A 75-year-old woman comes to the emergency department for shortness of breath. Her family reports she has been having difficulty breathing for the last few nights. About an hour ago, the patient's condition suddenly deteriorated and she developed significant respiratory distress. She has had no fever or cough but has been out of her regular medications for a week. Medical history is significant for hypertension, coronary artery disease, and heart failure. Temperature is 36.7 C (98.1 F) , blood pressure is 205/110 mm Hg, pulse is 112/min, and respirations are 32/min. Pulse oximetry is 89% on high-flow oxygen. The patient is awake but in distress and able to speak only in 1-word sentences. Bilateral crackles and an S3 gallop are present. She has 3+ pitting edema in her bilateral lower extremities. Noninvasive positive pressure mechanical ventilation is initiated. The beneficial effects of positive pressure ventilation in this patient are most likely through which of the following physiologic changes?

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Quizplus · Accepted Answer

Explanation: 11ec25c8_d1e8_46a5_bc81_3f226f453311_MD0003_00 The patient has acutely decompensated left heart failure with cardiogenic pulmonary edema (S3, crackles, and hypoxemia). Treatment of cardiogenic pulmonary edema is directed at improving left ventricle (LV) performance. Positive pressure ventilation (PPV) optimizes hemodynamics to achieve these goals. Pressure (typically 5-20 cm H₂O) is applied to the patient's airway, usually noninvasively through a face mask. ₂ face mask PPV decreases LV preload through the following series of events: Application of PPV directly raises the intrathoracic pressure, causing a drop in venous return to the right heart (decreased right ventricular [RV] preload). Elevated intrathoracic pressure compresses alveolar capillaries, raising the pulmonary vascular resistance (increased RV afterload). Due to these effects, there is less blood volume returning to the left atrium (ie, LV preload). In decompensated heart failure, LV end-diastolic volume is high. Lowering LV preload shrinks the ventricular cavity size, leading to a more advantageous Frank-Starling relationship (ie, improved myofibril overlap) to promote systolic ejection. PPV also decreases LV afterload through 2 mechanisms. To eject blood during systole, the LV must overcome 2 components of afterload: (1) the aortic pressure (ie, mean arterial pressure [MAP]) and (2) the LV transmural pressure gradient. PPV decreases MAP through baroreceptor effects and by lowering adrenergic tone through decreasing neurohormonal activation. This is particularly helpful with a superimposed hypertensive emergency, as in this patient. PPV decreases LV transmural pressure because, as the positive pressure is transmitted to the pleura, the LV contracts against a smaller transmural pressure gradient. This is especially beneficial in pulmonary edema, where massive inspiratory efforts generate deep negative pleural pressures to impede LV contraction. Therefore, PPV lowers LV preload and afterload. In patients with decompensated heart failure, this optimizes LV function across the entire cardiac cycle: there is enhanced stroke volume in systole (due to Frank-Starling effects and ↓ LV afterload) and improved filling in diastole (ie, improved LV relaxation), resulting in decongestion of the lungs and forward cardiac output. Along with increasing LV performance, PPV displaces interstitial lung water to improve oxygenation. Because cardiac output and oxygenation are both augmented, the overall effect of PPV is to increase global oxygen delivery to end organs. For these reasons, PPV has a major mortality benefit (number needed to treat = 13) in cardiogenic pulmonary edema. Educational objective: Positive pressure ventilation (PPV) is a mainstay lifesaving treatment for acute cardiogenic pulmonary edema. PPV lowers left ventricle (LV) preload and afterload to promote LV diastolic filling and systolic ejection, decreasing pulmonary congestion and enhancing oxygen delivery.

A 75-Year-Old Woman Comes to the Emergency Department for Shortness