnal inspired O2 fractions during noninvasive ventilation with daytime pharyngeal inspired O2 fractions during nasal cannula oxygen therapy (with the same O2 flow) in patients with COPD at home (ie, real-life conditions). METHODS: This single-center prospective observational study included 14 subjects with COPD who received long-term O2 therapy. We analyzed pharyngeal inspired O2 fractions in the evening, with a nasopharyngeal probe (sidestream gas analyzer). The O2 flow was measured with a precision flow meter, at the usual flow. Then, the same O2 flow was implemented for noninvasive ventilation with a study's home ventilator. The all-night noninvasive ventilation parameters were delivered in pressure mode with a single-limb leaking circuit. Daytime and nighttime inspired O2 fractions were compared. RESULTS: The mean ± SD daytime pharyngeal inspired O2 fraction, measured with normobaric basal O2 flow, 0.308 ± 0.026%, was significantly higher than the mean ± SD nighttime inspired O2 fraction, measured during noninvasive ventilation (0.251 ± 0.011; P < .001). CONCLUSIONS: The nighttime inspired O2 fraction decreased with a modern noninvasive ventilation pattern, pressure target, and intentional leaks. This partial lack of O2 therapy is likely to be harmful. It might explain the poor results in all but 2 randomized controlled trials on long-term noninvasive ventilation in COPD. (ClinicalTrials.gov registration NCT02599246.).

Clinical problem solving: Mental confusion and hypoxaemia after scuba diving.

INTRODUCTION: We report a case of a diving accident associating both cerebral symptoms and signs of respiratory impairment after two dives. The objective is to describe the process for obtaining the diagnosis. CASE REPORT: A 52-year-old man experienced mental confusion associated with hypoxaemia after surfacing. All decompression procedures were fully respected. The diver had a spatio-temporal disorientation accompanied by a marked tendency to fall asleep spontaneously. He had no dyspnoea and no cough, but crepitations at both lung bases were found with oxygen saturation at 80%. CONCLUSIONS: In this clinical case, cerebral magnetic resonance imaging and chest computed tomography scan helped to exclude other pathology that would have necessitated urgent transfer rather than urgent hyperbaric treatment. The imaging is particularly useful in case of cerebral and respiratory symptoms following scuba diving.

Is a 12-h Nitrox dive hazardous for pulmonary function?

PURPOSE: Prolonged exposure to a high partial pressure of oxygen leads to inflammation of pulmonary tissue [pulmonary oxygen toxicity (POT)], which is associated with tracheobronchial irritation, retrosternal pain and coughing, and decreases in vital capacity (VC). The nitric oxide (NO) concentration in exhaled gas (FeNO) has been used as an indicator of POT, but the effect of SCUBA diving on FeNO has rarely been studied. The study presented here aimed to assess alterations to pulmonary function and FeNO following a 12-h dive using breathing apparatus with a relatively high partial pressure of oxygen. METHODS: Six healthy, male, non-smoking military SCUBA divers were recruited (age 31.8 ± 2.7 years, height 179 ± 0.09 cm, and body weight 84.6 ± 14 kg). Each diver completed a 12-h dive using a demand-controlled semi-closed-circuit rebreather. During the 12 h of immersion, divers were subjected to 672 oxygen toxicity units (OTU). A complete pulmonary function test (PFT) was completed the day before and immediately after immersion. FeNO was measured using a Nobreath™ Quark (COSMED™, Rome, Italy), three times for each diver. The first datapoint was collected before the dive to establish the "basal state", a second was collected immediately after divers emerged from the water, and the final measurement was taken 24 h after the dive. RESULT: Despite prolonged inhalation of a hyperoxic hyperbaric gas mixture, no clinical pulmonary symptoms were observed, and no major changes in pulmonary function were detected. However, a major decrease in FeNO values was observed immediately after emersion [0-12 ppb (median, 3.8 ppb)], with a return to baseline [2-60 ppb (median, 26 ppb) 24 h later (3-73 ppb (median, 24.7 ppb)]. CONCLUSION: These results suggest that if the OTU remain below the recommended limit values, but does alter FeNO, this type of dive does not persistently impair lung function.

