When a rhythm goes bad, the treatment is a number in joules, and the number depends on what the rhythm is, whether you are shocking in sync or not, what device you are holding, and how much the patient weighs. That is a lot to assemble in the first thirty seconds of a code, which is exactly why it is worth having settled in advance.
Defibrillation: the unsynchronized shock
Defibrillation is for ventricular fibrillation and pulseless ventricular tachycardia, an unsynchronized shock delivered as soon as the device is charged. In adults, biphasic energy follows the manufacturer's recommended dose, typically 120 to 200 joules, defaulting to the maximum if you are unsure; an older monophasic device uses 360. Children start at 2 J/kg, move to 4 J/kg for the second shock, and may go above 4 J/kg thereafter without exceeding 10 J/kg or the adult dose.
Synchronized cardioversion: timed to the R wave
An unstable tachyarrhythmia that still has a pulse calls for synchronized cardioversion, with the shock timed to the R wave to avoid the vulnerable period. Adult doses run lower and rhythm-specific: roughly 50 to 100 joules for a narrow regular tachycardia, 120 to 200 biphasic for atrial fibrillation, and about 100 for monomorphic VT with a pulse. Children take 0.5 to 1 J/kg, escalating to 2 J/kg. The one easy-to-forget step is arming the sync mode.
Know your device
Biphasic defibrillators dominate modern practice and deliver effective rhythms at lower energy than the old monophasic units, so the right starting number is the one printed on the device in front of you. The guidelines set the framework; the manufacturer sets the dose.
Where Master Anesthesia comes in
Master Anesthesia turns the patient's weight into the actual joule numbers for defibrillation and cardioversion, initial and subsequent, adult and pediatric, so the dose is on the screen instead of in your head when the monitor alarms. See the Defibrillation Energy Calculator.