Some causes of ventricular bigeminy are definitely malignant while others are relatively benign.  Certain forms of ventricular bigeminy are virtually diagnostic of specific clinical disorders.  For instance, when correlated with other suggestive signs, the finding of alternating / bidirectional ventricular bigeminy or multiform ventricular bigeminy is all but diagnostic of digitalis intoxication.  (1,3)  At the very least, it should prompt you to investigate a little further and exclude digitalis toxicity as a potential cause.  Bidirectional ventricular bigeminy is defined as a bigeminal pattern where two distinctly different PVC morphologies alternate with one another.  (1)  Dr. Bill Nelson states that “ventricular bigeminy in the setting of atrial fibrillation and digitalis use should always suggest digitalis intoxication."  (2)        

Conversely, a relatively benign form of ventricular bigeminy is one that is caused by the not-too-often-heard-of or understood “Rule of Bigeminy” published way back in 1955:  http://circ.ahajournals.org/content/11/3/422.full.pdf   This term was coined by the late Dr. Richard Langendorf, et al. and is probably initiated by a very simple mechanism of micro-reentry in the ventricles.  I, myself, have seen this mechanism at work numerous times over the years.  An updated research paper published in 2006 by Dr. Ary Goldberger, et al. newly explores this topic and suggests that certain subsets of this cause of ventricular bigeminy are not so benign as they were previously believed to be: http://mprime-arrhythmias.idlab.dal.ca/pub_pdf/rule_2007.pdf

Years ago when I was working as a cardiac monitor technician, we had a patient being monitored on continuous cardiac telemetry who was in chronic ventricular bigeminy for several days and was frequently having their vital signs taken.  I was working several consecutive daily shifts and during that time I would routinely have RNs, LPNs, NAs, and PCTs coming out to the nursing station and asking me what I was getting for a ventricular rate on the telemetry monitor.  At any given time, the patient’s ventricular rate on the monitor was averaging around 80/min but all of the staff at the patient’s bedside were palpating a radial pulse rate of about half of that at right around 40/min.  A question I would frequently get was whether or not the telemetry monitor was factoring in the PVCs when it calculated the ventricular rate.  The answer was “yes”, the monitor does not exclude the PVCs and only count the conducted sinus beats.  All monitors are programmed to include all QRS complexes in the ventricular rate regardless of their origin.  The monitor does not distinguish between electrical and mechanical events.  It is only programmed to monitor electrical waveforms and label the QRS complexes according to anatomic location.  Most brands of telemetry monitors have a specific "yellow" alarm that is generated when ventricular bigeminy is detected.  Additionally, the patient was hooked up to a Dinamap machine which was also registering a heart rate of 40/min.  Despite all of the fuss this was generating amongst the healthcare staff, the patient was asymptomatic.

In a similar case, I recently performed a 12-lead ECG on a female patient a couple of weeks ago and she had an elective ablation for her PVCs because she said that on a few occasions she would develop ventricular bigeminy causing her pulse to drop in the 40’s and consequently she would have syncopal episodes.  Obviously her PVCs were depriving her of adequate perfusion.  I’m quoting from one of my textbooks accompanied by a tracing of ventricular bigeminy:  “The extrasystole [i.e., PVC] was sufficiently early that ventricular filling for it was inadequate and it was mechanically ineffective.  When the [PVC] conducted retrogradely, the resultant atrial contraction occurred during ventricular systole and could not provide contribution to ventricular filling."  (1)

It’s always worth emphasizing that ventricular bigeminy does have its mimics especially when PACs are conducting with ventricular aberration.  Whenever you see what you believe is ventricular bigeminy, you should think of and exclude the possibility of it actually being atrial bigeminy with aberrant ventricular conduction instead.  (4)  In certain patients who are not high-risk, voluntarily opting to have an Electrophysiologist perform an ablation is usually a viable treatment option for palpitations due to frequent PVCs and/or ventricular bigeminy.  A bit of trivia:  The first known use of the term bigeminy (or some choose to spell it bigemini) was in 1923.  Bigeminy comes from the Latin word bigeminus.  It begins with the prefix "bi" which, naturally, means "two".  The geminy (or gemini) is derived again from the Latin geminus and is best known as the astrological / Zodiac sign of Gemini which means "twin" as in the twin brothers.  (1)  All patterns of bigeminy share the same basic characteristic being that the beats are always grouped in pairs. 

REFERENCES / SOURCES:
1.)  Nelson WP, Marriott HJL, Schocken DD. Concepts & Cautions in Electrocardiography. Northglenn: MedInfo Inc., 2007, p. 230, 232, & 380
2.)  http://nelsonsekgsite.com/instruction.htm (click on Ventricular Rhythms PowerPoint presentation)
3.)  Marriott HJL. Marriott’s Manual in Electrocardiography. 2^nd ed. Naples: Trinity Press, 1999, p. 134
4.)  Marriott HJL. Pearls & Pitfalls in Electrocardiography. 2^nd ed. Baltimore: Williams & Wilkins, 1998, p. 52 & 58