The two types of arrhythmia that can lead to sudden cardiac arrest (SCA) are ventricular fibrillation and ventricular tachycardia. Both originate in the heart’s lower chambers (the ventricles) and both stop the heart from pumping blood effectively. They are also the only two cardiac arrest rhythms that a defibrillator can correct, which is why recognizing them matters so much.
Ventricular Fibrillation
Ventricular fibrillation, often shortened to V-fib, is the more immediately dangerous of the two. Instead of contracting in a coordinated squeeze, the ventricles receive rapid, chaotic electrical signals that cause them to quiver. No organized contraction means no blood leaves the heart. Within seconds, the person loses consciousness and has no pulse. Without treatment, death follows in minutes.
The electrical chaos behind V-fib involves disorganized signals looping back on themselves repeatedly, preventing the heart muscle from ever resetting to a normal beat. Once this pattern starts, it tends to sustain itself. Most people who experience ventricular fibrillation have an underlying heart condition or have suffered a serious cardiac injury, though it can occasionally strike an otherwise healthy heart.
Ventricular Tachycardia
Ventricular tachycardia (V-tach) is a dangerously fast heart rate, over 100 beats per minute, that originates in the ventricles rather than in the heart’s normal pacemaker. Brief episodes lasting only a few seconds may pass without harm. But when V-tach persists beyond about 30 seconds, it becomes a medical emergency.
The core problem is timing. At extremely high rates, the ventricles contract so rapidly that they never have a chance to fill with blood between beats. Cardiac output drops dramatically, blood pressure plummets, and the person can lose their pulse entirely. This is called pulseless ventricular tachycardia, and it is treated the same way as V-fib: with immediate defibrillation. Sustained V-tach can also degenerate into ventricular fibrillation, compounding the danger.
Why These Two Arrhythmias Are “Shockable”
Not every cardiac arrest rhythm responds to a defibrillator. Two other arrest rhythms, asystole (a flatline with no electrical activity) and pulseless electrical activity (where the heart’s electrical signals continue but produce no effective pumping), are classified as “nonshockable.” A defibrillator cannot help in those situations because there is no disorganized electrical activity to reset.
V-fib and pulseless V-tach are different. The heart still has electrical activity, just dangerously abnormal activity. A defibrillator delivers a controlled shock that momentarily stops all electrical signals, giving the heart a chance to restart with a normal rhythm. This is why automated external defibrillators (AEDs) placed in airports, gyms, and offices are so valuable. In one large study, 38% of people who received a shock from an AED before emergency services arrived survived, compared to far lower survival rates when defibrillation was delayed.
SCA Is Not the Same as a Heart Attack
People often use “cardiac arrest” and “heart attack” interchangeably, but they are fundamentally different events. A heart attack is a circulation problem: a blocked artery cuts off blood flow to part of the heart muscle. Sudden cardiac arrest is an electrical problem: the heart’s rhythm malfunctions and the heart stops pumping altogether. A heart attack can trigger a fatal arrhythmia and lead to SCA, but many cardiac arrests happen without a heart attack in progress. Out-of-hospital cardiac arrest strikes roughly 356,000 people per year in the United States, and the majority of those events are fatal, with overall survival rates historically below 10%.
What Makes These Arrhythmias More Likely
Certain underlying conditions raise the risk of V-fib and V-tach significantly. The three most common are coronary artery disease (blocked arteries that limit blood flow to the heart), scarring from a previous heart attack, and cardiomyopathy (structural changes to the heart muscle that alter how it conducts electricity). Each of these conditions can create areas of damaged tissue where electrical signals get delayed or rerouted, setting the stage for the kind of chaotic or overly rapid rhythms that lead to arrest.
Other risk factors include a family history of sudden cardiac death, inherited electrical disorders of the heart, and significant electrolyte imbalances. In younger people without obvious heart disease, genetic conditions affecting the heart’s electrical system are a more common trigger.
Warning Signs Before SCA
Sudden cardiac arrest often strikes without any warning at all, which is part of what makes it so deadly. In some cases, though, people experience symptoms in the minutes or hours beforehand. These can include chest discomfort, shortness of breath, a sensation of the heart racing or fluttering (palpitations), and sudden unexplained weakness. These symptoms are easy to dismiss, especially in someone who doesn’t know they have heart disease, but any combination of them, particularly palpitations accompanied by lightheadedness, warrants immediate attention.
Because the window for successful treatment is so narrow, bystander response is critical. Performing CPR and using an AED within the first few minutes gives someone in V-fib or pulseless V-tach the best chance of survival. Every minute without defibrillation reduces that chance substantially.