Which of the following features best distinguishes the increase in cardiac output seen during exercise from that seen in acute mitral regurgitation?

## Distinguishing Exercise-Induced vs. Pathologic Cardiac Output Increase ### Cardiac Output in Exercise **Key Point:** During exercise, cardiac output increases through coordinated increases in both heart rate and stroke volume, with preserved or improved ejection fraction (EF). 1. **Contractility increases** via sympathetic stimulation (β-adrenergic effect) 2. **Afterload decreases** due to vasodilation in exercising muscles 3. **Preload increases** via skeletal muscle pump and venoconstriction 4. **Ejection fraction remains ≥60%** or may increase slightly 5. **Pulmonary capillary wedge pressure (PCWP)** remains normal or only mildly elevated ### Cardiac Output in Acute Mitral Regurgitation **Key Point:** In acute MR, forward cardiac output is maintained by compensatory mechanisms, but ejection fraction is reduced because a large fraction of LV stroke volume is ejected backward into the left atrium. 1. **Regurgitant fraction** diverts blood away from systemic circulation 2. **Afterload is reduced** (low-impedance pathway into LA), which paradoxically increases total SV but reduces *forward* SV 3. **Ejection fraction decreases** (total SV ÷ LV volume is high, but forward SV is low) 4. **PCWP rises acutely** due to sudden volume overload of the left atrium 5. **Contractility may be normal initially** but forward output is compromised ### Comparison Table | Feature | Exercise | Acute Mitral Regurgitation | | --- | --- | --- | | **Ejection Fraction** | Normal or ↑ (>60%) | ↓ (<50%) | | **PCWP** | Normal or mildly ↑ | Acutely ↑↑ | | **Contractility** | ↑ (sympathetic) | Normal or ↓ | | **Afterload** | ↓ (vasodilation) | ↓ (regurgitant pathway) | | **Forward Cardiac Output** | ↑ (effective) | ↓ (compromised) | | **Mechanism of CO increase** | Coordinated HR + SV | Compensatory tachycardia + ↑ preload | **High-Yield:** The **ejection fraction is the single best discriminator**. In exercise, EF is preserved or supranormal; in acute MR, EF is reduced because the regurgitant volume inflates the total stroke volume while reducing forward output. **Clinical Pearl:** A patient with acute severe MR may have a *total* stroke volume that appears adequate on echocardiography, but the ejection fraction will be low because much of that volume is going backward. This is why acute MR can present with pulmonary edema despite a seemingly "normal" cardiac output — the forward output is insufficient and PCWP is dangerously elevated. [cite:Harrison 21e Ch 297]