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Aside from sinus tachycardia, all unstable patients with a wide or narrow‐complex tachydysrhythmia deserve countershock(s), irrespective of the exact source, ventricular or supraventricular. The QRS duration will help dictate care after countershock but does not fundamentally drive the initial care for unstable patients with tachydysrhythmias.

The initial energy level used to treat tachycardias is based on the QRS pattern. If the QRS pattern is regular or nearly regular in any unstable patient with a tachydysrhythmia and a palpable pulse, synchronized cardioversion with 100 J should be used, followed by stepwise energy increases to 200 J with a biphasic device or 360 J with a monophasic device, if necessary. Some rhythms may require less energy, but attempts to titrate this lifesaving therapy for unstable patients is of little practical benefit. Synchronized countershock is recommended to avoid postcountershock ventricular fibrillation (VF). However, sensing problems often make reliable identification of the QRS complex needed for synchronization impossible. We recommend an unsynchronized shock promptly if any sensing problem occurs. Any patient without pulses and an irregular tachydysrhythmia should be immediately given a high‐energy unsynchronized countershock.

Patients with internal pacemakers or automatic implantable cardioverter defibrillators (AICDs) are still at risk of cardiac dysrhythmias. Although meant to cardiovert dysrhythmias, AICDs do not always convert these rhythms, and sometimes these devices deliver shocks inappropriately. If a patient has an unstable tachydysrhythmia and the AICD is not firing or is ineffective, externally cardiovert as previously recommended, with pads in the anterior‐posterior configuration and 10 cm away from the internal device pouch. Postconversion care with medical therapy will be unaffected.

If an AICD is repeatedly firing absent a ventricular dysrhythmia, a magnet held over the device may inactivate it, simplifying patient care and improving patient comfort (see Chapter 11). Fortunately, these are rare events.

If countershock fails in an unstable patient with a wide‐complex tachycardia, give either IV amiodarone (150 mg or 5 mg/kg) or lidocaine (100 mg or 1‐2 mg/kg) as a bolus and repeat the countershock. The ALIVE trial and recent American Heart Association guidelines recommend amiodarone as the first‐line agent in unstable, and especially pulseless, wide‐complex tachydysrhythmia [1, 9]. Lidocaine is still the easiest to deliver quickly, but is considered a second‐line agent due to variable success in terminating ventricular tachycardia (VT).

If the QRS complexes are chaotic, the most common diagnosis is atrial fibrillation. When chaos and a QRS duration of more than three small boxes appear together, atrial fibrillation with altered conduction is the diagnosis. All unstable fast chaotic rhythms should be cardioverted with 100 J, attempting to synchronize first, recognizing that synchronization may fail and an unsynchronized countershock may be required. As always, titrate energy up as needed. No postcountershock medications are needed.

One practical point, if regularity versus irregularity cannot be established during assessment of a patient with an unstable wide or narrow tachydysrhythmia, 100 J remains an appropriate starting energy level for countershock. Similarly, if simplicity of treatment protocols is sought, 100 J is reasonable for all unstable nonsinus tachycardias, because the extra energy delivered to the rapid atrial fibrillation patient is unlikely to cause harm or worsen discomfort compared to 50 J.