Q. I watched a man fall unconscious on the sidewalk. A woman rushed up and started to do CPR on him and, later, I heard she may have saved his life. It made me sign up for a CPR course. You should tell your readers to take one of these courses.

If you would like to learn CPR, contact the American Heart Association at americanheart.org. Another CPR resource is the American Red Cross at redcross.org. Or, you can try a local hospital.

Here’s a troubling fact that is a motivation to take a course: About 80 percent of cardiac arrests happen at home near family members who often do not know CPR.

CPR, which stands for cardiopulmonary resuscitation, employs chest compression and mouth-to-mouth breathing to treat cardiac arrest, heart attack, drowning and electrocution. CPR can keep some blood flowing to the brain and heart during an emergency.

Maintaining blood flow can prevent brain injury and save a life. The brain suffers irreparable damage in a few minutes if it doesn’t get oxygenated blood. An unaided victim of cardiac arrest will die in five to 10 minutes.

The most common cause of sudden cardiac arrest is an abnormal heart rhythm called ventricular fibrillation (VF), which can be treated with a shock from a defibrillator. Defibrillation is not effective for all forms of cardiac arrest.

There are devices called automated external defibrillators (AEDs) that are about the size of a laptop computer. AEDs analyze the victim’s heart rhythm, determine if defibrillation is needed, then deliver a shock. There are training programs available that teach both CPR and operating AEDs. These portable defibrillators are available in many public places such as shopping malls, airports and stadiums.

To learn CPR properly, take an accredited first-aid training course that includes CPR and how to use an AED.

There is no substitute for taking a course from a trained instructor, but it would be helpful to understand the basics of CPR.

The University of Washington School of Medicine offers a free public service that explains CPR. Go to: depts.washington.edu/learncpr/

There are helpful illustrated guides and online videos on this website. The following is from one of these guides:

Call: Check the victim for unresponsiveness. If the person is not responsive and not breathing or not breathing normally. Call 911 and return to the victim. In most locations the emergency dispatcher can assist you with CPR instruction.

Pump: If the victim is still not breathing normally, coughing or moving, begin chest compressions. Push down in the center of the chest two inches 30 times. Pump hard and fast at the rate of at least 100 per minute, faster than once per second.

Blow: Tilt the head back and lift the chin. Pinch nose and cover the mouth with yours and blow until you see the chest rise. Give two breaths. Each breath should take one second.

Continue: With 30 pumps and two breaths until help arrives. his ratio is the same for one-person and two-person CPR. In two-person CPR, the person pumping the chest stops while the other gives mouth-to-mouth breathing.

There is also a hands-only version of CPR. You can learn this at: www.redcross.org/prepare/hands-only-cpr

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