Cardiac arrest occurs when the heart unexpectedly stops functioning, requiring immediate intervention to restore its pumping action and blood flow. Defibrillation, the delivery of an electrical shock to the heart, is a life-saving procedure in these situations.
Understanding Ventricular Fibrillation and Defibrillation
Ventricular fibrillation (VFib) is a chaotic electrical state within the heart’s lower chambers, the ventricles. Instead of coordinated contractions that pump blood effectively, the ventricles merely quiver, leading to an immediate cessation of blood circulation. This disorganized electrical activity prevents the heart from performing its essential function.
Defibrillation acts as a therapeutic electrical shock designed to reset this chaotic rhythm. It delivers a controlled electrical current that depolarizes a significant portion of the heart’s cells simultaneously. This aims to interrupt the abnormal electrical circuits that perpetuate VFib, allowing the heart’s natural pacemaker to potentially re-establish a normal, organized rhythm. Prompt defibrillation is important, as the chances of survival diminish rapidly with every minute of delay.
Energy Settings for Defibrillation
The energy delivered during defibrillation is measured in joules (J). Defibrillators use different technologies, primarily monophasic and biphasic, which influence the required energy settings. Monophasic defibrillators deliver electrical current in a single direction across the heart. For adults, the recommended energy for a first monophasic shock is 360 joules, with subsequent shocks typically delivered at the same level.
Biphasic defibrillators deliver current that flows in one direction and then reverses. This makes biphasic devices more efficient, often achieving effective defibrillation at lower energy levels. For adult patients, the recommended initial dose for biphasic defibrillation typically ranges from 120 to 200 joules, depending on the device. Biphasic waveforms are generally preferred due to their higher first-shock success rates compared to monophasic waveforms.
Factors Influencing Defibrillation Energy
The precise joule setting for defibrillation can vary based on several factors. Patient size and age are significant considerations, especially for pediatric patients. Children require substantially lower energy levels than adults; initial shocks for pediatric patients typically start at 2 joules per kilogram of body weight, with subsequent shocks escalating to 4 joules per kilogram, up to a maximum of 10 joules per kilogram or an adult dose. Adult defibrillation energy requirements, however, do not typically have a direct relationship with body size.
The type of defibrillator used, whether monophasic or biphasic, also dictates the energy selection, as biphasic devices achieve efficacy with lower joule settings. When a first shock is unsuccessful, medical guidelines address how subsequent shocks should be delivered. Some protocols recommend increasing the energy level for subsequent shocks, for instance, an escalating biphasic regimen might use 200, 300, and then 360 joules for successive attempts to improve success for refractory VFib. Other guidelines suggest repeating the initial energy level, noting that transthoracic impedance, or resistance to current flow, can decrease with repeated shocks, potentially increasing the effective current delivered even at the same energy setting. Underlying patient conditions, such as acidosis or prolonged hypoxia, can negatively influence the success of defibrillation.
Immediate Care After Defibrillation
Defibrillation is one component of the broader resuscitation effort for cardiac arrest. Immediate care after defibrillation is important, regardless of whether a normal heart rhythm is restored. The most important action following a defibrillation attempt is to promptly resume chest compressions.
Healthcare providers are instructed to continue cardiopulmonary resuscitation (CPR) for two minutes immediately after a shock, without pausing to check for a pulse or assess the heart rhythm. This uninterrupted CPR minimizes pauses in blood flow to the brain and other vital organs, which is crucial for overall survival. Many automated external defibrillators (AEDs) are programmed to provide voice prompts guiding rescuers to resume chest compressions immediately after a shock. Further interventions, such as securing an airway and administering medications like epinephrine or amiodarone, may also be necessary as part of comprehensive post-resuscitation care. Continuous monitoring of the patient’s condition and rapid transport to a hospital are essential next steps.