MEDizzy - A 60-year-old man is undergoing an electrophysiology study

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Disorders of the Cardiovascular System

A 60-year-old man is undergoing an electrophysiology study for the evaluation of syncope. After careful venous cannulation and placement of conductance and pacing catheters and after administration of 0.2 mg/kg of propranolol and 0.04 mg/kg of atropine, his heart rate is 65 bpm. After stopping the drugs and allowing adequate time for washout, his superior/lateral right atrium is paced at 140 bpm. On cessation of this overdrive pacing, his next sinus beat occurs 1800 msec later. Based on these observations, this patient can be diagnosed with which of the following?

ExplanationDetermining the intrinsic heart rate (IHR) may distinguish SA node dysfunction from slow heart rates that result from high vagal tone. The normal IHR after administration of 0.2 mg/kg of propranolol and 0.04 mg/kg of atropine is 117.2 – (0.53 × age) in bpm; a low IHR is indicative of SA disease. For this patient, his IHR should be approximately 85 bpm. Electrophysiologic testing may play a role in the assessment of patients with presumed SA node dysfunction and in the evaluation of syncope, particularly in the setting of structural heart disease. In this circumstance, electrophysiologic testing is used to rule out more malignant etiologies of syncope, such as ventricular tachyarrhythmias and AV conduction block. There are several ways to assess SA node function invasively. They include the sinus node recovery time (SNRT), which is defined as the longest pause after cessation of overdrive pacing of the right atrium near the SA node (normal: <1500 msec). This patient has no evidence of tachyarrhythmias (option E) or atrial fibrillation (option B). While amyloid cardiomyopathy might explain SA nodal dysfunction, further evidence (endomyocardial biopsy or serologic evidence of light chain disease) would be required to make this diagnosis.