Synchronized Cardioversion: What Happened?

Source:  Synchronized Cardioversion: What Happened?    Tag:  lead markers
EMS was dispatched for a 62 year old male with an altered mental status. Upon their arrival they found the patient to be non-communicative, responsive to verbal stimuli, in moderate respiratory distress, with pale, diaphoretic skin, and weakly palpable radial pulses. The patient was placed on the monitor during their initial assessment:

Wide complex tachycardia of unknown etiology.
A blood pressure was unobtainable, however a pulse of 150 was palpable at the carotid. Labored respirations were present, with clear breath sounds bilaterally. The patient had an extensive cardiac history, renal failure, and insulin dependent diabetes mellitus. The patient's blood sugar was 300 mg/dL.

A 12-Lead was obtained and interpreted as presumed ventricular tachycardia:

Wide complex tachycardia, interpreted as presumed ventricular tachycardia.
Differentials of a wide complex tachycardia at 150 bpm include: ventricular tachycardia, SVT with aberrancy, sinus tachycardia with aberrancy, and 2:1 atrial flutter with aberrancy. No previous 12-Lead was available for comparison.

Given the presence of a WCT with hemodynamic instability the patient was prepped for synchronized cardioversion. Combo-pads were placed anterio-laterally, the Sync button was pressed, and sync markers were noted with each QRS complex.

The patient was then synchronized cardioverted at 100J biphasic:

100J synchronized cardioversion.
A rhythm change was noted on the monitor:

Ventricular fibrillation post cardioversion.
With ventricular fibrillation present, the paramedic disabled synchronization and delivered a 200J biphasic shock:

200J defibrillation of ventricular fibrillation.
After defibrillation, the patient regained consciousness and palpable radial pulses were present. Emergency transport was initiated. During transport, a sustained run of ventricular tachycardia occurred and the patient was given 100 mg lidocaine IV with a subsequent conversion of a sinus rhythm (not captured). The patient experienced multiple episodes of non-sustained ventricular ectopy during transport.

In this case the paramedic did not appreciate that oversensing was present from the cardiac monitor's display. It was not until after the summary printed that the ineffective synchronization was discovered.

Oversensing during synchronized cardioversion--highlighted in red--resulting in therapy delivery during the vulnerable period.
As the ventricular myocardium repolarizes, it may not do so homogeonously. This window of non-uniformity, with both absolutely and relatively refractory myocardium present is known as the Vulnerable Period. Electrical stimulation during the vulnerable period of ventricular repolarization may result in ventricular tachyarrhythmias.

Illustration of the vulnerable period of ventricular repolarization. Adapted from Reilly et al. 1998 pp 188 Fig 5.19.
This is best appreciated during episodes of a prolonged QT interval. An early-cycle premature ventricular contraction may result in the so called "R-on-T" phenomenon initiating Torsades de Pointes.

A prolonged QT interval and an "R-on-T" PVC resulting in Torsades de Pointes. Used with permission from Dr. Ken Grauer's ECG Web Brain.
In this case, the electrical stimulation was provided by inappropriately synchronized biphasic shock. By default the synchronization used Lead II, which featured proportionately smaller negative complexes when compared to their T-waves. Sometimes atrial tachyarrhythmias, such as atrial flutter or atrial fibrillation, may produce deflections sufficient to trigger R-wave deflection as well.

Oversensing of atrial fibrillation. Adapted from Resuscitation 82 (2011):135-136,Fig.1.
Appropriate lead section is important when performing synchronized cardioversion in order to avoid delivering the therapy while the myocardium is vulnerable. If synchronization is not accurate the operator of the cardiac monitor should switch leads, increase the gain, or change pad placement.


  • Reilly J. Patrick. Applied Bioelectricity: From Electrical Stimulation to Electropathology. Springer-Verlag: New York (1998); pp 188.
  • Dr. Ken Grauer's ECG Web Brain. Accessed online 26 July 2012. [https://www.kg-ekgpress.com/]
  • Sodeck GH, Huber J, Stollberger C. Letter to the Editor: Electrical cardioversion - Misinterpretation of the R-wave. Resuscitation 82 (2011): 135-136. [PubMed]