Hormone replacement therapy improves cardiovascular risk by lowering plasma viscosity in postmenopausal women

Hormone replacement therapy may protect against cardiovascular disease thro ugh several mechanisms that have variable actions on the major determinants of plasma viscosity. Plasma viscosity is an important predictor of inciden t and recurrent cardiovascular events and mortality in coronary heart disea se patients. The effect of estrogen alone or in combination with progestin on plasma viscosity is not known. Using a randomized, double-blind design, we examined the impact of the following daily hormone regimens on plasma vi scosity in 23 women: (1) 1 mg estradiol and 2.5 mg medroxyprogesterone (n=7 ); (2) 1 mg estradiol alone (n=8); and (3) placebo (n=8). Plasma viscosity, fibrinogen, and standard lipoprotein levels were determined at baseline an d after 12 weeks of intervention. Plasma viscosity was measured at 37 degre es C with a coaxial microviscometer. Fibrinogen was measured by the Clauss method. Significant changes in plasma viscosity (mPa.s) levels occurred amo ng treatment groups (P<0.01) after the intervention. Plasma viscosity was s ignificantly reduced with estrogen replacement therapy (P<0.01). These data demonstrate that estrogen replacement therapy lowers plasma viscosity. Thi s study suggests an additional mechanism for the cardiovascular protection conferred to postmenopausal women on estrogen replacement therapy.