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Bigeminal rhythm V.

A 33-year-old man was admitted with exertional dyspnea, and an electrocardiogram (ECG) was recorded (Figure).

[FIGURE OMITTED]

DIAGNOSIS: Sinus rhythm, atrial bigeminy with right bundle branch block (RBBB) type aberrant ventricular conduction of alternate atrial premature complexes (APCs), and Q waves of inferior and anterolateral myocardial infarcts of indeterminate age.

Aberrant ventricular conduction was first described by Sir Thomas Lewis in 1910, (1) and Ashman (2,3) and Langendorf (4) subsequently elaborated on its various mechanisms. In what has come to be known as Ashman's phenomenon, because the refractory period after any QRS complex is proportional to the length of the preceding R-R interval, a premature impulse is most likely to find part of the conduction system refractory if its short R-R interval has been preceded by a long R-R interval. Because the right bundle branch (RBB) has the longest transmembrane action potential, and thus the longest refractory period, of any part of the conduction system, (5) supraventricular impulses resulting in a long R-R interval followed by a short R-R interval are likely to produce a RBBB configuration following the short interval.

Inspection of the figure shows a bigeminal rhythm with each of the long R-R intervals measuring 0.71 sec and each of the short R-R intervals, 0.46 sec. What then is the mechanism whereby RBBB occurs only with every other APC? The answer is in the transseptal conduction time, which ranges from 0.06 sec in normals to 0.10 sec in those with diseased ventricles. (2,6) With each APC with aberrant ventricular conduction, the RBB is not depolarized in an antegrade fashion, but instead is depolarized approximately 0.10 seconds later than usual by an impulse that crosses the ventricular septum from left to right and then makes its way into the RBB. As a consequence, so far as the RBB is concerned, the R-R interval from the RBBB complex to the next sinus QRS is less than 0.71 seconds, resulting in a shortened refractory period that is over by the time the next APC-initiated impulse arrives. (7)

The part of the conduction system with the second longest refractory period is the atrioventricular node. Because all of the APCs fall in the relative refractory period of the atrioventricular node, the P-R intervals following APCs (0.20 sec) are slightly longer than the P-R intervals following sinus P waves (0.17 sec).

This young Vietnamese man was first seen two years before this ECG when he presented with chest pain, dyspnea, and an elevated serum troponin I that peaked at 4.33 ng/mL. Coronary arteriography revealed triple-vessel disease with a chronic proximal occlusion of the right coronary artery and a subtotal occlusion by thrombus of the mid portion of the left anterior descending coronary artery, which was stented. His ECG was similar to the one shown here. Echocardiograms then and subsequently have shown a left ventricular ejection fraction of 20% with inferoposterior akinesis, a restrictive filling pattern consistent with elevated left atrial and left ventricular end-diastolic pressures, severe mitral and tricuspid regurgitation, and a pulmonary arterial systolic pressure of 70 mm Hg. His only demonstrated risk factor for coronary arterial disease has been a low HDL that has ranged from 26 to 18 mg/dL (reference [greater than or equal to] 40). His dyspnea both two years ago and on this admission was due to cardiac failure.

REFERENCES

(1.) Lewis T. Paroxysmal tachycardia, the result of ectopic impulse formation. Heart 1910;1:262.

(2.) Ashman R, Byer E. Aberration in the conduction of premature suprventricular impulses. J La State Univ Sch Med 1946;8:62-65.

(3.) Gouaux JL, Ashman R. Auricular fibrillation with aberration simulating ventricular paroxymal tachycardia. Am Heart J 1947;34:373.

(4.) Langendorf R. Aberrant ventricular conduction. Am Heart J 1951;41:700-707.

(5.) Fisch C, Knoebel SB. Electrocardiography of Clinical Arrhythmias. Armonk, NY: Futura;2000:175.

(6.) Katz AM, Pick A. The transseptal conduction time in the human heart. An evaluation of fusion beats in ventricular parasystole. Circulation 1963;27:1061-1070.

(7.) Fisch C, Knoebel SB. Electrocardiography of Clinical Arrhythmias. Armonk, NY: Futura;2000:178-179, 189-190.

D. Luke Glancy, MD; and C. Carmen Ilie, MD

Dr. Glancy is a professor in the Section of Cardiology in the Department of Medicine, Louisiana State University Health Sciences Center, New Orleans (LSUHSC-NO). Dr. Ilie is a fellow in the Section of Cardiology in the Department of Medicine, LSUHSC-NO.
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Title Annotation:ECG of the Month
Author:Glancy, D. Luke; Ilie, C. Carmen
Publication:The Journal of the Louisiana State Medical Society
Date:Sep 1, 2010
Words:735
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