What is Sudden Cardiac Arrest?

Sudden cardiac arrest (SCA) is one of the leading causes of death among adults in North America. It is estimated that up to 350,000 die each year from SCA, with 50% of the victims having no prior symptoms or cardiac history.

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How the Heart Works

The heart muscle is both a mechanical and an electrical muscle. Meaning it has a mechanical response (pumping/contraction) to an electrical current (impulse).

For circulation to occur, the heart muscle must pump blood in an organized synchronic manner. Normally this pumping action is controlled by electrical stimuli that originates in the heart's natural pacemaker in the upper chambers of the heart (atria). As the stimuli is produced in the atria, it then travels rapidly through specialized conduction pathways to the lower portion of the heart (ventricles). This stimuli causes a coordinated mechanical response that contracts the blood filled chambers and forces blood to flow throughout the body. This is termed a Normal Sinus Rhythm and is what provides us with consciousness and life.

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Ventricular Fibrillation

Most SCA victims experience ventricular fibrillation; in fact, 75-85% of SCA is from ventricular fibrillation. During ventricular fibrillation, the normal electrical pathway of the heart is disrupted and fails to control the heart muscle in an organized synchronic manner. Instead, thousands of electrical impulses discharge from many different parts of the heart. The impulses are chaotic and repetitive, causing the heart muscle to lose all pumping capability as the chambers of the heart fail to work together. The heart muscle can only fibrillate (quiver) causing blood flow to stop. The victim suddenly collapses or becomes unconscious, respirations and pulses are lost as the brain and muscles stop receiving blood flow from the heart. The fibrillation of the heart will continue until the muscle depletes it energy and dies or a dose of electric medicine (defibrillation) is delivered across the heart stopping all chaotic impulses. Left untreated the victim will surely die. While many victims have no prior warning signs or history of heart disease, many have risk factors for cardiovascular disease such as smoking, hypertension, diabetes and a sedentary life style.

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Treatment

Immediate treatment is needed for a victim of SCA to survive. Currently, it is estimated that 95% of the victims die before reaching a hospital. Emergency Medical Services have carried defibrillators for years while only achieving a 5% survival rate nationally. The reason is response time. For each minute that a person suffers SCA, the chance of survival is decreased by 7-10%. As you can imagine, the current system is somewhat inefficient in response to SCA victims. By the time the victim is noticed to be in SCA, the responders must find a phone and activate EMS. Time is lost as the dispatcher enters information into the system and the EMS agency responds through traffic. When the patient if finally treated with defibrillation by EMS, the patient chance of survival is down to 5%.

A nearby defibrillator that can quickly be deployed and deliver therapy to the patient within 0-4 minutes is optimal for the victim to survive.

Every minute counts

	Time from notification to defibrillation	Survival rate *
Rehabilitation centers	Immediate defibrillation (1-2 minutes)	90%
Model community	Early defibrillation (6 minutes)	45%
	Early defibrillation (7 minutes)	30%
Typical community	Delayed defibrillation (>10 minutes)	<5%
* For victims of sudden cardiac arrest in witnessed ventricular fibrillation

It is impossible in most community settings to respond consistently to sudden cardiac arrest victims within the one to two minute timeframe documented in some cardiac rehabilitation centers. But all communities can aim to reduce the critical interval from collapse to defibrillation. Since the exact time of collapse often is difficult to identify and measure, communities can focus on minimizing the interval from the 911 call to the first defibrillatory shock. Communities that reduce this "call-to-shock" time to five minutes or less can expect as many as one-third to one-half of sudden cardiac arrest victims found in ventricular fibrillation to be resuscitated. Reducing "call-to-shock" time by even one minute can mean the difference between life and death.

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What is Defibrillation?

Defibrillation is simply electric therapy for the heart in ventricular fibrillation. It delivers an electrical shock to stop the chaotic, non-productive activity within the heart muscle. The shock is passed between two disposable adhesive pads that are usually positioned on the chest as shown in the illustration. The placement allows a fast response in an SCA emergency. The shock passes through the heart as it travels from one pad to the other.

Defibrillation stops the chaotic electrical activity of fibrillation and causes the heart to pause, allowing the heart's natural pacemaker to regain control of the heart muscle and therefore the normal rhythm. That is why, immediately after a successful defibrillation shock, it is normal for the heart to briefly experience asystole (flatline) before the return of a spontaneous rhythm.

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Chain of Survival

In 1990, the American Heart Association developed the Chain of Survival. This protocol addresses the fact that most SCA episodes occur outside of a hospital, with death occurring within minutes of onset. For the Chain to be effective, quick execution of each and every link is critical. With each minute that passes, the likelihood of survival decreases 7-10%.

To provide the best opportunity for survival, each of these four links must be put into motion within the first few minutes of SCA onset:

• Early Access to Emergency Care must be provided by calling 911.
• Early CPR should be started and maintained until emergency medical services (EMS) arrive.
• Early Defibrillation is the only one that can re-start the heart function of a person with ventricular fibrillation (VF). If an automated external defibrillator (AED) is available, a trained operator should administer defibrillation as quickly as possible until EMS personnel arrive.
• Early Advanced Care, the final link, can then be administered as needed by EMS personnel. When each link in the chain works successfully, the chance of surviving SCA increases greatly.

Click the image above or here for a detailed description of the Chain of Survival.

Since more than 70% of SCA cases occur at home, and another 10% to 15% occur at work 2 , trained EMS personnel are unlikely to be at the scene at onset. Therefore, trained lay responders with quick access to defibrillation units can be a vital asset when SCA strikes. In certain environments, where the Chain is strong and when defibrillation occurs within the first few minutes of cardiac arrest, survival rates can approach 80% to 100%.

People who survive sudden cardiac arrest have an excellent prognosis: 83% survive for at least one year, and 57% survive for five years or longer. In fact, when analyzed by age group, survival rates for SCA survivors are comparable to survival rates of people who have never had an event. Clearly, early intervention can offer years of productivity and fulfillment to victims of SCA.

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