Six months after orthotopic cardiac transplantation, a patient's resting heart rate is 105 bpm. Which statement best explains his cardiovascular physiology?
- A Atropine will slow his heart rate toward normal
- B His heart rate cannot rise during exercise because sympathetic nerves are absent
- C His resting tachycardia reflects loss of vagal tone, and he lacks reflex bradycardia to phenylephrine infusion ✓
- D Baroreceptor reflex changes in heart rate are preserved but blunted
Correct answer: C. His resting tachycardia reflects loss of vagal tone, and he lacks reflex bradycardia to phenylephrine infusion
Explanation
The transplanted heart is surgically denervated, so resting rate rises to 100 to 110 bpm because tonic vagal slowing is gone, and intravenous phenylephrine produces no reflex bradycardia since the efferent vagal limb cannot reach the graft. Exercise response is preserved but delayed because circulating adrenal catecholamines act directly on the graft, which eliminates option B. Atropine is ineffective without vagal innervation, eliminating option A.
Reference: Ganong Review of Medical Physiology, 26th ed.
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