Awareness
When someone goes into cardiac arrest, every second matters. But not every cardiac arrest rhythm can be treated with an electrical shock.
Understanding the difference between shockable and non-shockable rhythms helps explain why paramedics use different treatments during resuscitation, and why early CPR and defibrillation are so important.
ER24 ALS Paramedic Ruan van der Westhuizen from the Pretoria branch explains what happens during these different types of cardiac arrest and what members of the public should know.
What is a shockable cardiac arrest?
A shockable rhythm is one that can potentially be treated with defibrillation, an electrical shock delivered to the heart.
The two shockable cardiac arrest rhythms are ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT).
“In ventricular fibrillation, the electrical activity within the heart’s lower chambers is very chaotic,” says Van der Westhuizen. “The heart muscle contracts in an uncoordinated way, causing the ventricles to quiver instead of producing a coordinated squeeze.”
With pVT, abnormal electrical signals cause the ventricles to beat very rapidly. This leaves less time for the chambers to fill between beats, so the heart cannot pump enough blood to maintain circulation.
Although the two rhythms look different, both result in cardiac arrest because there is no effective circulation or detectable pulse.
What is a non-shockable cardiac arrest?
The two non-shockable rhythms are asystole and pulseless electrical activity (PEA).
Asystole means there is no detectable cardiac electrical activity, while PEA occurs when electrical activity is present but does not result in effective mechanical pumping.
“Electrical signals are present, but they are not producing effective circulation,” explains Van der Westhuizen. “A shock cannot correct problems such as severe blood loss, oxygen deprivation or an obstruction to blood flow.”
This is why PEA requires CPR and treatment of its underlying cause rather than defibrillation.
How do paramedics identify the rhythm?
Paramedics assess whether the person is responsive, whether they are breathing normally and whether they have a pulse. If they are unresponsive, not breathing normally and have no definite pulse, CPR is started immediately.
Defibrillator pads are placed on the chest so the heart rhythm can be analysed. CPR interruptions are kept as short as possible while paramedics identify the rhythm. VF and pVT are shockable, while asystole and PEA are non-shockable. (cpr.heart.org)
An AED can perform the rhythm analysis and advise whether a shock is appropriate, meaning members of the public do not need to interpret the rhythm themselves.
What happens during a non-shockable cardiac arrest?
Treatment includes high-quality CPR, breathing support, adrenaline and the identification and treatment of reversible causes.
“The defibrillator stays connected to monitor the heart’s rhythm, even though no shock is currently indicated,” says Van der Westhuizen. “If it changes to VF or pulseless VT, a shock is then delivered.”
Adult cardiac-arrest guidance recommends adrenaline as soon as feasible for non-shockable rhythms, with further doses given every three to five minutes during resuscitation. (cpr.heart.org)
If circulation returns, this is known as return of spontaneous circulation (ROSC), after which care focuses on supporting breathing and blood pressure, treating the underlying cause and arranging further hospital care.
Why do CPR and defibrillation need to happen quickly?
“There is a saying in the healthcare world, ‘time is tissue’ or ‘time is myocardium’,” says Van der Westhuizen.
When effective circulation stops, the brain and heart are deprived of oxygen. CPR helps maintain blood flow to these vital organs, while early defibrillation can stop VF or pVT and give the heart a chance to regain a coordinated rhythm.
The longer cardiac arrest continues, the more difficult successful resuscitation can become.
Can anyone use an AED?
One of the biggest misconceptions, according to Van der Westhuizen, is that you need medical training to use an AED.
“AEDs are designed for public use, with voice prompts and pictures that guide you through each step,” he explains.
The device analyses the rhythm and only advises a shock when appropriate. If it says “no shock advised”, this does not mean the person has recovered. If they remain unresponsive and are not breathing normally, continue CPR and follow the AED’s instructions.
“Using it promptly and following its instructions can save a life,” says Van der Westhuizen.
If someone is unresponsive and not breathing normally, call emergency services, start CPR and use an AED if one is available.