Positive cultures from cardiopulmonary bypass: prevalence and relevance regarding postoperative infection

Postoperative infections due to cardiopulmonary bypass (CPB) are associated with high morbidity and mortality. The value of positive cultures taken from CPB priming fluid and CPB blood samples, however, is unclear. This study investigates the epidemiology of positive cultures from CPB and their relation to the occurrence of postoperative infection.

The study was conducted at the Maastricht University Medical Centre, a 715-bed teaching hospital with 900-1000 surgeries requiring CPB annually. From 1 January 1998 until 31 March 2010, all patients with positive CPB cultures drawn either from priming fluid or blood were retrospectively studied. Second, 335 patients with a positive CPB culture were compared with 335 randomly assigned patients who underwent cardiovascular surgery using CPB and had negative CPB cultures. Patients with active endocarditis were excluded. Demographic data and perioperative parameters were documented. Outcome measures were: a relevant infection (acute infectious valve endocarditis, wound infection, intravascular catheter-related infection, and bloodstream infection), occurrence of fever of unknown origin, and 30-day mortality.

A total of 21840 cultures were analyzed, half being priming fluid and half CPB blood cultures. As many as 111 out of 10920 (1.0%) priming fluid cultures and 598 out of 10920 (5.6%) blood cultures tested positive. Gram-positive cocci predominated both priming fluid and blood cultures. Relevant postoperative infections within 30 days after surgery were seen in 47/663 (7.1%) of patients overall, in 27/330 in the CPB-culture-positive group (8.2%) and 20/333 in the CPB-culture-negative group (6.0%), p=0.275. As many as 38 out of 363 patients (5.7%) were affected by fever of unknown origin (CPB-culture-positive group 4.5%, and CPB-culture-negative 6.9%; p=0.191). The 30-day mortality was 16/330 (4.8%) in the CPB-culture-positive group and 13/333 (3.9%) in the CPB-culture-negative group (p=0.552).

Positive cultures from both CPB priming fluid and CPB blood samples were not a rarity and mainly involved skin bacteria, arguing that contamination may have played a role. The risk of postoperative infection within 30 days after surgery was not increased in CPB-culture-positive patients. Therefore, no evidence was found to support routine culturing of CPB samples in patients undergoing cardiothoracic surgery.