Bigeminy

Bigeminy as seen on a 12 lead ECG

A rhythm strip demonstrating bigeminy

simple ECG of a Supraventricular Bigeminy

Bigeminy (Latin: bi-two, gemini-twins) is a descriptor for a heart arrhythmia in which there is a continuous alternation of long and short heart beats. Most often this is due to ectopic heart beats occurring so frequently that there is one after each sinus beat, typically premature ventricular contractions (PVCs). For example, a sinus beat is shortly followed by a PVC, then followed by a pause. The normal beat then returns, only to be followed by another PVC.^[1]

Etiology

After any PVC there is a pause that can lead to the development of bigeminy. A PVC wavefront often encounters a refractory AV node that does not conduct the wavefront retrograde. Thus the atrium is not depolarized and the sinus node is not reset. Since the sinus p wave to PVC interval is less than the normal P-P interval, the interval between the PVC and the next p wave is prolonged to equal the normal time elapsed during two P-P intervals. This is called a "compensatory" pause. The pause after the PVC leads to a longer recovery time which is associated with a higher likelihood of myocardium being in different stages of re-polarization. This then allows for re-entrant circuits and sets up the ventricle for another PVC after the next sinus beat.^[2] The constant interval between the sinus beat and PVC suggests a reentrant etiology rather than spontaneous automaticity of the ventricle.^[3]

Atrial premature complexes (APCs) do not have a compensatory pause since they reset the sinus node but atrial or supraventricular bigeminy can occur. If the APCs are very premature, the wave front can encounter a refractory AV node and not be conducted. This can be mistaken for sinus bradycardia if the APC is buried in the t wave since the APC will reset the SA node and lead to a long P-P interval.^[2]

Rule of bigeminy

When the atrial rhythm is irregular (as in atrial fibrillation or sinus arrythmia) the presence of bigeminy depends on the length of the P-P interval and happens more frequently with a longer interval. As with post PVC pauses, a longer P-P interval leads to a higher chance of re-entrant circuits and thus PVCs. The term "rule of bigeminy" is used to refer to the dependence of bigeminy on the ventricular cycle length in irregular rhythms.^[3]

Classification

There can be similar patterns depending on the frequency of abnormal beats. If every other beat is abnormal, it is described as bigeminal. If every third beat is aberrant, it is trigeminal; every fourth would be quadrigeminal. Typically, if every fifth or more beat is abnormal, the aberrant beat would be termed occasional.^[1]

Bigeminy is contrasted with couplets, which are paired abnormal beats. Groups of three abnormal beats are called triplets and are considered as a brief run of non-sustained ventricular tachycardia (NSVT) and if the grouping last for more than 30 seconds, it is ventricular tachycardia (VT).^[2]

Treatment

In people without underlying heart disease and who do not have any symptoms, bigeminy in itself does not require any treatment. If it does become symptomatic, beta-blockers can be used to try and suppress ventricular ectopy. Class I and III agents are generally avoided as they can provoke more serious arrhythmias.^[4]

References

1. 1 2 Shvilkin, Ary L. Goldberger, Zachary D. Goldberger, Alexei (2013). Goldberger's Clinical Electrocardiography: A Simplified Approach (8th ed.). Philadelphia, PA: Elsevier/Saunders. ISBN 9780323087865. 
2. 1 2 3 Wagner, Galen S. (2001). Marriott's Practical Electrocardiography (10th ed.). Philadelphia, PA: Williams & Wilkins. ISBN 0683307460. 
3. 1 2 Langendorf, R.; Pick, A.; Winternitz, M. (1 March 1955). "Mechanisms of Intermittent Ventricular Bigeminy: I. Appearance of Ectopic Beats Dependent Upon Length of the Ventricular Cycle, the "Rule of Bigeminy"". Circulation 11 (3): 422–430. doi:10.1161/01.CIR.11.3.422. 
4. ↑ Papadakis, Maxine A.; Stephen J. McPhee; Michael W. Rabow, eds. (2013). Current Medical Diagnosis and Treatment 2014 (53 ed.). New York: McGraw-Hill Medical. ISBN 9780071806336.