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When possible, delivering a countershock synchronized with the intrinsic QRS complexes is preferred. Synchronization helps avoid depolarization during the vulnerable phases of repolarization, theoretically decreasing the risk of postcountershock VF. During most dysrhythmias, the defibrillator unit senses the underlying QRS pattern and delivers the shock at the appropriate time. When the rhythm is extremely fast or irregular or the QRS complexes are markedly abnormal (i.e., very wide or small), sensing is difficult. In these cases, an unsynchronized countershock is appropriate. Electrophysiological data do not support the notion that this will increase the likelihood of VF. If postcountershock VF occurs, repeat countershock is usually successful in restoring an organized rhythm.

The usual controversy surrounding field countershock is the awake unstable patient. Medical oversight must clearly communicate the need for this unpleasant but lifesaving intervention for appropriate patients. Sedation with a benzodiazepine before countershock may improve patient comfort. However, countershock should not be delayed for unstable patients while awaiting clinical sedation.