ARDS Pathophysiology: Shunt, Gas Exchange and Lung Compliance
How does ARDS cause hypoxemia and altered lung compliance?
ARDS involves inflammatory injury to the alveolar-capillary barrier. Protein-rich fluid and debris disrupt airspaces and surfactant, producing uneven aeration, low ventilation-perfusion regions and shunt. Blood passing poorly ventilated units lowers arterial oxygenation. Carbon dioxide clearance and overall respiratory-system compliance sometimes look less abnormal because they measure different features of the injured lung.
Key takeaways
- Barrier injury increases permeability rather than requiring visible tears in every alveolus.
- Low V/Q regions receive some ventilation, while shunt regions receive none.
- A normal PaCO2 does not rule out a serious oxygenation problem.
- Overall respiratory-system compliance includes the chest wall and hides differences among lung regions.
In this video
- ARDS: more than white lungs on the X-ray
- The normal alveolar-capillary barrier
- Barrier injury and protein-rich fluid
- Flooding, collapse and shunt
- Why the lung is uneven
- Oxygen and carbon dioxide can diverge
- What lung compliance means
- Two regions and one total compliance number
- Permeability versus hydrostatic edema
- Returning to the pneumonia patient
- Explaining ARDS in an interview
- Practice questions
- Recap: barrier, shunt and compliance
What happens at the alveolar-capillary barrier?
The pulmonary capillary endothelium and alveolar epithelium separate blood from air while allowing gas exchange. Inflammatory injury increases barrier permeability. Fluid, protein and cellular material accumulate in interstitial tissue and airspaces. Direct lung insults and systemic illnesses contribute through different routes.
Surfactant lowers surface tension and supports airspace stability. Injury and accumulated material impair that function, promoting flooding and collapse. The lungs do not become one uniformly affected compartment. Better-aerated and poorly aerated regions coexist, and their relative sizes and behavior change over time.
Why does shunt respond differently from low V/Q to more oxygen?
A low ventilation-perfusion region still receives some fresh gas relative to its blood flow. Increasing inspired oxygen reaches that region. A true shunt region receives blood but no ventilation, so newly inspired oxygen does not enter that unit. Its less-oxygenated blood mixes into the arterial circulation.
Whole-patient oxygen response depends on the mix of regions and the rest of the circulation. Saying oxygen never helps a patient with shunt is too broad. Some lung regions still receive inspired gas, even while unventilated units continue contributing venous admixture. A mechanism does not independently determine the oxygen or ventilator prescription.
Why does carbon dioxide sometimes stay normal?
Better-ventilated regions continue clearing carbon dioxide even while poorly ventilated regions impair oxygenation. PaCO2 therefore does not need to rise whenever arterial oxygen falls. Overall effective ventilation and carbon dioxide production influence the arterial value.
A later PaCO2 rise also has several explanations, including changes in ventilation, dead space, mechanics and patient effort. It does not identify one mandatory disease stage. Read oxygenation and carbon dioxide as separate physiological results, with the actual ventilator support and measurement conditions attached.
What does one compliance number leave out?
Compliance is volume change divided by the relevant distending-pressure change. Respiratory-system compliance combines lung and chest-wall behavior. It is not a direct measurement of alveolar injury or regional stretch. Airway resistance and patient effort also affect the interpretation of real pressure measurements.
The recording uses two hypothetical parallel regions exposed to a 5 cmH2O change. At compliances of 30 and 10 mL/cmH2O, their volume changes are 150 and 50 mL. The combined value hides that unequal expansion. These simplified values illustrate the calculation and do not prescribe ventilator settings, diagnose recruitability or measure overdistension.
Do bilateral opacities and hypoxemia establish ARDS?
Those findings support concern but do not complete the diagnosis. Timing, oxygenation under defined support, imaging and the assessment of competing or coexisting causes matter. Hydrostatic edema from elevated pulmonary venous pressure also impairs gas exchange and sometimes coexists with inflammatory permeability injury.
A normal-looking ejection fraction, one filling pressure or one compliance value does not settle the entire explanation. Current ARDS treatment guidance addresses defined patient groups and interventions. This lesson explains the barrier, gas exchange and mechanics, while a full assessment determines diagnosis and patient-specific support.
