A patient is found to have a cardiac output of 4 L/min with a heart rate of 80 bpm. Compared to a healthy individual with cardiac output of 5 L/min at the same heart rate, what is the most likely physiological explanation for the reduced output?
- A Reduced stroke volume due to decreased preload ✓
- B Increased venous return
- C Increased sympathetic tone
- D Decreased systemic vascular resistance
Correct answer: A. Reduced stroke volume due to decreased preload
Explanation
Cardiac output = Heart Rate × Stroke Volume. With a heart rate of 80 bpm and CO of 4 L/min, the stroke volume is 50 mL versus 62.5 mL in the healthy individual. The most common cause of reduced stroke volume is decreased preload (reduced end-diastolic filling), which decreases sarcomere stretch and thus contractile force per the Frank-Starling mechanism. Increased SVR or decreased contractility are other possibilities, but reduced preload is the most likely primary cause.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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