🫁 Gas exchange in ARDS: why oxygenation is not simply a pulmonary problem In ARDS, we frequently interpret PaO₂/FiO₂ as a measure of lung injury and PaCO₂ as an indicator of ventilation. Yet neither parameter can be understood independently of pulmonary perfusion, cardiac output and ventilation-perfusion relationships. Collino, Camporota and Pinsky revisit these principles in an excellent 2026 physiological review, a must read! 1. Hypoxemia is predominantly a shunt problem. Perfusion of nonventilated or poorly ventilated alveoli produces venous admixture, reducing arterial oxygenation. Increasing FiO₂ may improve oxygenation in low V̇A/Q̇ regions, but has limited effectiveness in true shunt. Moreover, excessive FiO₂ may promote absorption atelectasis and attenuate hypoxic pulmonary vasoconstriction, potentially worsening shunt. 2. Cardiac output changes PaO₂ even when pulmonary pathology remains unchanged. This is particularly important in septic shock and ARDS. According to the Fick principle, reduced cardiac output lowers mixed venous oxygen content when oxygen consumption remains constant. With a fixed shunt fraction, lower mixed venous oxygenation produces lower PaO₂. The authors illustrate that with a 20% shunt and FiO₂ 1.0, PaO₂ may exceed 400 mmHg at normal cardiac output but fall below 150 mmHg when cardiac output decreases. Therefore, deteriorating oxygenation does not necessarily indicate worsening lung injury. 3. PaO₂/FiO₂ is not a fixed characteristic of the lung. It changes with FiO₂, PEEP and cardiac output. The same patient may move between ARDS severity categories without a proportional change in underlying pulmonary injury. Standardizing ventilator conditions improves interpretation. 4. Hypercapnia is frequently a dead space problem. Increased alveolar dead space reduces effective alveolar ventilation, requiring greater minute ventilation to maintain PaCO₂. However, not all dead space indices measure the same physiology. The Bohr equation estimates true physiological dead space, whereas the Enghoff modification incorporates the effects of shunt and V̇A/Q̇ heterogeneity. Consequently, elevated Enghoff VD/VT does not necessarily mean increased anatomical or alveolar dead space alone. The ventilatory ratio and EtCO₂/PaCO₂ provide practical bedside information, although neither replaces direct physiological assessment. The clinical implication is fundamental. When PaO₂ decreases, investigate not only recruitment and shunt, but also cardiac output and mixed venous oxygenation. When PaCO₂ increases, assess dead space, CO₂ production and effective alveolar ventilation before simply increasing respiratory rate. Reference 📚 Collino F, et al. Intensive Care Medicine. 2026. doi.org/10.1007/s00134-026-0… #ARDS #CriticalCare #RespiratoryPhysiology #MechanicalVentilation #Hemodynamics #GasExchange

Critical Physiology Series #93 Catastrophic Antiphospholipid Syndrome A patient develops acute kidney injury, respiratory failure, neurological dysfunction and thrombocytopenia within days. Sepsis? DIC? TTP? Consider also catastrophic antiphospholipid syndrome (CAPS). Pathophysiology CAPS is a rare, life-threatening manifestation of antiphospholipid syndrome characterized by rapidly progressive thrombosis, predominantly affecting the microcirculation. Antiphospholipid antibodies, particularly those targeting β2-glycoprotein I, activate endothelial cells, monocytes and platelets. Antibody binding → complement activation → endothelial dysfunction → tissue-factor expression → thrombin generation Neutrophil extracellular traps and inflammatory mediators amplify thrombosis. The result: microvascular occlusion → tissue ischemia → organ dysfunction Unlike isolated venous thromboembolism, CAPS may simultaneously damage kidneys, lungs, brain, heart and other organs. Risk factors and triggers CAPS may develop in established APS or be its first manifestation. Important associations include: systemic lupus erythematosus, previous thrombosis and high-risk antiphospholipid antibody profiles. Common triggers: infection, surgery, trauma, pregnancy complications and interruption of anticoagulation. Diagnosis Suspect CAPS when thrombosis and organ dysfunction develop rapidly across multiple systems. Classical classification criteria include: ≥3 organs involved manifestations