The heart’s electrical system is vital for pumping blood. Malfunctions can cause abnormal heart rhythms (arrhythmias), sometimes leading to cardiac arrest. Cardiac arrest is a sudden, complete cessation of the heart’s pumping function, causing loss of consciousness and breathing. While defibrillation aims to restore a normal heart rhythm during cardiac arrest, not all abnormal rhythms respond to this treatment.
How Defibrillation Works
Defibrillation delivers an electrical shock to the heart. This impulse aims to “reset” chaotic electrical activity, allowing the heart’s natural pacemaker to restore a normal rhythm. The procedure depolarizes heart muscle, stopping disorganized electrical signals. This brief interruption gives the heart a chance to restart with a coordinated beat.
Defibrillation is effective for rhythms with chaotic electrical activity, such as ventricular fibrillation. In ventricular fibrillation, the heart’s lower chambers quiver ineffectively instead of pumping blood. An electrical shock can synchronize these chaotic signals, potentially restoring a functional heartbeat. Automated external defibrillators (AEDs) analyze heart rhythms and deliver a shock only when a shockable rhythm is detected.
Rhythms Not Treated by Shock
The two primary non-shockable rhythms are asystole and pulseless electrical activity (PEA). Asystole, commonly called a “flatline,” appears as a straight line on an electrocardiogram (ECG), indicating a complete absence of electrical activity. This means the heart muscle is not contracting and no blood is pumped.
PEA is distinct from asystole; it involves organized electrical activity on the ECG, but the heart does not effectively pump blood. Despite electrical signals, there is no detectable pulse, signifying that the electrical signals and mechanical pumping action are uncoupled.
Why These Rhythms Are Not Shockable
Asystole is not shockable due to its complete lack of electrical activity. A defibrillator reorganizes chaotic electrical signals; if none exist, there is nothing to “reset.” Shocking asystole is ineffective and will not restart the heart. Media portrayals of shocking a “flatline” are inaccurate.
For PEA, electrical activity is present, but the underlying problem is not an electrical disorganization that a shock can fix. PEA stems from severe mechanical, metabolic, or circulatory issues that prevent the heart from effectively pumping blood despite electrical signals. These issues can include severe blood loss, collapsed lungs, blood clots, or severe electrolyte imbalances. A shock cannot resolve these physical or chemical problems.
Interventions for Non-Shockable Rhythms
When asystole or PEA is identified, immediate, high-quality cardiopulmonary resuscitation (CPR) is the primary intervention. CPR manually circulates blood and oxygen to vital organs, buying time until more definitive treatment can be provided. Continuous chest compressions are crucial, with minimal interruptions.
Beyond CPR, the focus shifts to identifying and treating reversible underlying causes. This often involves a systematic search for conditions such as low oxygen (hypoxia), severe fluid loss (hypovolemia), electrolyte imbalances, or drug overdose. Medications, such as epinephrine, are also administered to stimulate the heart and improve blood flow. Successful outcomes depend on how quickly these underlying causes can be identified and corrected.