Thursday, March 15, 2007

In Cardiac Arrest, Higher Defibrillation Energies Are Better

Take any drug, give more of it, then usually you’ll see a bigger physiologic response. Most of us in medical school knew this as a dose-response curve.

As a cardiac electrophysiologist, I have always been interested (but never had the patience to test) why the American Heart Association’s guidelines were always suggesting “start low and work your way up” with defibrillation (shock) energies when a patient has the life-threatening heart rhythm disturbance, ventricular fibrillation. During ventricular fibrillation, the heart is only barely quivering and generates no effective cardiac output or blood pressure. Time is of the essence when correcting this arrhythmia to improve patient survival: without cardiopulmonary resuscitation (CPR) during ventricular fibrillation, irreversible brain injury can begin in just four to five minutes.

So it was refreshing to see the results of the effectiveness of out-of-hospital defibrillation with two different energy regimens tested side-by-side by Canadian researchers in this month’s Circulation. They compared fixed lower (150J-150J-150J) defibrillation versus higher escalating doses of defibrillation (200J-300J-360J) in 221 patients requiring more than one shock with a biphasic defibrillator during out-of-hospital cardiac arrest. Their results were predictable: higher energies work better. Much better. 25% vs. 37% better (p<0.035).

The dose-response curve held true: improved success was seen when higher defibrillation energies were applied.

Now the question becomes, why not just start delivering shocks at the maximum output of the defibrillators during cardiac arrest? If a 10% improvement was seen with escalating doses of defibrillation, could additional success be identified using a fixed maximum defibrillation energy? In animals, it has been demonstrated that ventricular fibrillation in the setting of acute ischemia (lack of blood flow to the heart) requires higher energies to achieve successful defibrillation than non-ischemic ventricular fibrillation.

I guess we’ll have to wait for another study for my answer. But for now, dial up those defibrillators when shocking ventricular fibrillation.