Children as young as nine can learn CPR in school, researchers say.

Cardiopulmonary resuscitation is a first-aid procedure for an unconscious person whose breathing or pulse has stopped. The technique aims to protect the heart and brain until the heart can resume pumping in a normal rhythm.

Bystander CPR is known to more than double the chance of survival in sudden cardiac arrest. But the usefulness of CPR training in schools has been questioned, given concerns that young children may not be strong enough to give chest compressions or understand the steps.

In Thursday's online issue of the journal Critical Care, Fritz Sterz of the Medical University of Vienna and his colleagues studied 147 school children who received six hours of life-support training from their teachers.

Four months after the training, 86 per cent performed CPR correctly, Sterz said.

"Students as young as nine years are able to successfully and effectively learn basic life-support skills, including AED [automated external defibrillators] deployment, correct recovery position and emergency calling," the study's authors concluded.

"Given the excellent performance by the students evaluated in this study, the data support the concept that CPR training can be taught and learnt by school children and that CPR education can be implemented effectively in primary schools at all levels."

As with adults, physical strength may limit the depth of chest compressions and volume of breaths, but skill retention is good, Sterz said.

In 2007, British researchers concluded children as young as 13 can perform CPR as well as adults, and suggested the lessons start in elementary school.

The Ottawa-based ACT Foundation started teaching CPR to high school students in 1994. Since then, 900,000 high school students across Canada have learned the life-saving technique.