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What should you do if you are using an AED and the device says “no shock advised?” If you are a designated first responder or expect that you might one day treat a victim of sudden cardiac arrest, it’s important to understand:
- How an automated defibrillator works
- When an electric shock is appropriate
- What to do in the case of an AED “no shock advised” message
How Defibrillators Help With Sudden Cardiac Arrest
Sudden cardiac arrest (SCA) is a condition that affects around 1,000 Americans every day and is usually fatal within 10 minutes if the victim does not receive quick intervention. When an automated external defibrillator is used within five minutes of the onset of SCA, it can double the victim’s chance of survival compared to CPR alone.
Sudden cardiac arrest is different from a heart attack because heart attacks are caused by a blockage to the heart rather than changes to the heart’s electrical activity. Someone can also suffer a heart attack without collapsing—and sometimes without even knowing they’ve had one. When a person suffers a cardiac arrest, the person:
- Suddenly collapses
- Becomes unconscious
- Is not breathing
If you see all of these signs occur, you should presume that the person has suffered a cardiac arrest and commence life-saving measures immediately.
Sudden Cardiac Arrest and Ventricular Fibrillation
When SCA occurs, the victim’s heart goes into a pattern known as ventricular fibrillation. This means that the brain is still sending electrical impulses to the heart muscle, but the ventricles quiver in a sort of uncoordinated twitching instead of beating normally. If cardiac arrest patients do not receive CPR within minutes, their organs suffer from oxygen deprivation and their brain cells begin to die.
If there is an AED nearby, a bystander should fetch the device, turn on the machine, and attach the electrode pads to the victim’s chest using correct AED pad placement while someone else calls 9-1-1. The AED will instruct you to complete one cycle of CPR chest compressions before it tells you to “stand clear” and analyzes the victim’s heart rhythm.
If the machine determines that the heart is in a shockable rhythm, it will audibly instruct you to press the “Shock” button (for semi-automatic AEDs such as the HeartSine Samaritan PAD 350P) or will automatically deliver an electric shock (for fully automatic AEDs such as the Physio-Control LIFEPAK CR2). After the first shock, the heart may resume beating or you may need to continue CPR until the defibrillator analyzes the victim’s cardiac rhythm again.
In some cases, the voice prompts may say “no shock advised” after analyzing the victim’s heart. There are two possible reasons that the automated external defibrillator might say this, and you need to know what to do in each case.
The Heart Does Not Have a Shockable Rhythm
Shockable Rhythms: Ventricular Fibrillation and Pulseless Ventricular Tachycardia
There are two heart rhythms that an AED can shock:
- Ventricular fibrillation or V-fib
- Pulseless Ventricular tachycardia or V-tach
Ventricular fibrillation is shockable because the heart is still receiving nerve impulses from the brain. The ventricles are simply firing erratically. In this situation, the shock may help to reset the heart’s natural pacemaker—the sinoatrial node.
Pulseless ventricular tachycardia, while different from v-fib, also responds well to a shock. In this pattern, the electrical impulses are organized, but originating in the ventricles, and firing too quickly for the heart to react, resulting in a non-beating heart with no blood circulation.. This rhythm can quickly turn into v-fib if a normal pattern is not restored.
Non-Shockable Rhythms: Asystole and Pulseless Electrical Activity
There are two additional rhythms that are common with SCA but non-shockable with an AED machine:
- Asystole
- Pulseless Electrical Activity
Asystole occurs when the heart is not beating and is no longer receiving electrical impulses. In this situation, the heart rhythm won’t respond to a shock, and it’s highly unlikely that the victim will survive.
Pulseless electrical activity describes an abnormal heart rhythm in which the electrical activity of the heart is normal but the ventricles aren’t responding. Because there are no problems with the electric signaling, automatic external defibrillators can’t help.
What to Do
If the AED machine delivers a “no shock advised” message, leave the automated external defibrillator connected to the patient’s chest via the adhesive pads and continue to perform CPR. All AEDs on the market today include helpful voice prompts with metronome to assist you in properly performing CPR, and some will even provide feedback letting you know if your compressions are too shallow, deep, fast or slow. If the machine later detects a shockable rhythm, the voice prompts will tell you to stand clear while the AED delivers a shock.
A Normal Rhythm Has Been Restored
Ideally, the AED won’t advise a shock because the heart has resumed a normal pattern with chest compressions and rescue breaths and/or with the delivery of the first shock. An AED will never shock a person with a normal heart rhythm.
Once you are sure a normal pattern has been restored and the person is breathing on their own, it’s okay to stop CPR. Continue to check the victim’s pulse and breathing and monitor the victim’s airway until healthcare providers arrive.
Important note: Whether the AED “no shock advised” message occurs because of a non-shockable heart rhythm or because the victim’s heart is now beating at a normal pace, it’s essential to leave the electrode pads connected to the patient’s chest. It’s possible that a patient may go back into ventricular fibrillation and need an electrical shock. Conversely, a non-shockable rhythm may turn into a shockable one. In both cases, the voice prompts will continue to provide instructions on what to do.
AED No Shock Advised – Remember to Stay Calm
The important thing is to keep calm and listen carefully to the instructions provided. Automated external defibrillators are designed to analyze the heart pattern of cardiac arrest victims and deliver a shock if appropriate. They will not shock a person who is conscious and breathing, and they will not shock a victim of SCA who doesn’t have a shockable heart rhythm.
If you use an AED and it says “no shock advised,” keep the adhesive AED pads in place and continue performing CPR. While you might not be an expert on the circulatory system and how it works, you can still save someone’s life in an emergency.