within approximately one week small-vessel thrombosis on histology positive antiphospholipid antibodies However, incomplete criteria do not exclude probable CAPS. Request: lupus anticoagulant anticardiolipin IgG/IgM anti-β2-glycoprotein I IgG/IgM Interpret testing cautiously during acute illness and anticoagulation. Differentiate CAPS from DIC, TTP/HUS, HIT and sepsis-associated microangiopathy. Monitoring Follow: platelets and hemolysis markers PT/aPTT, fibrinogen creatinine/urine output lactate and organ perfusion ECG/troponin/echocardiography neurological and respiratory function Management CAPS requires urgent multidisciplinary treatment, often before complete diagnostic confirmation. The conventional approach combines: Therapeutic heparin → suppress ongoing thrombosis High-dose corticosteroids → suppress inflammation Plasma exchange and/or IVIG → modulate pathogenic immune activity Treat the precipitating infection or other trigger. In SLE-associated CAPS, cyclophosphamide may be indicated. Refractory cases may require rituximab or complement inhibition with eculizumab in selected circumstances. Evidence remains largely observational because randomized trials are lacking. Recommended lectures 📚 Jacobs, 2024. doi.org/10.3390/antib1301002… Raschi, 2024. doi.org/10.1093/rheumatology… Legault, 2018. doi.org/10.1111/jth.14192 Kazzaz, 2016. doi.org/10.1097/BOR.00000000…

👅The nasogastric tube: routine does not mean harmless Few interventions in acute care are as familiar as the nasogastric tube. Perhaps that familiarity is exactly why its indication is sometimes insufficiently questioned. This review revisits NG tubes from indication to removal and challenges several practices that have persisted more through tradition than evidence. In small bowel obstruction, decompression remains important. In adhesive small bowel obstruction without ischemia, strangulation or peritonitis, nonoperative management including NG decompression resolves approximately 75% of cases. But strangulation, closed loop obstruction, perforation or peritonitis changes the problem entirely: decompression must not delay definitive surgery. Water soluble contrast is also interesting. Its diagnostic and prognostic role remains useful, but recent meta-analysis does not demonstrate a convincing therapeutic reduction in surgery, ischemia, bowel resection or mortality. The more disruptive message concerns postoperative care. Routine NG decompression after elective gastrointestinal surgery should increasingly be abandoned. Across 37 studies involving 5711 patients, avoiding routine NG tubes was associated with earlier return of bowel function and fewer pulmonary complications, without an increase in anastomotic leakage. This philosophy is now embedded in ERAS pathways across multiple surgical populations. Placement itself deserves more respect. Tracheobronchial misplacement occurs in approximately 2% to 7% of insertions and can result in aspiration, pneumonitis or pneumothorax. Resistance should never be overcome with force. And the traditional “whoosh test” should disappear from clinical reasoning. Air insufflation with auscultation is unreliable. When the tube will be used for feeding or instillation, radiographic confirmation is recommended, with the tip demonstrated below the diaphragm. The tube also becomes a physiological intervention once suction begins. Gastric losses can produce volume depletion, hypokalemia and metabolic alkalosis. Output therefore belongs in the fluid balance, and water and electrolyte replacement should be deliberate rather than reactive. Perhaps the most useful concept in this review is the simplest: An NG tube should have an indication not only on the day it is inserted, but on every subsequent day it remains. The proposed safety bundle therefore includes a daily removal screen, assessing symptoms, distension, output, imaging and diet tolerance, with removal at the first safe opportunity. In ICU practice, we routinely ask whether the patient still needs antibiotics, vasopressors, sedation or a urinary catheter. Perhaps the nasogastric tube deserves the same question. Reference 📚 Alverdy JC, Boermeester MA, Salminen P, Zuckerbraun BS. Nasogastric Tubes: Indications, Placement, and Management: A Review. JAMA Surgery. 2026. DOI: 10.1001/jamasurg.2026.3223