Clinical Marketing Manager Blaine Krusor summarizes the findings of this important study.

“Timing of defibrillation shocks for resuscitation of rapid ventricular tachycardia: Does it make a difference?” I. Turner, S. Turner. Resuscitation, Jan 2009.

Why is shock timing (not time to shock) in an AED potentially important?

Timing of a biphasic defibrillation shock is important in two ways. It’s well known that faster shock times increase the likelihood of successful defibrillation. For certain common types of ventricular rhythms in sudden cardiac death (SCD), it’s equally important that the shock timing be accurate. Monomorphic ventricular tachycardia (VT w/ a uniform ECG shape) is one of the three potential lethal rhythms that occur during a typical SCD event. This type of ventricular rhythm is considered to be electrically organized, originating from scar tissue, can vary in rates (140 – 200 bpm) and will vary in symptom otology based on EF% (overall heart function). Left untreated, monomorphic VT progresses to polymorphic VT (multiple ECG shapes), which then progresses to VF.

Considering that LV dysfunction (poor EF%) is known to be a powerful predictor of SCD, it’s likely that those who have a monomorphic VT event will be symptomatic and experience loss of consciousness (LOC). An AED capable of recognizing and appropriately treating this rhythm is needed.

It’s well established that an R-wave synchronized shock (shock within the QRS) is the most effective therapy for monomorphic VT and corresponding LOC. The electrically organized ventricular contractions (QRS) in VT are secondary to an abnormal re-entry circuit which often requires precise shock timing. This is standard VT therapy programming in nearly every ICD (implantable intracardiac defibrillator). Furthermore, unsynchronized shocks in VT can have a pro-arrhythmic effect resulting in progression to VF. This results in a second shock which delays conversion to perfusing rhythms.

+ Click to enlarge images

What did the researchers want to find?

• The researchers performed a retrospective study addressing the relative effectiveness of monophasic shock timing in rapid VT. The shock timing relative to the QRS complex (R-synchronized) was used to define whether each shock was acting as a ‘synchronized’ or unsynchronized’ shock.

What did they find?

• Shocks within the QRS complex (R-synchronized) had a success rate of 93% compared to a success rate of 42% for shocks outside the QRS complex (unsynchronized).

• Out of 271 patients; 144 had induced monomorphic VT; 42 of them required defibrillation:

30 patients were given shocks within a
100ms window of the peak R-wave deflection (QRS):
-after the shock 28 patients converted to a perfusing rhythm
-after the shock 2 patients deteriorated from VT to VF (6%)

12 patients received shocks outside this window:
-after the shock 5 converted to a perfusing rhythm
-after the shock 7 deteriorated to VF (58%)

Potential impacts?

The rate of occurrence of VT is significant and warrants consideration for appropriate therapy options, even in AEDs. Patients with SCD episodes caused by sustained monomorphic VT often have cardiac output inadequate to maintain consciousness but remain in VT for considerably longer periods, with blood flow that’s marginally sufficient to maintain viability. Therefore, defibrillation therapy for this type of lethal rhythm should be appropriately programmed.

Get the full study at http://www.elsevier.com

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