Reduction in the Level of Plasma Mitochondrial DNA in Human Diving, Followed by an Increase in the Event of an Accident.

Circulating mitochondrial DNA (mtDNA) is receiving increasing attention as a danger-associated molecular pattern in conditions such as autoimmunity or trauma. In the context of decompression sickness (DCS), the course of which is sometimes erratic, we hypothesize that mtDNA plays a not insignificant role particularly in neurological type accidents. This study is based on the comparison of circulating mtDNA levels in humans presenting with various types of diving accidents, and punctured upon their admission at the hyperbaric facility. One hundred and fourteen volunteers took part in the study. According to the clinical criteria there were 12 Cerebro DCS, 57 Medullary DCS, 15 Vestibular DCS, 8 Ctrl+ (accident-free divers), and 22 Ctrl- (non-divers). This work demonstrates that accident-free divers have less mtDNA than non-divers, which leads to the assumption that hyperbaric exposure degrades the mtDNA. mtDNA levels are on average greater in divers with DCS compared with accident-free divers. On another hand, the amount of double strand DNA (dsDNA) is neither significantly different between controls, nor between the different DCS types. Initially the increase in circulating oligonucleotides was attributed to the destruction of cells by bubble abrasion following necrotic phenomena. If there really is a significant difference between the Medullary DCS and the Ctrl-, this difference is not significant between these same DCS and the Ctrl+. This refutes the idea of massive degassing and suggests the need for new research in order to verify that oxidative stress could be a key element without necessarily being sufficient for the occurrence of a neurological type of accident.

Immersion pulmonary oedema in a healthy diver not exposed to cold or strenuous exercise.

In healthy divers, the occurrence of immersion pulmonary oedema (IPE) is commonly caused by contributory factors including strenuous exercise, cold water and negative-pressure breathing. Contrary to this established paradigm, this case reports on a 26-year-old, well-trained combat swimmer who succumbed to acute IPE during static immersion in temperate (21°C) water, while using a front-mounted counterlung rebreather. The incident occurred during repeated depth-controlled ascent practice at the French military diving school. It was discovered that the diver had attempted to stop any gas leakage into the system by over-tightening the automatic diluent valve (ADV) (25th notch of 27) during the dive, thus causing a high resistance to inspiratory flow. The ventilatory constraints imposed by this ADV setting were assessed as a 3.2 Joules·L⁻¹ inspiratory work of breathing and -5 kPa (-50 mbar) transpulmonary pressure. This report confirms the key role of negative pressure breathing in the development of interstitial pulmonary oedema. Such a breathing pattern can cause a lowering of thoracic, airway and interstitial lung pressure, leading to high capillary pressure during each inspiration. Repetition of the diving drills resulted in an accumulation of interstitial lung water extravasation, causing pathological decompensation and proven symptoms.

The Key Roles of Negative Pressure Breathing and Exercise in the Development of Interstitial Pulmonary Edema in Professional Male SCUBA Divers.

BACKGROUND: Immersion pulmonary edema is potentially a catastrophic condition; however, the pathophysiological mechanisms are ill-defined. This study assessed the individual and combined effects of exertion and negative pressure breathing on the cardiovascular system during the development of pulmonary edema in SCUBA divers. METHODS: Sixteen male professional SCUBA divers performed four SCUBA dives in a freshwater pool at 1 m depth while breathing air at either a positive or negative pressure both at rest or with exercise. Echocardiography and lung ultrasound were used to assess the cardiovascular changes and lung comet score (a measure of interstitial pulmonary edema). RESULTS: The ultrasound lung comet score was 0 following both the dives at rest regardless of breathing pressure. Following exercise, the mean comet score rose to 4.2 with positive pressure breathing and increased to 15.1 with negative pressure breathing. The development of interstitial pulmonary edema was significantly related to inferior vena cava diameter, right atrial area, tricuspid annular plane systolic excursion, right ventricular fractional area change, and pulmonary artery pressure. Exercise combined with negative pressure breathing induced the greatest changes in these cardiovascular indices and lung comet score. CONCLUSIONS: A diver using negative pressure breathing while exercising is at greatest risk of developing interstitial pulmonary edema. The development of immersion pulmonary edema is closely related to hemodynamic changes in the right but not the left ventricle. Our findings have important implications for divers and understanding the mechanisms of pulmonary edema in other clinical settings.

