The correct diagnosis is isorhythmic dissociation with junctional rhythm (Figure 2).
Figure 2. Courtesy of Dr Podrid.
The RR intervals are regular (┌┐), at a rate of 90 beats/min. The QRS complexes are narrow and have a normal morphology and axis, so they are supraventricular. P waves are not seen initially, but a distinct P wave precedes the fifth QRS complex (+) and is present before all of the subsequent QRS complexes (+). The PP intervals are regular (└┘), at a rate of 90 beats/min. The PR intervals are not constant, indicating atrioventricular (AV) dissociation. Because the atrial and ventricular rates are the same, this is termed "isorhythmic dissociation."
The etiology of atrioventricular dissociation may be complete heart block, in which the atrial rate is faster than the rate of the QRS complexes, or accelerated junctional rhythm, in which the atrial rate is slower than the rate of the QRS complexes. Because the two rates are the same, however, the etiology is not clear. Therefore, it is called "isorhythmic dissociation with junctional rhythm."
Philip J. Podrid, MD, is an electrophysiologist, a professor of medicine and pharmacology at Boston University School of Medicine, and a lecturer in medicine at Harvard Medical School. Although retired from clinical practice, he continues to teach clinical cardiology and especially ECGs to medical students, house staff, and cardiology fellows at many major teaching hospitals in Massachusetts. In his limited free time he enjoys photography, music, and reading.
Follow Dr Podrid on Twitter: @PPodrid
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