The Food and Drug Administration's approval of cardiovascular devices is often based on weak studies or may be prone to bias, U.S. researchers said on Tuesday.

The researchers reviewed 123 FDA summaries of studies for 78 applications from manufacturers for cardiovascular devices, such as pacemakers, stents and heart-valve replacements approved between January 2000 and December 2007. The information was posted on the agency's Website.

An analysis found that 65 percent of the devices were supported by a single study, the researchers wrote in the Journal of the American Medical Association.

The researchers said they also found that only 27 percent of the studies were randomized and 14 percent were blinded.

These are high-risk cardiovascular devices. We were expecting to see kind of a more common use of randomized, blinded studies which are considered to be the highest quality studies, lead author Dr. Rita Redberg said in a telephone interview.

The FDA oversees medical devices ranging from simple bandages and tongue depressors to the most complex products such as pacemakers and heart-valve replacements.

The agency said the study was flawed because it did not consider the feasibility of device clinical trials and because it did not have access to all of the evidence from studies on the devices.

They took the assumption that a randomized double-blind, placebo controlled trial is the kind of study you need to do for devices. It is the kind of study you would do in the case of drugs, but for medical devices that may not necessarily be either feasible or ethical, said Dr. Jeffrey Shuren, head of the FDA's Center for Devices and Radiological Health, said in a telephone interview.

Expedited reviews of devices shown to be similar to existing products allow companies to get products to market more quickly and with less expense.

Consumer watchdog groups, lawmakers and others have criticized the FDA program that gives fast-track approval to certain medical devices.

It would be helpful to have stronger legislation because the standards for approval of devices are not as stringent as approval for drugs as pointed out in this paper, said Sidney Wolfe, director of the consumer advocacy group Public Citizen.