A 37-year-old woman presents to urgent care with complaints of dizziness and palpitations. She was in her usual state of health when symptoms started abruptly about 30 minutes prior to her arrival at urgent care. The symptoms began with only light exertion while she was shopping. On questioning, she states that her dizziness is much more of a “light-headed” feeling than a sensation of spinning or abnormal motion. There is mild associated shortness of breath, but no chest pain or headache. She is on no medications, but drinks caffeinated drinks in moderate amounts; however, she says she did not have any more caffeine today than on a typical day.
On examination, the patient is conversant and pleasant, but somewhat anxious appearing. Her vital signs on the automated blood pressure (BP) cuff are: blood pressure, 88/60 mm Hg; heart rate, 112 beats/min; respiratory rate, 24 breaths/min; temperature, 97.9°F; and oxygen saturation, 96% on room air. Upon auscultation of the chest, her heart rate seems much faster than the 112 beats/min registered by the automated BP cuff, but heart sounds are regular with no obvious murmur or extra sounds. The following ECG is obtained:
What is your interpretation of the ECG, and what is the best explanation for this patient’s hypotension and near syncope?
A. Atrial fibrillation with rapid ventricular response and diffuse ST abnormality; decreased myocardial contractility
B. AV nodal reentry tachycardia; rate-related decrease in cardiac output due to inadequate left ventricle filling
C. Sinus tachycardia with diffuse ST depression; decreased myocardial contractility due to ischemia
D. Atrial flutter with 2:1 ventricular conduction; rate-related decreased cardiac output due to inadequate left ventricle filling
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Answer: B. The tracing shows a regular, narrow QRS complex tachycardia with a ventricular rate of about 220 beats/min. There are no discernible P waves and diffuse ST depression is present. This ECG is a classic presentation of atrioventricular nodal reentrant tachycardia (AVNRT), which is commonly but less specifically termed paroxysmal supraventricular tachycardia. The re-entry circuit is a functional one located within the atrioventricular node, not an anatomic one as with atrioventricular re-entry tachycardias such as Wolff-Parkinson-White syndrome. AVNRT is the most common cause of sudden, persistent palpitations in patients with structurally normal hearts. It predisposes to abrupt-onset tachycardias and associated symptoms (palpitations, dyspnea, syncope, and/or near syncope), which may or may not resolve spontaneously.
Atrial fibrillation with rapid ventricular response is an irregular, narrow-QRS tachycardia (unless other conduction abnormalities such as bundle-branch blocks are present). It is highly unusual to see sustained sinus tachycardia at an extraordinarily fast rate in the absence of heavy exercise. No P waves are present in the ECG tracing (although they would be difficult to discern at such a fast rate), and there is no beat-to-beat rate variability as would be expected with sinus tachycardia.
Atrial flutter with 2:1 conduction is also a regular, narrow QRS complex tachycardia like the one shown the patient’s ECG. However, the hallmark is a consistent ventricular rate of at or around 150 beats/min. The sawtooth P waves of classic atrial flutter are usually not evident when conduction is at a 2:1 rate, as they are hidden by QRS complexes. When there appears to be a sinus tachycardia with a solid regular rate at or near 150 beats/min, strongly consider atrial flutter with 2:1 conduction.
When hypotension, syncope, or near syncope are present in patients with abrupt-onset AVNRT, decreased cardiac output is the cause. At a significantly rapid ventricular rate, there is not enough time for the left ventricle to fill, and the stroke volume falls. This phenomenon is almost always the culprit when a drop in mean arterial pressure occurs in cases of AVNRT. Automated BP cuffs (and pulse oximeters) will frequently miscount heart rate in cases of extreme tachycardia since the devices may not sense every pulse when stroke volume is dramatically reduced. Listen to the chest, and get an ECG when there is any doubt or discrepancy.
Last Updated on May 11, 2022