A 53-year-old woman is admitted to the hospital due to a 3-day history of worsening dyspnea. On examination, the patient is tachypneic; diffuse, wet, inspiratory crackles are present on pulmonary auscultation. Chest x-ray reveals bilateral patchy opacities. Arterial blood gas analysis shows that the patient's alveolar-arterial gradient (P_AO₂ − P_aO₂) is markedly increased. Although the patient receives 100% oxygen via a face mask, she has persistent hypoxemia and respiratory distress. Consequently, she is intubated and placed on mechanical ventilation with 100% oxygen and positive end-expiratory pressure (PEEP). Results of arterial oxygen measurements are as follows: The patient's improved oxygenation following PEEP application is most likely due to a decrease in which of the following?

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11ec160b_3b2b_f383_9249_57d413220a97_MD0001_00 This patient has diffuse pulmonary edema (inspiratory wet crackles, bilateral patchy infiltrates) that can be cardiogenic (eg, decompensated left ventricular failure) or noncardiogenic (eg, acute respiratory distress syndrome). With pulmonary edema, the alveoli become flooded and unable to fill with gas (ventilation: V ≈ 0). However, they are still being perfused (Q), resulting in many alveoli with ventilation-perfusion (V/Q) ratios approaching zero. Because blood passing through the lungs returns to the left atrium without being oxygenated, this situation is analogous to a true right-to-left anatomic shunt (eg, atrial septal defect with flow from the right to the left atrium) and is termed intrapulmonary shunt effect. Increasing the fraction of inspired oxygen (FiO₂) to 100% does not improve oxygenation because the shunted blood never "sees" the alveolar gas. Therefore, this patient's low arterial partial pressure of oxygen (PaO₂) has not corrected with 100% supplemental oxygen. Because pure oxygen alone cannot improve hypoxemia, treatment depends on reducing the shunt effect. This can be done by pressurizing the inspired gas in a closed circuit (eg, mechanical ventilator). Positive end-expiratory pressure (PEEP) reduces the shunt by propping the collapsed alveoli open, displacing extracellular lung fluid and exposing more alveolar surface area for gas exchange/oxygen uptake. (Choice A) Anatomical dead space refers to the volume of air in the conducting airways (eg, trachea, bronchi) that never reaches alveoli. It is increased, not decreased, in an intubated patient due to the extra volume of the ventilator tubing. At any given tidal volume, increased anatomic dead space causes less air to reach alveoli with each breath, contributing to hypoventilation and worsening hypoxemia. (Choice C) When fluid enters the alveolar airspace (ie, pulmonary edema), the surface tension increases (due to dilution of surfactant) causing the alveolar walls to cling together and collapse (ie, atelectasis), which decreases lung compliance. PEEP props open ("recruits") the partially flooded alveoli, improving lung compliance. (Choices D and E) PEEP raises intrathoracic pressure, which compresses pulmonary capillaries and raises pulmonary vascular resistance. This increases pressure within the right heart chambers and can increase right-to-left shunting through an existing intracardiac shunt (eg, atrial or ventricular septal defect), worsening hypoxemia. Therefore, PEEP is generally avoided in patients with intracardiac shunts. Educational objective: Severe hypoxemia that does not correct with 100% supplemental oxygen is consistent with a right-to-left shunt. Intrapulmonary shunt effect occurs when an alveolar filling process (eg, diffuse pulmonary edema) causes poor alveolar ventilation despite continued perfusion; this improves with positive end-expiratory pressure.

A 53-Year-Old Woman Is Admitted to the Hospital Due to a 3-Day