Jan 4, 2017 
Sudden cardiac arrest can occur like a bolt from the blue. It can happen to anyone at any time; victims often seem healthy and may not have pre-existing heart conditions. Approximately 350,000 people suffered cardiac arrest outside of a hospital in 2016, with a further 209,000 cardiac arrests occurring at hospitals.
People who experience cardiac arrest outside of a hospital have about a 12% chance of survival. The chances of survival are doubled or even tripled if the victim receives CPR from a bystander—even one with no prior medical training—but unfortunately, less than half of cardiac arrest victims are lucky enough to receive CPR.
The key to survival for victims of cardiac arrest is often receiving CPR immediately—those who have to wait for an ambulance have very low chances. There’s a good reason for that. Here’s an overview of exactly how cardiac arrest works—and why CPR is often the only means of countering it.
What is Sudden Cardiac Arrest?
The heart runs on electricity. Each heart has a natural “pacemaker,” called the SA node, that sends out an electrical signal to the surrounding cells. This signal sets off a complex process by which the chambers of the heart contract and release, causing blood to pump at regular intervals. The electrical signals control the heart rate, which can increase or decrease depending on activity level.
While the heart rate can speed up or slow down, it is essential to maintain the steady pulse—no matter the speed. A cardiac arrest occurs when there is a malfunction in the heart’s electrical system, causing the heartbeat to become irregular. This disrupts the flow of blood throughout the body and into the brain. The body can only take a few seconds of this before the victim will lose consciousness. A cardiac arrest victim will have no pulse, and will die in minutes if the condition is not treated.
Cardiac arrest can occur out of the blue, or it can be caused by an event such as severe blood loss, drowning, drug overdose, asphyxia, or major physical trauma.
What is a Heart Attack?
Cardiac arrest and heart attacks are often confused with each other, but they are not the same thing. A heart attack occurs when the flow of blood to the heart is blocked, often by a clogged artery. If the blockage is not removed, the part of the heart that receives blood from that artery will start to die.
Some heart attacks resemble cardiac arrest in that the attack comes quickly and is very intense. However, heart attacks can also feel fairly mild and the symptoms can sometimes be spread out over hours, days, or even weeks. Unlike with a cardiac arrest, the heart generally continues to beat during a heart attack. In addition, unlike with cardiac arrest, the symptoms of heart attack vary between men and women.
Heart attacks are often the end result of atherosclerosis, a condition in which plaque builds up in the arteries due to high cholesterol in the blood. Unlike with cardiac arrest, victims of heart attacks often have coronary heart disease.
This isn’t always the case, however. Heart attacks can also be caused by coronary artery spasms that interrupt blood flow through an artery. These spasms can occur even in people who do not have coronary heart disease. Other causes include use of drugs such as cocaine, extreme pain or emotional stress, and exposure to extreme cold.
Heart attacks and cardiac arrest are not the same, but they are causally connected. A heart attack victim is at heightened risk of sudden cardiac arrest during recovery from a heart attack.
Why Does CPR Work on Sudden Cardiac Arrest?
Sudden cardiac arrest is so deadly because without a heartbeat, the brain loses its supply of oxygenated blood. When that happens, irreversible brain damage and death can occur within minutes. That’s why it’s so crucial for cardiac arrest victims to receive CPR immediately—even CPR by an inexperienced bystander is better than waiting for an ambulance.
When rescuers deliver CPR to cardiac arrest victims, they essentially become the victim’s heartbeat. The chest compressions manually squeeze blood out of the heart and push it throughout the body, delivering some oxygen-rich blood to the victim’s brain and organs.
In the past, rescue breaths were believed to be crucial to restoring oxygen to the blood of a sudden cardiac arrest victim. However, recently the guidelines were changed to eliminate the teaching of rescue breaths to lay CPR students. Over ten years of statistics determined that the outcome of survival for CPR was better for those who received hands-only CPR from a bystander.
There is good reason for this. The body’s cardiovascular system needs to maintain a minimum amount of pressure to keep blood flowing through the body. Studies show that during CPR, it takes about 10 consecutive chest compressions at the right depth to raise the blood pressure to adequate levels and start the flow of blood to the brain.
Rescuers stop delivering chest compressions in order to perform rescue breathing—and that makes the blood pressure drop. That means each time a rescuer stops performing chest compressions to provide rescue breaths, the victim’s blood pressure drops and the process of restoring blood flow has to start all over again.
In addition, fear over delivering rescue breaths and increased anxiety over a more complicated process can prevent bystanders from providing CPR altogether, which is bad for cardiac arrest outcomes.
A cardiac arrest victim’s brain cells start dying within 4-6 minutes of losing blood flow. Each minute afterward, the chances of restoring those brain cells drops by approximately 10%. After 10-16 minutes, the cardiac arrest victim is functionally brain dead. That’s why it’s so crucial to get the blood flowing again via CPR—and why it’s so important for everyone, including non-medical professionals, to learn this lifesaving technique.