Cardiovascular Mechanisms of Extravascular Lung Water Accumulation in Divers.

This study assessed the relation between altered cardiac function and the development of interstitial pulmonary edema in scuba divers. Fifteen healthy men performed a 30-minute scuba dive in open sea. They were instructed to fin for 30 minutes and were wearing wet suits. Before and immediately after immersion, cardiac indexes and extravascular lung water were measured using echocardiography and lung ultrasound, respectively. The mean ultrasound lung comet score increased from 0 to 4.6 ± 3.4. The diameter of the inferior caval vein increased by 47 ± 5.2%, systolic pulmonary artery pressure by 105 ± 8.6%, left atrial volume by 18.0 ± 3.3%, and left ventricle end-diastolic volume by 10 ± 2.4% suggesting that both right and left ventricular (LV) filling pressures were elevated. Doppler studies showed an increased mitral E peak (+2.5 ± 0.3%) and E/A ratio (+22.5 ± 3.4%) with a decreased mitral A peak (-16.4 ± 2.7%), E peak deceleration time (-14.5 ± 2.4%) consistent with rapid early LV filling but without a change in LV stroke volume. There was an increase in right/left ventricle diameter ratio (+33.6 ± 4.8%) suggesting a relative increase in right-sided heart output compared with the left. Furthermore, the lung comet score correlated significantly with inferior caval vein diameter, systolic pulmonary artery pressure, right/left ventricle diameter ratio, and E-wave deceleration time. In conclusion, the altered right/left heart stroke volume balance could play an essential role in the development of immersion pulmonary edema. Our findings have important implications for the pathogenesis of cardiogenic pulmonary edema.

Submarine rescue decompression procedure from hyperbaric exposures up to 6 bar of absolute pressure in man: effects on bubble formation and pulmonary function.

Recent advances in submarine rescue systems have allowed a transfer under pressure of crew members being rescued from a disabled submarine. The choice of a safe decompression procedure for pressurised rescuees has been previously discussed, but no schedule has been validated when the internal submarine pressure is significantly increased i.e. exceeding 2.8 bar absolute pressure. This study tested a saturation decompression procedure from hyperbaric exposures up to 6 bar, the maximum operating pressure of the NATO submarine rescue system. The objective was to investigate the incidence of decompression sickness (DCS) and clinical and spirometric indices of pulmonary oxygen toxicity. Two groups were exposed to a Nitrogen-Oxygen atmosphere (pO2 = 0.5 bar) at either 5 bar (N = 14) or 6 bar (N = 12) for 12 h followed by 56 h 40 min resp. 60 h of decompression. When chamber pressure reached 2.5 bar, the subjects breathed oxygen intermittently, otherwise compressed air. Repeated clinical examinations, ultrasound monitoring of venous gas embolism and spirometry were performed during decompression. During exposures to 5 bar, 3 subjects had minor subjective symptoms i.e. sensation of joint discomfort, regressing spontaneously, and after surfacing 2 subjects also experienced joint discomfort disappearing without treatment. Only 3 subjects had detectable intravascular bubbles during decompression (low grades). No bubbles were detected after surfacing. About 40% of subjects felt chest tightness when inspiring deeply during the initial phase of decompression. Precordial burning sensations were reported during oxygen periods. During decompression, vital capacity decreased by about 8% and forced expiratory flow rates decreased significantly. After surfacing, changes in the peripheral airways were still noticed; Lung Diffusion for carbon monoxide was slightly reduced by 1% while vital capacity was normalized. The procedure did not result in serious symptoms of DCS or pulmonary oxygen toxicity and may be considered for use when the internal submarine pressure is significantly increased.