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Defibrillation of ventricular fibrillation or ventricular tachycardia (VF/VT) is the most effective intervention for resuscitation from cardiac arrest. Ideally, automated external defibrillators (AEDs) are present at the site of the arrest for use by willing trained or untrained bystanders, perhaps at the prompting of the 9‐1‐1 call taker. All medical first responders and BLS personnel, including all BLS ambulances, should be equipped with AEDs. Most ALS clinicians deliver shocks by manually operating a cardiac monitor‐defibrillator after determining the patient has a shockable rhythm.

An important technical consideration is the type of electrical waveform delivered by the defibrillator [19]. Older defibrillators use monophasic electrical current. In this mode, the device delivers electrical current in a single direction only. To compensate for increased impedance (electrical resistance), older protocols specified escalating energy levels for each successive rescue shock. Now that shocks are given one at a time, each shock should be at the maximum energy output of the specific device.

Defibrillators that are more recent use a biphasic waveform, in which electrical current flows first in one direction, then in the opposite direction. This modality more thoroughly eradicates electrical activity in the heart and so more effectively “defibrillates” the myocardium. Biphasic defibrillators measure the impedance across the chest and adjust the voltage and/or duration of current appropriately. Different models use different patterns of delivered current, most often using a rectilinear or truncated exponential waveform. Compared with monophasic defibrillators, biphasic defibrillators demonstrate higher rescue shock success at lower energy levels and have been associated with increased rates of ROSC [20]. Current research is looking at the effectiveness of higher frequency waveforms [21].

Energy levels for biphasic defibrillators are device specific because each model has different waveform and delivery characteristics that affect shock efficacy. Although some manufacturers endorse nonescalating low‐energy shocks (150 J), recent data suggest that higher energy biphasic shocks may increase success without impairing cardiac function [22].

Another consideration is the interface between BLS and ALS defibrillation equipment. Some AEDs can be converted to manual mode by ALS personnel. This feature is an important logistical consideration because switching from the BLS AED to the ALS defibrillator may cause delays. Some brands use the same defibrillation pads on BLS and ALS models, allowing personnel to simply unplug the connector from one device and plug into the other. The EMS physician should be aware that the AED “analyze and shock” algorithm might add 49 to 59 seconds of hands‐off time, although this may be shorter when used by professional rescuers [23–25].

This is an important consideration when ALS rescuers care for a patient with an AED already attached. Technology to analyze rhythm during compressions is under development. One method to minimize hands‐off time is to continue chest compressions until just before defibrillation. Recent data suggest that continuing chest compressions during defibrillation may be feasible [26]. However, standard examination gloves may be insufficient protection from electrical shock during external defibrillation [27]. Balancing safety and minimizing hands‐off time remains an active area of research.

A current scientific controversy is whether initial defibrillation should precede or follow an initial course of chest compressions. Prior AHA ECC algorithms specified rescue shocks first for VF, regardless of arrest duration [28]. However, several factors support performing initial chest compressions before rescue shocks, including the prolonged duration of most out‐of‐hospital cardiac arrests before treatment is initiated, which leads to the depletion of myocardial high‐energy phosphates, cellular damage resulting from accumulated free radicals, and the development of severe acidosis [29, 30]. Theoretically, a period of chest compressions may perfuse the heart and reduce the severity of these anomalies, better preparing the heart for defibrillation.

The largest randomized clinical trial between chest compressions before rhythm analysis and immediate rhythm analysis showed no difference in the rate of good neurologic outcome between groups (19.4% in early analysis vs. 18.5% in chest compressions before analysis cohorts) [31]. Older studies demonstrated improved outcomes in prolonged (>4 minutes) VF arrests when chest compressions were delivered before defibrillation [32, 33]. Current ACLS guidelines recommend immediate defibrillation in clinician‐witnessed cardiac arrests or those when a defibrillator is immediately available, but delivering chest compressions for about 2 minutes before rhythm analysis otherwise [1].