Myocardial tissue hematocrit: Existence of a transmural gradient and alterations after fibrinogen infusions

Abstract

Tissue hematocrit was determined in left ventricular myocardium slices in in situ frozen rat hearts and the influence of experimental plasma fibrinogen increments was investigated. Hematocrit values in 100 μm thick myocardial slices were estimated by measuring the activity of two different radionuclides labelling plasma (¹²⁵I-Albumin) and red blood cells (^99mTc). In the control group with physiological levels of fibrinogen, myocardial tissue hematocrit was highest at the layer adjacent to epicardium. It decreased linearly through the depth of myocardium, approaching a minimum at the myocardial layer closest to the endocardium. In the group with about two folds higher plasma fibrinogen concentration, plasma viscosity and red blood cell aggregation were about 50% higher. In this group, tissue hematocrit difference between epicardial and endocardial layers existed, however this difference was more likely due to a step change, rather than a linear gradient. These findings confirm those physiological mechanisms underlying the hematocrit reduction in small blood vessels might be affected by hemorheological alterations.