A 58-year-old man with a 10-year history of hypertension presents to the emergency department with acute dyspnea, orthopnea, and bilateral ankle edema. His blood pressure is 165/105 mmHg, heart rate 112 bpm, and JVP is elevated at 8 cm H₂O. Echocardiography reveals a left ventricular ejection fraction (LVEF) of 35% with global hypokinesis. Cardiac catheterization shows elevated pulmonary capillary wedge pressure (PCWP) of 28 mmHg. Which of the following mechanisms BEST explains the reduced cardiac output in this patient?

Explanation

## Pathophysiology of Reduced Cardiac Output in Heart Failure **Key Point:** In systolic heart failure (LVEF <40%), the primary mechanism of reduced cardiac output is **decreased myocardial contractility**, not preload or afterload alone. ### Mechanism of Contractility Loss In chronic hypertension progressing to dilated cardiomyopathy: 1. **Calcium handling defects** — Impaired sarcoplasmic reticulum calcium release and reuptake (SERCA2a downregulation) reduce the force-generating capacity of each sarcomere. 2. **Sympathetic hyperactivation** — Chronic β-adrenergic overstimulation leads to receptor desensitization, reduced cAMP signaling, and myocyte apoptosis. 3. **Myocardial remodeling** — Eccentric hypertrophy and fibrosis impair the ventricle's ability to generate force at any given fiber length. ### Why the Frank-Starling Mechanism Fails Although preload is elevated (PCWP 28 mmHg, JVP 8 cm H₂O), the ventricle operates on a **flattened Frank-Starling curve**. The failing myocardium cannot generate adequate stroke volume despite high filling pressures — this is the hallmark of systolic dysfunction. **Cardiac Output Formula:** $$CO = HR \times SV = HR \times (EDV - ESV)$$ In this patient: - EDV is increased (dilated ventricle) - ESV is markedly increased (poor contractility) - Therefore, SV is reduced → CO is reduced ### Compensatory Mechanisms (Maladaptive) | Mechanism | Effect | Outcome | |-----------|--------|----------| | Sympathetic activation | ↑ HR, ↑ contractility (initially) | Tachycardia, arrhythmia risk | | RAAS activation | Vasoconstriction, fluid retention | ↑ Preload, ↑ Afterload (worsens CO) | | Ventricular dilation | ↑ Preload (Frank-Starling) | Limited benefit; increases wall stress | **Clinical Pearl:** The elevated PCWP (28 mmHg) and JVP (8 cm) indicate the ventricle is operating at the **steep portion of the diastolic pressure-volume curve**, yet CO remains low — proof that contractility, not preload, is the limiting factor. **High-Yield:** In systolic HF, inotropic support (dobutamine, milrinone) or contractility enhancers (ACE-I, β-blockers) are indicated because the primary defect is force generation, not filling. [cite:Harrison 21e Ch 297]