Critical: Timing of a defibrillator shock

To shock or not to shock? That is the question… in timing your defibrillator shock.

There is a common misconception that an AED will shock the heart, even if it is not supposed to, but one of your defibrillator’s key skills is knowing when to shock, and when to not. Once the pads are attached to the patient, the device will analyse their heart’s rhythm and inform the user if the rhythm is shockable or non-shockable.

How does it know when it should, or should not shock?

When analysing the heart, the AED will be searching for four different rhythms. If it detects Ventricular Tachycardia (VT) or Ventricular Fibrillation (VF), the device will either instruct the user to shock or shock the patient itself (depending on if it is a semi or fully automatic device).

If it detects Pulseless Electrical Activity (PEA) or an Asystole, the AED will alert the user and say, “No shock advised”. By accurately identifying whether a rhythm is shockable or non-shockable, the AED ensures that the appropriate treatment is administered promptly. When every minute counts, this is crucial and can mean the difference between life and death.

Shockable Rhythms

– V-Tach, also known as ventricular tachycardia or VT, is characterised by an abnormally rapid heartbeat. Unlike the normal heart rate of 60-100 beats per minute, a heart experiencing VT usually beats over 100 times or more per minute. V-Tach is caused by irregular electrical impulses within the lower chambers of the heart. When these irregular electrical impulses disrupt the heart’s normal rhythm, it can lead to a disruption of blood flow throughout the body.

– V-Fib, also known as ventricular fibrillation or VF, is characterised by the twitching of the heart’s lower chambers, without effectively performing an effective heartbeat. This life-threatening condition often occurs during or shortly after a heart attack and is the primary cause of sudden cardiac death.
In 22.4% of cases of sudden cardiac arrest in 2021, the initial rhythm was either ventricular fibrillation or ventricular tachycardia.

Non-Shockable Rhythms

– Asystole is the lack of a heartbeat; it is also known as a flatline and is due to the heart’s electrical system being shut down. Asystole is usually the result of untreated ventricular fibrillation or ventricular tachycardia. In 51.9% of sudden cardiac arrest cases in 2021, the initial rhythm was Asystole.

– PEA, also known as pulseless electrical activity, is where the heart’s electrical activity is too weak to pump blood throughout the body. In 21.4% of sudden cardiac arrest cases in 2021, the initial rhythm was PEA.

Regardless of the type of arrhythmia, the main issue during sudden cardiac arrest is that the heart cannot pump blood effectively to the rest of the body. This means that vital organs, such as the brain, do not receive enough oxygen and nutrients to function properly. This can result in serious and potentially irreversible damage to the organs, especially if the blood flow is not restored quickly. For example, neurological deficits, such as cognitive impairment.

This is why it is crucial to perform cardiopulmonary resuscitation (CPR) as soon as possible after a cardiac arrest. Good CPR can increase the chances of survival and recovery for cardiac arrest victims and can also prevent further damage to the victim’s organs by providing manual circulation of blood through the body.
In addition to this, in instances where the arrhythmia cannot be treated by fibrillation, such as asystole and PEA, CPR can buy time until medical professionals arrive.

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