Bigeminy means beating in groups of two (gemini means twins) There are four basic mechanisms of bigeminy:
Escape capture bigeminy
Ectopic bigeminy (commonest)
In severe sinus bradycardia, junctional escape beats may occur during the sinus pause, followed by sinus capture. If this occurs in a bigeminal pattern, it is called escape capture bigeminy. Other mechanisms of bigeminy are ectopic bigeminy, block bigeminy and echo bigeminy. Each mechanism can have different types. Ectopic bigeminy could be atrial ectopic bigeminy or ventricular ectopic bigeminy. Block bigeminy could be due to 3:2 conduction at the sinoatrial junction or atrioventricular junction. The escape beat in escape capture bigeminy could be either junctional or ventricular. A rare form of block bigeminy is one in which atrial flutter or tachycardia gets conducted with alternate 4:1 and 2:1 ratio. This can also occur in other patterned sequences of conduction ratio. In echo bigeminy, the rhythm is typically a junctional rhythm. Retrograde conduction of the junctional beat gets echoed back and produces another QRS complex. A retrograde P wave is sandwiched between the QRS complexes. Retrograde P waves are negative in inferior leads.
Atrial parasystole has been reported as a rare cause of escape capture bigeminy. Sinus capture was followed by capture of the atria by a parasystolic focus . Reference
Friedberg HD, Schamroth L. Atrial parasystole. Br Heart J. 1970 Mar;32(2):172-80.