‘Cardiac arrest.’ It’s a word you often hear. Yet it’s not always clear what exactly is meant by it, as there are different types of cardiac arrest.
And when you use an Automated External Defibrillator (AED), it first analyses the patient’s heart. Sometimes, it advises a shock, and sometimes it doesn’t. Have you ever wondered why?
The answer is that ‘cardiac arrest’ isn’t a single condition. It’s a general term for when the heart stops pumping blood effectively, but there are different types, or ‘rhythms’. An AED is designed to treat only specific, life-threatening electrical problems in the heart.
Understanding these different rhythms helps explain the crucial roles of both defibrillation (the shock) and CPR (cardiopulmonary resuscitation). This blog post, based on the principles behind the Resuscitation Council UK (RCUK) guidelines, breaks down the main types into two key groups.
What is cardiac arrest?
The term ‘cardiac arrest’ means that the heart stops effectively pumping blood. As a result, the brain and other vital organs no longer receive oxygen. In particular, the lack of oxygen in the brain quickly leads to serious problems. That’s why it’s very important to act quickly.
Shockable Rhythms: When an AED Can Help
A ‘shockable’ rhythm is a chaotic or abnormally fast electrical pattern that prevents the heart from pumping. An AED can deliver a controlled electrical shock to try and reset this chaos. The two main shockable rhythms are:
1. Ventricular Fibrillation (VF)
Ventricular Fibrillation is a common cause of cardiac arrest. In VF, the electrical signals in the heart’s ventricles become completely chaotic and disorganized. Instead of pumping, the heart muscle just quivers or ‘fibrillates’. This means no blood is being circulated.
- Can an AED help? Yes. A shock is essential to stop this electrical chaos and give the heart a chance to restart a normal rhythm. Immediate CPR is vital to keep blood flowing until the AED can deliver a shock.
2. Ventricular Tachycardia (VT)
In Ventricular Tachycardia, the heart beats dangerously fast due to a faulty electrical signal. The ventricles contract so rapidly that they don’t have time to fill with blood between beats, leading to a drop in blood flow. In severe cases, this leads to a cardiac arrest where no pulse can be found.
- Can an AED help? Sometimes. If the person is in cardiac arrest with no pulse, the AED will usually advise a shock to correct the runaway rhythm.
A ‘non-shockable’ rhythm is when a shock from an AED would have no benefit. In these cases, the problem isn’t chaotic electrical activity that needs resetting. The only treatment is high-quality, uninterrupted CPR to manually pump blood to the brain and organs.
Non-Shockable Rhythms: Why CPR is Key
3. Asystole
Often called a ‘flatline’, Asystole is the complete absence of any electrical activity in the heart. The heart has totally stopped.
- Can an AED help? No. A shock is useless because there is no electrical activity to reset. The AED will state “no shock advised” and instruct you to continue CPR. CPR is the only action that can help.
4. Pulseless Electrical Activity (PEA)
PEA is a deceptive rhythm. The heart’s electrical system is often working correctly, showing a normal or near-normal rhythm on a monitor. However, the heart muscle itself is not responding to these signals—it doesn’t contract, so no blood is pumped.
- Can an AED help? No. Since the electrical signal isn’t the problem, a shock has no effect. The heart muscle simply can’t be shocked into contracting. Again, the only course of action is immediate, high-quality CPR.
Your Role in All Different Types of Cardiac Arrest
As a rescuer, you don’t need to know which rhythm is causing the cardiac arrest. The AED will figure that out for you. Your job is simple and life-saving:
- Call 999 (or 112) immediately.
- Start CPR as soon as possible.
- Use an AED as soon as it arrives and follow its voice commands.
Whether a shock is advised or not, the AED is an essential tool that will guide you on what to do next to give the person the best possible chance of survival. You don’t need to be trained or know what’s going on. The AED will tell you what to do.
Are you considering to buy or rent an AED? Discover the key features to compare in our article “How to Choose the Right AED“.
