According to the CDC, someone in the U.S. has a heart attack every 43 seconds. It happens when part of the heart muscle doesn't receive enough blood flow. The longer it takes to get treatment, the greater the damage to the heart muscle.

How do you know if someone is having a heart attack? It's important to recognize the difference between a heart attack and sudden cardiac arrest. When someone is experiencing a heart attack, he or she is conscious and might complain of chest pain or other symptoms. When a person is experiencing sudden cardiac arrest, the heart has unexpectedly stopped beating and blood is no longer pumping throughout the body or the brain. The individual may lose consciousness and appear lifeless. Some victims gasp and shake as if they're having a seizure. Death can occur within minutes.

Here are the five things you should do right away, before help arrives, if you think someone is experiencing heart trouble:

1. Call 9-1-1.

Whether it's a heart attack or sudden cardiac arrest, step one is always calling 911 dispatchers who can coach you through life-saving measures.

2. Ease the strain on the heart.

If the person is still conscious and possibly suffering a heart attack, Dr. Mehmet Oz recommended a comfortable position — half-sitting, with head and shoulders well supported and knees bent, to ease the strain on the heart. Loosen the clothing at the neck, chest and waist.

3. Chew and swallow an aspirin.

If the person is fully conscious, provide a full dose (300 mg) of an Aspirin tablet. Advise the person to chew it slowly so that it dissolves and is absorbed into the bloodstream more quickly when it reaches the stomach. Aspirin helps to break down blood clots, minimizing muscle damage during a heart attack.

4. Perform CPR.

The next step is to start chest compressions. Press hard (about two inches deep) and fast (100-120 times per minute) on the center of the chest.

5. Look for an automated external defibrillator.

These devices have clear instructions and are designed for use by the public. Here is a brief overview of how to use it: Attach the pads as indicated on the machine, and then the machine will talk you through the process. It will only deliver a shock if the patient's condition indicates that it is necessary. Leave the machine switched on at all times, and leave the pads attached — even if the patient has recovered.