Atrial Fibrillation

Blood viscosity has significant potential to serve as a predictor of ischemic stroke in patients with atrial fibrillation. Blood viscosity has been established to correlate with the presence of spontaneous echo contrast. Spontaneous echo contrast (smoke-like swirling echo) in transesophageal echocardiography has been observed during paroxysmal atrial fibrillation, indicating atrial hemostasis [1]. Spontaneous echo contrast is a rheological marker for left atrial thrombus and embolism in nonvalvular atrial fibrillation [2]. In a prospective study of 272 patients with nonvalvular atrial fibrillation, patients with spontaneous echo contrast had a fourfold increased incidence of subsequent stroke or embolism than those without spontaneous echo contrast (p=0.002), [3]. In a clinical study of 185 atrial fibrillation patients, a significant positive correlation was observed between the higher grades of spontaneous echo contrast and blood viscosity measurements (p<0.001), [4]. Separately, a study involving 156 patients with nonvalvular atrial fibrillation and 130 healthy controls showed an independent association between blood viscosity and ischemic events in multivariate analysis: blood hyperviscosity was observed in patients with prior ischemic events more frequently than in those who had not (odds ratio 3.19, p=0.023), [5].

A number of outcome studies have demonstrated the risk of major cardiovascular events increased with blood viscosity [6,7]. In a random population of 1,106 adults, both blood viscosity and hematocrit-corrected viscosity were shown to be linearly related to common carotid intima-media thickness (IMT) (p≤0.001), and upon multivariate analysis blood viscosity remained significantly associated with carotid IMT (p≤0.01) [8]. Stroke patients and those with stroke risk factors were shown to have chronically elevated blood viscosity relative to healthy controls [9]. The link between blood viscosity and cardiovascular disease events was reported in a study of a random population of 1,592 adults followed for a mean of 5 years: After adjustment for age and gender, both blood viscosity and hematocrit-corrected blood viscosity were higher in patients who experienced cardiovascular disease events (ischemic heart disease and stroke) than in those who did not. The difference in blood viscosity between the two groups of patients was significant (p=0.0003), and the relationship of viscosity to the occurrence of cardiovascular disease events was at least as strong as that of diastolic blood pressure and LDL cholesterol and stronger than that of smoking. With further adjustment for conventional cardiovascular risk factors, the association with blood viscosity remained significant only for stroke [10].

The findings support the central concept that hyperviscosity is an important risk factor for ischemic stroke. Additional clinical research is needed in atrial fibrillation—a disease state where viscosity has been shown to be independently associated with ischemic events and there is significant increased patient risk of stroke—to show whether or not blood viscosity can serve as a more accurate predictor of stroke.

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