April 2 (Bloomberg) -- Zoll Medical Corp. said some of its AED Plus external defibrillators, used in public settings such as airports, have defective batteries and software, leading to failures to deliver a shock and two patient deaths.

Zoll began on Feb. 12 asking customers to download new software for 180,000 units that will help detect a potential defective battery, the company said. About 80,000 units that have been installed for at least three years are at the highest risk, said the Chelmsford, Massachusetts-based company.

The two patient deaths were among four reported cases Zoll reviewed in which a shock was not delivered to a patient and “the battery may have experienced the identified problem,” the company said in its statement. Company officials said they couldn’t give any further details on the deceased.

“We did an extensive and thorough investigation,” said Ward Hamilton, Zoll’s senior vice president of marketing, in a telephone interview. “We’re still investigating but there’s a point where you have to let people know.”

Zoll fell as much as $1.64, or 12 percent, to $12.49 in extended Nasdaq trading. The company declined 47 percent in the past 12 months.

Each year 250,000 to 450,000 Americans suffer a sudden heart stoppage known as a cardiac arrest due to a rhythm disturbance or silent coronary artery disease, according to the National Heart Lung and Blood Institute. About 95 percent of such patients die, according to the agency. Defibrillators deliver a shock aimed at restoring a normal heart rhythm.

Survival Rate

“Studies show the rate of survival remains low, sometimes in the range of between 5 percent and 7 percent on average in the United States” even with resuscitation efforts, Hamilton said.

The AED Plus devices are located in health clubs, airports, schools and other public places, and used by emergency medical services personnel, Hamilton said. The malfunctions include batteries that don’t work and self-test software that fails to detect battery problems, according to the company’s statement.

Zoll asked customers to download free software updates from its Web site at www.zollaedplusbatteryhelp.com. The updated software will diagnose whether batteries need replacement, the company said.

To contact the reporter on this story: Marilyn Chase in San Francisco at mchase6@bloomberg.net

Last Updated: April 2, 2009 20:19 EDT