A 68-year-old man with long-standing diabetes mellitus develops orthostatic hypotension (BP drop >20/10 mmHg on standing) with a fixed heart rate. This autonomic neuropathy is characterized by loss of which reflex arc?

• A Loss of parasympathetic cardiac innervation causing fixed tachycardia and impaired vasodilation
• B Loss of sympathetic cardiovascular efferents: impaired norepinephrine release to splanchnic/leg vessels prevents compensatory vasoconstriction; cardiac parasympathetic loss causes fixed heart rate ✓
• C Loss of baroreceptor afferents only, with intact efferent sympathetic function
• D Adrenomedullary failure causing reduced circulating epinephrine as the sole mechanism

Explanation

Diabetic autonomic neuropathy classically affects both sympathetic and parasympathetic fibers. The orthostatic hypotension results from loss of sympathetic efferents to splanchnic and lower limb vasculature — when standing, blood pools in dependent veins and splanchnic bed, but the compensatory arteriolar vasoconstriction (normally via NE/alpha-1 receptors) is absent. The fixed (unchanging) heart rate results from loss of cardiac parasympathetic innervation (vagus), eliminating normal HR variability and the reflex tachycardia response to hypotension. The resting HR is typically 90–100 due to parasympathetic loss. Both limbs are involved — option B is the complete explanation.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.