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It argues that the classic “baby lung” concept can underestimate chest wall mechanics and elevated pleural pressure, showing how mean wall pressure—rather than transpulmonary pressure alone—governs tissue compression. The discussion links airway and tissue dynamics, surfactant effects, prone positioning, and contrasts PPV with negative pressure ventilation, highlighting impacts on recruitment and ventilation 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can the “baby lung” concept underestimate mechanisms in ARDS?","Question",{"text":63,"@type":64},"The discussion emphasizes that chest wall mechanics and elevated pleural pressure (Ppl) meaningfully alter how stress is distributed, beyond what the conventional concept captures.","Answer",{"name":66,"@type":61,"acceptedAnswer":67},"What determines lung tissue compression under PPV according to the letter?",{"text":68,"@type":64},"Tissue compression is described as depending on mean wall pressure, using the mean of Paw and Ppl, which increases compressive load even when transpulmonary pressure is preserved.",{"name":70,"@type":61,"acceptedAnswer":71},"How does prone positioning change the “baby lung” and pleural pressure behavior?",{"text":72,"@type":64},"Proper prone positioning is presented as reducing diaphragmatic compression, lowering Ppl, reopening collapsed dorsal alveoli, and shifting the “baby lung” from ventral to dorsal regions while increasing its 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pressure ventilation and reduced pulmonary compliance  \nJan van Egmond1,2^, Michael Seltz Kristensen3^, Jan Paul Mulier4,5^  \n1Department of Anaesthesiology, Radboud University Medical Centre, Nijmegen, The Netherlands; 2Donders Institute for Brain, Cognition and Behaviour, Donders Centre for Cognition, Radboud University, Nijmegen, The Netherlands; 3Department of Anaesthesia, Centre of Head and Orthopaedics, Rigshospitalet, University Hospital of Copenhagen, Copenhagen, Denmark; 4Faculty of Medicine and Health Sciences, Basic and Applied Medical Sciences, Ghent University, Ghent, Belgium; 5Department of Anaesthesiology and Pain Clinic, Ghent University Hospital, Ghent, Belgium  \nCorrespondence to: Jan van Egmond, PhD. Clinical Physicist, Department of Anaesthesiology, Radboud University Medical Centre, Geert Grooteplein Zuid 10, 6525 GA, Nijmegen, The Netherlands; Donders Institute for Brain, Cognition and Behaviour, Donders Centre for Cognition, Radboud University, Nijmegen, [The Netherlands. Email: Jan.vanEgmond.kf@gmail.com](The Netherlands. Email: Jan.vanEgmond.kf@gmail.com).  \nSubmitted Aug 18, 2025. Accepted for publication Nov 04, 2025. Published online Dec 24, 2025.  \ndoi: 10.21037/jtd-2025-1693  \nView this article at: [https://dx.doi.org/10.21037/jtd-2025-1693](https://dx.doi.org/10.21037/jtd-2025-1693)  \nIntroduction  \nAcute respiratory distress syndrome (ARDS) is defined by diffuse alveolar damage, pulmonary edema, and severely reduced lung compliance. Positive pressure ventilation (PPV) is often essential for survival, yet it concentrates mechanical stress on the limited portion of aerated parenchyma—the “baby lung”(1) . This conventional concept, however, underestimates the role of chest wall mechanics and elevated pleural pressure (Ppl) (2) and airway pressure (Paw) .  \nMechanical perspective on PPV in lowcompliance lungs  \nIn healthy lungs during spontaneous breathing, endexpiratory Ppl is negative (~−6 cmH2O), keeping alveoli open. In ARDS, under mechanical ventilation, Ppl may become markedly positive (3-5) (+15 to +20 cmH2O), especially in dependent regions. When Paw is increased to maintain ventilation [including positive end-expiratory pressure (PEEP)], Ppl rises in parallel.  \nTissue compression and alveolar interdependence  \nLung tissue compression depends not on transpulmonary pressure (Ptp = Paw − Ppl) but on mean wall pressure. Theparenchyma is “sandwiched” between Ppl and Paw; thus, the mean of both,(Paw + Ppl)/2, determines compressive load. Increases in both Paw and Ppl preserve Ptp but elevate interstitial pressure, favouring collapse of gravity-dependent alveoli.  \nNon-dependent alveoli, exposed to lower Ppl are more expanded at rest and therefore less compliant. When two alveolar units experience the same pressure change, the more compliant unit will show the largest volume change. This asymmetry drives redistribution under positive pressure: gas leaves compliant dependent alveoli toward stiffer non-dependent regions, worsening heterogeneity.  \nSurfactant dynamics reinforce this: as an alveolus shrinks, its radius decreases, surfactant layer thickens, and compliance rises, making collapse more likely. Expansion has the opposite effect. Thus, regional shifts accelerate and contribute to the “baby lung” phenomenon.  \n^ ORCID: Jan van Egmond, 0000-0001-9561-5777; Michael Seltz Kristensen, 0000-0001-9462-1665; Jan Paul Mulier, 0000-0001-6664- 8270.  \n© AME Publishing Company. J Thorac Dis 2025;17(12):11520-11523 | [https://dx.doi.org/10.21037/jtd-2025-1693](https://dx.doi.org/10.21037/jtd-2025-1693)  \nJournal of Thoracic Disease, Vol 17, No 12 December 2025 11521  \nGiven that the dependent regions of the lung (those in the lower parts with respect to gravity) generally have higher compliance, the drop in Ppl during natural inspiration preferentially inflates these lower regions. This compensates partly for ","cbCainDT46MREi3d","https://ap.wps.com/l/cbCainDT46MREi3d","pdf",136116,"English","# Introduction\n## Mechanical perspective on PPV in low-compliance lungs\n## Tissue compression and alveolar interdependence\n## Prone positioning and Ppl\n## Negative pressure ventilation (NPV) versus PPV: experimental insights\n## Airway resistance dynamics","[{\"question\":\"Why can the “baby lung” concept underestimate mechanisms in ARDS?\",\"answer\":\"The discussion emphasizes that chest wall mechanics and elevated pleural pressure (Ppl) meaningfully alter how stress is distributed, beyond what the conventional concept captures.\"},{\"question\":\"What determines lung tissue compression under PPV according to the letter?\",\"answer\":\"Tissue compression is described as depending on mean wall pressure, using the mean of Paw and Ppl, which increases compressive load even when transpulmonary pressure is preserved.\"},{\"question\":\"How does prone positioning change the “baby lung” and pleural pressure behavior?\",\"answer\":\"Proper prone positioning is presented as reducing diaphragmatic compression, lowering Ppl, reopening collapsed dorsal alveoli, and shifting the “baby lung” from ventral to dorsal regions while increasing its size.\"}]","The emergence of the “baby lung”: a mechanical consequence of positive pressure ventilation and reduced pulmonary compliance - Letter to the Editor | PDF",1790748304]