A 68-year-old man presents to the emergency department with worsening shortness of breath … — USMLE Step 2 CK

A $68$ -year-old man presents to the emergency department with worsening shortness of breath and nocturnal awakenings over the past $5$ days. He reports needing to sleep in a reclining chair to breathe comfortably and frequently wakes up gasping for air. The patient has a history of hypertension, type 2 diabetes mellitus, and a $40$ -pack-year smoking history. He has tried using an over-the-counter bronchodilator inhaler without relief. Vital signs show a blood pressure of $165/95 \text{ mm Hg}$ , pulse of $105\text{/min}$ , and respirations of $22\text{/min}$ . Pulse oximetry is $91\%$ on room air. On physical examination, the body mass index is $32 \text{ kg/m}^2$ . The jugular venous pulse is visible $5 \text{ cm}$ above the sternal angle while seated at a $45^\circ$ angle. Lung auscultation reveals prolonged expiration, diffuse bilateral wheezes, and dullness to percussion at the lung bases. Cardiac examination reveals a regular tachycardia with a prominent early diastolic extra heart sound at the apex. Trace bilateral pitting edema is present at the ankles. Which of the following pathophysiologic processes best explains the primary cause of this patient's respiratory findings?

Increased pulmonary venous hydrostatic pressure leading to peribronchial fluid accumulationAnswer
B
Reversible bronchial smooth muscle constriction and airway mucosal inflammation
C
Hypertrophy of bronchial submucosal glands leading to mucous hypersecretion
D
Destruction of interalveolar septa leading to decreased pulmonary elastic recoil
E
Acute occlusion of the pulmonary arterial tree causing alveolar dead space ventilation

Answer

The correct answer is increased pulmonary venous hydrostatic pressure leading to peribronchial fluid accumulation, which accurately describes the mechanism of cardiogenic pulmonary edema (cardiac asthma).

The correct answer correctly identifies the pathophysiology of cardiac asthma. This patient presents with classic signs of acute decompensated heart failure, including orthopnea, paroxysmal nocturnal dyspnea, elevated jugular venous pressure, and an S3 gallop. The S3 gallop is a highly specific sign of elevated left ventricular filling pressures. When left ventricular end-diastolic pressure rises acutely, the pressure is transmitted backward to the left atrium and pulmonary veins. The resulting increased pulmonary venous hydrostatic pressure pushes fluid into the pulmonary interstitium. When this fluid accumulates around the bronchioles (peribronchial edema), it compresses the small airways and causes reactive bronchospasm, resulting in expiratory wheezing. This phenomenon is known as 'cardiac asthma' and is critical to differentiate from a primary obstructive lung disease exacerbation.

Step-by-Step Solution

Analyze the patient's history and acute symptoms.

The patient has acute dyspnea, orthopnea, and paroxysmal nocturnal dyspnea (PND).

These are classic historical hallmarks of volume overload and acute decompensated heart failure (ADHF).

Interpret the physical examination findings.

Exam shows elevated JVP, an S3 gallop, basilar dullness (suggesting pleural effusions), trace edema, and diffuse wheezing.

An S3 gallop is highly specific for elevated left ventricular end-diastolic pressure. Elevated JVP and peripheral edema confirm systemic volume overload.

Determine the etiology of the respiratory findings (wheezing).

The wheezing is caused by peribronchial edema (cardiac asthma) due to elevated pulmonary venous hydrostatic pressure, not a primary obstructive airway disease.

Backward transmission of elevated left heart pressures causes transudation of fluid into the pulmonary interstitium. Fluid surrounding the bronchioles causes extrinsic compression and mucosal swelling, leading to expiratory wheezing that mimics an asthma or COPD exacerbation.

Key Concept

Cardiac asthma occurs in acute decompensated heart failure when elevated pulmonary venous pressures cause peribronchial edema, leading to airway narrowing and diffuse expiratory wheezes that can be clinically confused with a primary COPD or asthma exacerbation.

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