Monday, 14 November 2016

Ventricular Bigeminy

Bigeminy or bigemini is a heart rhythm problem in which there is a continuous alternation of long and short heart beats. Most often this is due to ectopic beats occurring so frequently that there is one after each sinus beat. The two beats are figuratively two twins (hence bi- + gemini). The ectopic beat is typically a premature ventricular contraction (PVC). For example, in ventricular bigeminy, a sinus beat is shortly followed by a PVC, a pause, another normal beat, and then another PVC. In atrial bigeminy, the other "twin" is a premature atrial contraction (PAC).

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. The constant interval between the sinus beat and PVC suggests a reentrant etiology rather than spontaneous automaticity of the ventricle.

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.