Cardiac arrest is a sudden medical emergency where the heart unexpectedly stops pumping blood effectively. This condition is primarily an electrical problem within the heart, differing significantly from a heart attack, which involves a blockage of blood flow to the heart muscle. The heart relies on precise electrical signals to coordinate its pumping action. When these electrical signals become chaotic or cease, the heart can no longer function, leading to cardiac arrest. Some electrical disruptions can be corrected by delivering a controlled electrical shock to the chest.
Understanding Cardiac Rhythms
The heart’s rhythmic pumping is orchestrated by its intrinsic electrical system, starting with a specialized group of cells called the sinoatrial (SA) node. Located in the upper right chamber of the heart, the SA node acts as the heart’s natural pacemaker, generating regular electrical impulses. These impulses spread through the heart muscle in a coordinated wave, causing the upper chambers (atria) to contract, followed by the lower chambers (ventricles). This organized electrical activity ensures efficient blood circulation. During cardiac arrest, this normal, organized electrical activity is disrupted, becoming chaotic, extremely rapid, or completely absent, preventing the heart from pumping blood.
The Two Shockable Rhythms: Ventricular Fibrillation and Pulseless Ventricular Tachycardia
Medical guidelines identify two primary rhythms that respond to an electrical shock during cardiac arrest. These “shockable” rhythms involve electrical activity that is present but disorganized or too rapid for effective pumping. Delivering a controlled electrical shock can reset the heart’s electrical system, allowing it to resume a normal rhythm.
Ventricular Fibrillation (VF)
Ventricular Fibrillation (VF) is a chaotic and disorganized electrical activity in the heart’s lower chambers, the ventricles. Instead of contracting effectively to pump blood, the ventricles merely quiver. This uncoordinated electrical activity prevents the heart from pushing blood out to the body. Defibrillation momentarily stops all electrical activity, allowing the heart’s natural pacemaker to re-establish a normal rhythm. VF is often the initial rhythm observed in sudden cardiac arrest cases.
Pulseless Ventricular Tachycardia (pVT)
Pulseless Ventricular Tachycardia (pVT) occurs when the ventricles beat very rapidly and in a somewhat organized manner. The contractions are so fast that the heart cannot fill with blood or pump it effectively. Despite electrical activity, there is no detectable pulse. This rapid, ineffective rhythm prevents blood from circulating to vital organs. Similar to VF, defibrillation is effective for pVT as it interrupts this rapid, unstable rhythm, allowing the heart to regain a functional pace.
Non-Shockable Rhythms: Asystole and Pulseless Electrical Activity
Two other rhythms encountered in cardiac arrest are considered “non-shockable,” meaning that an electrical shock from a defibrillator will not be effective. In these cases, the problem is not an electrical disorganization that can be reset. Instead, treatment focuses on cardiopulmonary resuscitation (CPR) and addressing any underlying causes.
Asystole
Asystole, commonly known as “flatline,” represents a complete absence of electrical activity in the heart. On an electrocardiogram (ECG) monitor, asystole appears as a flat line, indicating no electrical impulses are being generated. Since there is no electrical activity to reset, defibrillation is ineffective. In these situations, medical efforts concentrate on high-quality CPR and administering medications like epinephrine to stimulate electrical activity.
Pulseless Electrical Activity (PEA)
Pulseless Electrical Activity (PEA) is a condition where the heart shows organized electrical activity on an ECG monitor, but fails to produce a mechanical contraction strong enough to generate a pulse. This means the electrical signals are present, but the heart muscle itself is not effectively pumping blood. Defibrillation is not a treatment for PEA, as the issue is mechanical failure or another cause preventing the heart from pumping. Treatment involves continuous CPR and identifying and treating reversible causes like severe blood loss, oxygen deprivation, or metabolic imbalances.
The Critical Role of Defibrillation
Defibrillation involves delivering a controlled electrical shock to the heart to interrupt abnormal electrical activity. This momentarily stops electrical chaos, allowing the heart’s natural pacemaker to resume a normal rhythm. For shockable rhythms like ventricular fibrillation and pulseless ventricular tachycardia, early defibrillation is vital for survival.
Automated External Defibrillators (AEDs) allow trained bystanders to deliver a life-saving shock. These devices analyze the heart’s rhythm and only advise a shock if a shockable rhythm is detected, preventing accidental or unnecessary shocks. The prompt use of AEDs in public spaces is important, as survival rates decrease with every minute that passes without defibrillation for these rhythms. While defibrillation treats shockable rhythms, non-shockable rhythms require continuous chest compressions and addressing the underlying cause.