WASHINGTON -- The FDA has elevated the status of a recall of a semi-automatic external defibrillator series to class I -- the agency's most serious -- because of a defect that cancels the shock it is meant to deliver. Manufacturer Defibtech said in a statement that the Lifeline AED and ReviveR AED brands of the DDU-100 series with 2.004 software and earlier may fail to provide therapy to patients as the device cancels its shock while charging. The devices may also cancel a shock-ready charge in high humidity and condensing conditions. The error cannot be detected by the periodic self test, the statement said. Defibtech will issue free software upgrades on affected AEDs to address the charging issues, the statement said. Device owners can check if their device is affected by the recall on a company website. The company added that the error occurs very rarely and that the devices should be kept in service until the software upgrade, the statement said. The AEDs were distributed globally to fire departments, emergency medical services, health clubs, schools, and other locations. Class I recalls affect devices with a reasonable risk of serious adverse event or death with use.