During a treadmill stress test, a patient's cardiac output increases from 5 L/min to 20 L/min. If mean arterial pressure increases by only 20%, the calculated drop in total peripheral resistance (TPR) is approximately:
- A 50% reduction
- B 65% reduction
- C 75% reduction ✓
- D 25% reduction
Correct answer: C. 75% reduction
Explanation
TPR = MAP / CO. If resting MAP is 100 mmHg and CO is 5 L/min, resting TPR = 100/5 = 20 units. During exercise, MAP rises 20% to 120 mmHg and CO rises to 20 L/min, so new TPR = 120/20 = 6 units. Percentage reduction = (20-6)/20 × 100 = 70%, closest to 75% among the options. The large drop in TPR reflects massive vasodilation in exercising skeletal muscle, the dominant physiological response during dynamic exercise.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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