Electrical Pattern and Duration of Ventricular Fibrillation. Ventricular fibrillation is the most common ECG pattern found during cardiac arrest in adults, and the only effective treatment is electrical defibrillation. Defibrillation should be performed as soon as the ventricular fibrillation is diagnosed and equipment is available. Immediate defibrillation is only effective when applied within 4 to 5 minutes of collapse. Otherwise, a brief period of 2 to 3 minutes of chest compressions before defibrillation is necessary.
The most important controllable determinant of failure to resuscitate a patient with ventricular fibrillation is the duration of fibrillation (The fibrillating heart has a high oxygen consumption).
If defibrillation occurs within 1 minute of fibrillation, CPR is not necessary.
Defibrillation should not be delayed for epinephrine administration (There is no evidence that epinephrine improves the success of defibrillation or decreases the energy setting needed for defibrillation).
Fibrillation amplitude on an ECG lead varies with the orientation of that lead to the vector of the fibrillatory wave (A flat line can be present if lead is oriented at right angles to the fibrillatory wave).
A nonfibrillatory rhythm will not respond to defibrillation.
The typical defibrillator consists of a variable transformer that allows selection of a variable voltage potential, an AC to DC converter to provide a direct current that is stored in a capacitor, a switch to charge the capacitor, and discharge switches to complete the circuit from the capacitor to the paddle electrodes.
Defibrillation is accomplished by direct current passing through a critical mass of myocardium, resulting in simultaneous depolarization of the myofibrils.
Even at a constant delivered energy, the delivered current (critical determinant of defibrillation) is decreased as impedance (resistance) increases.
Repeated defibrillation with high-energy shocks, especially if repeated at short intervals, may result in myocardial damage.
Current recommendations for adults are to use 200 J for the initial shock followed by a second shock at 200 to 300 J if the first is unsuccessful. If both fail to defibrillate the patient's heart, additional shocks should be given at 300 to 360 J.