A patient with autonomic neuropathy (e.g., in diabetes mellitus) has a fixed resting heart rate of 90 bpm with no change on standing or with the Valsalva maneuver. Which autonomic abnormality primarily explains this?
- A Loss of sympathetic innervation to the SA node causing resting tachycardia
- B Loss of both parasympathetic and sympathetic innervation; the SA node fires at its intrinsic rate (~90–100 bpm), and heart rate no longer responds to postural or Valsalva stimuli ✓
- C Enhanced sympathetic tone due to loss of baroreceptor input
- D Parasympathetic overactivity causing persistent tachycardia via M2 receptor stimulation
Correct answer: B. Loss of both parasympathetic and sympathetic innervation; the SA node fires at its intrinsic rate (~90–100 bpm), and heart rate no longer responds to postural or Valsalva stimuli
Explanation
Cardiac denervation (loss of both parasympathetic and sympathetic input) results in the SA node firing at its intrinsic automaticity rate of approximately 90–100 bpm. Normally, vagal tone predominates and slows the resting heart rate to 60–80 bpm; loss of vagal tone causes resting tachycardia. The absence of R-R interval variation with standing (loss of the 30:15 ratio), Valsalva, and deep breathing are clinical tests that reveal cardiac autonomic neuropathy.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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