How the Heart Works
There are two parts to the normal heart (or cardiac) function: mechanical and electrical. The mechanical part is the pumping action, which is felt as a pulse, each time the heart muscle contracts. The heart has its own pacemaker, a group of cells called the sinus node (or sino-atrial node). These cells send out electrical signals, usually between 60 and 100 times each minute. The signals travel along special conduction pathways in the heart walls and stimulate the muscle tissue to contract. The rate of the signals changes depending on a person's needs.
For example, when exercising, the sinus node tells your heart to beat faster, and when sleeping, your heart rate slows down.
The heart's electrical system regulates the mechanical system, and in a healthy heart, the result is the rhythmic and coordinated beating of the heart.
Picture source: Oregon Museum of Science & Industry 
When the Heart Malfunctions
SCA occurs when the heart's pumping chambers (ventricles) suddenly stop contracting effectively. As a result, the heart assumes an abnormal, chaotic rhythm, causing the heart muscle to twitch spasmodically. The heart then loses its ability to effectively pump blood. This activity is called ventricular fibrillation (VF). Because the heart is not pumping blood, a person in VF usually has no detectable pulse. They suddenly become unresponsive, stop breathing, and could die if treatment is not received in minutes. VF causes more cardiac arrests than any other life-threatening rhythm and can occur suddenly, without warning.
If Not Treated In Time
Brain death and permanent death start to occur in just four to six minutes after someone experiences SCA. However, SCA can be reversed if it's treated within a few minutes. This treatment, called defibrillation, is an immediate electric shock to the heart to restore a normal heartbeat.
Defibrillation, the third link in the Chain of Survival , can stop VF before it turns into sudden cardiac death. However, defibrillation must occur soon after the onset of an attack. A victim's chances of surviving SCA decrease by 7-10% with every minute that passes without this life-saving shock. Few attempts at resuscitation succeed after 10 minutes.
Source: http://www.chainofsurvival.com/
Did you know...
- Rapid heart rhythms (arrhythmias) account for the great majority of sudden cardiac deaths. Read more...
- An estimated 2.2 million Americans are living with atrial fibrillation. Read more...
- Learn to recognize a heart attack and act quickly when you notice the warning signs. Call 9-1-1 immediately if you experience symptoms! Read more...
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