A defibrillator delivers a controlled electrical shock (defibrillation) to the heart. This intervention is used during sudden cardiac arrest caused by uncoordinated electrical rhythms, such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (pVT). These chaotic rhythms prevent the heart muscle from contracting effectively, halting blood flow. The shock aims to momentarily stop all electrical activity, allowing the heart’s natural pacemaker a chance to reset and restore an organized rhythm. Successful outcomes depend heavily on the immediate and structured steps that follow the electrical discharge.
Immediate Resumption of Chest Compressions
The most important step immediately following a defibrillation attempt is the instant resumption of high-quality chest compressions. There should be no delay to check for a pulse or analyze the heart’s rhythm immediately after the shock. The focus shifts back to maintaining blood flow to the brain and heart muscle.
A brief pause in compressions, even for a few seconds, causes a significant drop in coronary perfusion pressure, which drives blood into the heart muscle. Resuscitation efforts emphasize minimizing these interruptions to maximize the chest compression fraction (the proportion of time spent actively compressing the chest). Rescuers should aim to keep this pause under 10 seconds, ideally restarting compressions instantly.
The mechanical action of chest compressions helps circulate blood that was stagnant during the shock and perfuses the heart muscle. This support is necessary because, even if the shock was successful, the heart muscle often takes time to regain enough energy to pump effectively. Resuming compressions ensures that oxygen and energy substrates are delivered to the heart, preparing it for the next resuscitation cycle.
The Two-Minute CPR Cycle and Rhythm Check
Following the resumption of compressions, the resuscitation effort enters a structured, two-minute cycle of cardiopulmonary resuscitation (CPR). This period of uninterrupted, high-quality chest compressions and ventilations circulates oxygenated blood and prepares the heart for subsequent interventions. High-quality compressions are delivered at a rate of 100 to 120 per minute, with a depth of at least two inches in an adult, allowing for full chest recoil.
The two-minute cycle serves as the interval between planned rhythm checks and subsequent defibrillation attempts. This fixed duration simplifies the process and ensures the patient receives a sustained period of perfusion. This interval ensures the heart muscle has the best chance to recover and respond to the next action, whether that is a shock or medication.
At the conclusion of the two-minute cycle, the team pauses briefly for a rhythm check, which must take no longer than 10 seconds to minimize the interruption. The analysis determines the heart’s electrical status, guiding the next decision. This determines whether the rhythm remains shockable (VF or pVT) or has converted to a non-shockable rhythm, such as asystole or pulseless electrical activity (PEA). If the rhythm is still shockable, the defibrillator is charged immediately for the next attempt.
Pharmacological Support During Resuscitation
Medications are administered during the two-minute CPR cycle to improve the chances of successful resuscitation. The primary class of drugs used is vasopressors, specifically Epinephrine (adrenaline), which improves blood pressure and blood flow to the heart and brain. Epinephrine acts by stimulating alpha-adrenergic receptors, causing vasoconstriction that shunts blood toward the central organs.
For a shockable rhythm, Epinephrine is typically administered after the second or third unsuccessful defibrillation attempt and repeated every three to five minutes. This timing prioritizes initial defibrillation efforts before drug introduction. If the cardiac arrest presents with a non-shockable rhythm (asystole or PEA), Epinephrine is administered as soon as intravenous or intraosseous access is established.
A second class of drugs, antiarrhythmics, is reserved for refractory rhythms that continue to be shockable despite multiple defibrillation attempts and Epinephrine. The primary agents used are Amiodarone or Lidocaine, which stabilize the electrical activity of the heart muscle. Amiodarone is often the first-line choice, given as a bolus dose to suppress electrical chaos.
If the rhythm remains shockable after the initial dose of Amiodarone, a second, smaller dose may be administered later. Lidocaine is an acceptable alternative if Amiodarone is unavailable or contraindicated. These medications alter the electrical threshold of the heart muscle, making it more responsive to defibrillation and less likely to revert to a life-threatening rhythm.
Recognizing the Return of Spontaneous Circulation (ROSC)
The goal of resuscitation efforts is the Return of Spontaneous Circulation (ROSC), defined as the sustained return of a perfusing heart rhythm that generates a pulse and measurable blood pressure. Recognizing ROSC signals the need to stop chest compressions and transition immediately to post-resuscitation care protocols. Common signs include a palpable pulse, measurable blood pressure, or spontaneous breathing, movement, or coughing.
In monitored patients, the most objective indicator of ROSC is often a sudden, sustained increase in end-tidal carbon dioxide (EtCO2) concentration. EtCO2 is the amount of carbon dioxide in the exhaled breath. A sudden rise, often above 40 millimeters of mercury, indicates the heart is effectively circulating blood and delivering CO2 to the lungs.
Once ROSC is confirmed, the focus shifts to stabilizing the patient’s overall condition. This involves optimizing ventilation and oxygenation, and managing blood pressure to ensure adequate perfusion to vital organs, especially the brain. Achieving ROSC signals the beginning of a new phase of care aimed at preventing the heart from stopping again and minimizing damage caused by oxygen deprivation.