Negative Static Lung Load and Immersion Pulmonary Edema in Healthy Divers: The Determinant Role of Breathing-Gas Source Position

Purpose: Immersion pulmonary edema (IPE) affects divers and swimmers across the age and fitness spectrum, including young, trained individuals, indicating that mechanical factors act alongside any cardiovascular predisposition. We hypothesized that an extreme negative static lung load (SLL), produced when the breathing-gas source sits shallower than the pulmonary centroid during prone swimming, increases extravascular lung water as a function of gas-source position rather than device type. Methods: This translational study combined a retrospective analysis of 21 rebreather-associated IPE cases in healthy professional divers (2009–2015) and a prospective cross-over study in 15 healthy volunteers who performed a standardized 30-minute prone finning exercise in cold water twice: with an open-circuit system (near-neutral SLL) and a back-mounted rebreather (negative SLL). Extravascular lung water was quantified by ultrasound lung comets (ULC); inspiratory work of breathing (WOB) and mean hydrostatic load were measured in immersion with a pneumo-baro-tachograph across body positions. Results: All 21 cases (median age 32) occurred in divers free of cardiovascular disease with a back-mounted gas source generating negative SLL; 34% developed without exertion, and depth was not distinguishing (median 21 m, range 7-80). The prone position imposed a mean hydrostatic load near -32 cmH2O and an inspiratory WOB of approximately 3.3 J/L, versus 1.3-1.5 J/L when vertical. After diving, increased extravascular lung water (ULC > 5) was more prevalent with negative SLL than open-circuit (80% vs 27%; McNemar P =.008), despite a greater distance covered with open-circuit. Conclusions: An extreme negative SLL set by gas-source position is associated with markedly increased inspiratory loading and extravascular lung water, independent of exertion. These findings support continued consideration of SLL in the physiological evaluation and safety assessment of occupational life-support systems.