last authored: July 2013, Brandon Cook
Introduction
Using electricity to revert dangerous heart rhythms is one of the foundational treatments in cardiovascular resuscitation. Unstable or pulseless ventricular tachycardia, or ventricular fibrillation, are rapidly life-threatening heart rhythms. When these rhythms occur, electrical cardioversion and defibrillation can be used to depolarize the bulk of myocardial tissue, interrupting pathological re-entry circuits and allowing the heart’s normal tissues to regain pacemaker control.
With cardiac arrest, early defibrillation is critical for maximizing chances of survival. With bystander CPR given, the chance of survival decreases by 3-4% per minute until defibrillation is given, or 7-10% per minute if CPR is not given.
Defibrillation does not restart the heart; rather, it temporarily stops all electrical activity, allowing existing the heart’s existing pacemaker to reset and begin a perfusing rhythm again.
This rhythm is typically quite slow initially, which is why CPR is required for several minutes following a shock.
Automatic External Defibrillation
Automatic External Defibrillators (AEDs) are small, affordable devices for use in cardiac arrest. AEDs are intended to be used with minimal training by members of the public. Audible instructions to follow a protocol similar to this one are given when an AED is turned on:
Power on
Attach electrodes to patient
- one on upper right chest, beside sternum and below clavicle
- one to left of nipple, just below left armpit
Analyze rhythm
- ensure no one is touching patient
Shock if advised
- ensure everyone is clear of patient; state “I’m clear, you’re clear, we’re all clear” and then look at the patient/bed
It is vitally important to continue CPR with as few interruptions as possible. Do not stop CPR until pads are connected and in place. Do not stop CPR to troubleshoot AED. As soon as the shock is delivered, start CPR immediately with chest compressions for 2 minutes until the AED prompts a rhythm check. At this point, the AED will again determine whether or not a shock is indicated.
There are many models of AEDs that all operate in a similar manner.
Shock first, or CPR first?
If arrest is witnessed with an AED available, CPR should be started immediately and continued with as few interruptions as possible, while the AED should be set up and used as soon as possible. If an AED is not immediately available, provide CPR until it arrives, applying the pads and initiating the AED as soon as possible.
Special Considerations
Hairy chest: Attempt pad placement by pressing firmly. If connection is insufficient, a ‘check pads or electrodes’ message will occur. Quickly remove pads to remove hair; if this does not work, quickly shave with a razor if available. Use a new set of pads to try again.
Pacemaker or implanted cardioverter defibrillator (ICD): Avoid placing AED pads directly on top of pacemaker or ICD; place at least one inch away. If ICD is delivering shocks, delay AED attachment 30 to 60 seconds to allow the ICD to complete its treatment cycle.
Transdermal medication patch: Avoid placing AED pads directly on top of patch. Remove patches on the chest and wipe area clean, wearing gloves to protect yourself from potentially harmful medication. Act quickly to reduce delays in compressions.
Victim is in water, snow, or on ice: AEDs can be used if the patient is lying in minimal water, or on snow or ice. If there is sufficient water that you can splash the patient with it, they should be moved to a drier environment.
Chest is wet: quickly dry victim’s chest before attaching AED.
Children: For children ages 1 to 8, an AED with a pediatric dose attenuator system or a manual defibrillator should be used if available. If an AED with a dose attenuator is not available, an AED without a dose attenuator may be used instead.
Infants: A manual defibrillator is preferred for infants. If one is not available, an AED with a pediatric dose attenuator should be used. If neither is available, an AED without a dose attenuator may be used as a last resort, with adult pads placed on the front and back of the chest.
Manual Defibrillation
Manual defibrillators, unlike AEDs, allow a medical professional to determine the precise timing and energy used.
Monophasic defibrillators deliver high energy (up to 360 J) current in only one direction, while biphasic models use a variety of biphasic waveforms to deliver lower energy (120 to 200J) current in two directions. While biphasic defibrillators use lower energy doses than monophasic defibrillators, there is no evidence to show that any one waveform is more effective than others. Most newer defibrillators are biphasic.
Paddles may be used, but require electrode paste to prevent burns. Self-adhesive electrode pads may also be used, and these are preferred as they reduce the risk of arcing, can also be used for monitoring, and allow for more rapid shock delivery.
Once a shockable rhythm is identified, charge the device while continuing CPR. Stop compressions before the shock and ensure everyone is clear of the patient. Immediately begin CPR after the shock. If the first shock fails to restore a perfusing rhythm, it is more effective to promptly resume CPR than to deliver additional shocks before resuming CPR. If the first shock has failed, the incremental benefit of additional shocks is low, and the increased delay before resuming CPR is harmful to the patient. It is important to resume CPR immediately after a shock has been delivered, as interruption of CPR is associated with a decreased probability of successfully converting VF to a perfusing rhythm.
Cardioversion
Synchronized cardioversion is the delivery of a shock for an unstable heart rhythm when a pulse is present. The shock is synchronized to occur with ventricular contraction (the QRS complex), rather than during repolarization (the T wave), as the latter can result in cardiac arrest.
Cardioversion is used to treat supraventricular tachycardia (SVT), atrial fibrillation, atrial flutter, and atrial tachycardia. Typically, atrial flutter and SVT require lower energy doses than atrial fibrillation (50 to 100 J versus 100 to 200 J, respectively, when using a biphasic defibrillator). Monomorphic, or regular, VT in patients with a pulse is also treated with synchronized cardioversion.
Pulseless VT is treated with unsynchronized defibrillation, as well as ventricular fibrillation. Defibrillators may not recognize QRS complexes in VF, resulting in delayed shock delivery if synchronized cardioversion is attempted on a patient in VF.