Abstract
Background: Atrial fibrillation (AF) is associated with a systemic pro-coagulant state. Coagulation involves complex interactions between endothelial and circulating cells, cell-derived microparticles and plasma proteins.
Methods: We investigated the levels of endothelial and platelet microparticles, leukocyte and platelet activation, and cell interactions in 48 patients with non-valvular AF and compared to normal controls. We performed flow cytometry in periheral venous blood to measure (1) Platelet expression of P-selectin, (2) Platelet microparticles; (3) Platelet-leukocyte conjugates; (4) Leukocyte expression of CD11b; (5) Endothelial microparticles (EMP) positive for CD31 and negative for CD42 (EMP31+), EMP positive for E-selectin (EMP62E+), CD54 (EMP54+), or CD51 (EMP51+); (6) EMP54+- and EMP62E+-leukocyte conjugates. Atrial fibrillation was confirmed electrocardiographically and left atrial volume was estimated by echocardiography. The mean age of patients with AF was 61.3±12 years. Congestive heart failure was present in 54.2% of patients, hypertension in 72.9%, and diabetes mellitus in 22.9%. Most patients (81%) were receiving anticoagulant therapy and 62.5% were receiving antiplatelet medications. Calcium channel blockers or digoxin were given to 43.8% and 54.2% of patients, respectively. The mean ventricular rate was 85.5±17 beats per minute, and the mean left atrial volume was 97.9 ml.
Results: Patients with AF had significantly higher levels of platelet P-selectin expression and platelet-leukocyte conjugates. P-selectin expression strongly correlated with platelet-leukocyte conjugate levels (r=0.77; p<0.001; Figure 1). Patients with AF also demonstrated increased levels of free EMP31+, EMP62E+ and EMP51+, as well as EMP62E+-lymphocyte, -monocyte and -neutrophil conjugates. Levels of free EMP54+ were lower, whereas EMP54+- neutrophil and EMP54+-lymphocyte conjugates were higher among patients with AF. EMP62E+-neutrophil conjugates correlated with leukocyte expression of activation marker CD11b (r=0.42; p=0.003; Figure 2) and predicted CD11b expression independently of platelet-leukocyte conjugates (r2 increase=16%; p=0.003).
Conclusions: Patients with AF have increased levels of platelet and leukocyte activation, circulating endothelial microparticles, and binding of endothelial microparticles and platelets to leukocytes. Platelet activation is a determinant of platelet-leukocyte conjugate formation in AF. The formation of EMP62E-neutrophil conjugates is a major determining factor for circulating leukocyte activation in AF. Further studies are needed to assess the correlation of these markers with the risk of thromboembolism in AF.
