Mean Platelet Volume and Cardiac Surgery-Associated Atrial Fibrillation

Postoperative atrial fibrillation (AF) is the most common complication after cardiac surgery. Although reported incidences vary widely, postoperative AF occurs in as many as 30% to 50% of patients. Mean platelet volume (MPV)—a marker of platelet activation and peri-surgical inflammation— recently was reported to be associated with postoperative AF in a small-scale study involving patients undergoing isolated coronary artery bypass grafting surgery. Higher MPV values reflect increased platelet activity, a key inflammatory response that has been associated with numerous adverse outcomes in critical illness settings, as well as after cardiac surgery.^, Extensive cross-talk between activated platelets and other cell types has been described in the early phase of the inflammatory response reaction. Of note, we recently demonstrated an independent association between the magnitude of postoperative MPV changes and the development of postoperative acute kidney injury and its severity. We tested our primary hypothesis that postoperative changes in MPV after cardiac surgery are associated independently with postoperative AF. We conducted a single-center, retrospective observational study at a tertiary academic center on 4,071 patients older than 18 years who underwent cardiac surgery between December 12, 2011, and June 5, 2018. A standard set of perioperative data were abstracted and collected from the institutional Society of Thoracic Surgeons Adult Cardiac Surgery Database, the Perioperative Data Warehouse (developed and maintained by the institutional Department of Anesthesiology and Perioperative Informatics Research Division), and the patients’ electronic medical records.

Demographics, clinical characteristics, preoperative and postoperative medication use, cardiopulmonary bypass duration, intraoperative and postoperative blood product administration, platelet counts, and MPV were considered risk factors for postoperative AF. Platelet counts and MPV were measured by the institutional clinical pathology laboratory preoperatively and for the first 10 days postoperatively or until discharge, whichever came first. The baseline MPV values were defined as the value available closest to the date of surgery. The average postoperative MPV values were mean values of the daily measurements before the occurrence of postoperative AF within 10 days after surgery for patients with postoperative AF, or mean values of the daily measurements during the first two days (ie, the median follow-up time or time to postoperative AF) for patients without AF. Subsequently, changes in postoperative MPV values were calculated as the difference between baseline values and average postoperative values.

New-onset postoperative AF was determined based on documentation of postoperative electrocardiogram or rhythm strip or at least two of the following forms of documentation in the electronic health records of the patients: progress notes, nursing notes, discharge summary, or change in medication. A logistic regression model was used to determine associations among changes in postoperative MPV, changes in postoperative platelet counts, and postoperative AF as a binary variable after adjusting for the prespecified clinical risk factors. The median (25th-75th) postoperative MPV change was 0.23 fL (0.04-0.40) for patients with postoperative AF and 0.20 fL (0.07-0.37) for patients without postoperative AF (p = 0.032), and the median platelets count change was –69 × 10⁹/L (–108 to –39) for patients with postoperative AF, and –66 × 10⁹/L (–102 to –35) for patients without postoperative AF (p = 0.044). Demographic, clinical, and perioperative characteristics of the patients with postoperative AF and without postoperative AF are presented in Table 1.

The incidence of postoperative AF was 33.6% (n = 1368). The multivariate analysis showed that there was no significant association between MPV and postoperative AF (Fig 1). In the multivariate analysis, older age, male sex, higher body mass index, obstructive sleep apnea, and longer cardiopulmonary bypass duration were associated with elevated odds for postoperative AF. In a separate sensitivity analysis when we adjusted for postoperative blood product administration, the association among preoperative (baseline) average MPV and platelet counts, postoperative changes in MPV and platelet counts, and postoperative AF remained unchanged (Fig 2). Thus, this study of cardiac surgery patients at high risk for postoperative AF found no evidence that a postoperative increase in MPV was associated with a higher risk for postoperative AF.