Bacteria Spread to Bloodstream by Contaminated Cardiac Devices

ROCHESTER, Minn., April 12 -- The source of Staphylococcus aureus bloodstream infections that develop after cardiac devices are implanted may be the devices themselves, researchers here reported.

Implantable defibrillators were a greater source than permanent pacemakers for S. aureus infections stemming from the devices or their electrode leads, according to a report in the April 9 issue of the Archives of Internal Medicine.

"Surprisingly," Dr. Uslan's team said, "we found that the cumulative probability of device infection was significantly higher in patients with defibrillators." Infection from defibrillators was almost nine times greater than infection from pacemakers."

It is unclear, they said, whether this is because of underlying comorbidities or demographics of defibrillator patients, or differences in devices or physical properties of the lead, or implantation-associated risks.

If there is S. aureus bloodstream infection, then an infected device was the source in 54.6% of cases, while device infection was only responsible for 12% of Gram-negative bacilli bloodstream infections, said Daniel Z. Uslan, M.D., of the Mayo Clinic here, now at the University of California Los Angeles, and colleagues.

Device infection was microbiologically confirmed if findings from culture samples or Gram stain from the generator pocket or electrode lead were positive for organisms, the researchers said.

Accurate identification of infective organisms in a device is essential to help avoid unnecessary device removal in patients who are dependent on it, Dr. Uslan said. A correct diagnosis is equally important for patients who do not have infection stemming from a device so that unnecessary device removal does not occur.

But until now, he said, neither the rate of bloodstream infection from devices with infective organisms nor the impact on treatment has been adequately evaluated.

The findings emerged from a large 30-year retrospective population-based cohort study of 1,524 patients with cardiac devices living in Olmsted County, Minn., home of the Mayo Clinic, from 1975 to 2004.

Of the patients, 1,300 had permanent pacemakers, 203 patients had defibrillators, and 21 patients had both. The mean age at device placement was 74.6 years overall, although there was a significant difference between those with pacemakers (76.2) and those with defibrillators (64.3), the researchers said. Of the pacemaker patients, 51% were female, compared with only 19% of defibrillator recipients.

Total person-time of follow-up was 7,578 years. Overall, the incidence of definite device infection was 1.9 per 1,000 device-years (95% confidence interval [CI], 1.1-3.1).

The incidence of pocket infection without bloodstream infection was 1.37 per 1,000 device-years (95% CI, 0.62-3.05), and pocket infection with blood infection or device-related endocarditis was 1.14 per 1,000 device years (95% CI, 0.47-2.74), the researchers reported.

The cumulative probability of device infection was higher among patients with defibrillators compared with pacemaker patients (P

Twelve (54.6%) of 22 cases of S aureus bloodstream infection had definite or possible cardiac device infection versus three (12.0%) of 25 cases of blood infection because of Gram-negative bacilli (P=.004), the researchers reported.

The estimated rate of S aureus bloodstream infections caused by primary infection from devices was as much as 54.6%, similar to previously reported studies, the researchers said, while the low-risk of Gram-negative bacilli bloodstream infection caused by infected devices was similar to their previously published results in non-population-based studies.

One explanation, they said is that although device infection does occur with Gram-negative bacilli, antimicrobial therapy is curative, even if the system is infected and is not removed.

Study limitations included its retrospective design. Also, Olmstead County has relatively homogeneous demographics, which limited the generalizability to populations underrepresented in the largely white community.

Although the lack of universal echocardiography precluded an exact calculation of the rate of device infection due to varying pathogens, they said, the long duration of follow-up available with a population-based study excluded device infection in most of these patients. Even if device infection were present in all patients who did not have echocardiography, it would not have had a treatment effect, because relapses were uncommon, they wrote.

To their knowledge, the researchers said, this is the first population-based study to describe the incidence of cardiac-device infection in patients with a bloodstream infection caused by varying pathogens. Further understanding of the specific risk factors for device infection and differing risks between pacemakers and defibrillators will be crucial, they said.

"Prior studies have supported the systematic extraction of cardiac devices among patients with device infection, followed by prolonged therapy with antimicrobial drugs prior to device reimplantation at a new site," Dr. Uslan and his colleagues wrote.

"Our data support the consideration of device infection among patients with S aureus bloodstream infection, but device infection, or the necessity for device removal, does not seem to be commonly found in patients with bloodstream infection caused by gram-negative bacilli."