Structural-Mechanical Changes in the Pericardium During Pregnancy

The pericardium's collagenous structure limits over-expansion of the heart, couples the left and right ventricles, and keeps the heart in place in the chest cavity. Experimental volume overload studies in dogs have demonstrated consequent pericardial remodeling. By comparison, the maternal heart is subjected to large, natural volume overload but with altered hormonal levels. No current literature exists on the resulting pericardial changes. This study investigates those effects in a bovine model that mimics human pregnancy. Pericardial samples were collected from 10 never-pregnant and 17 pregnant cattle from 13 weeks gestation or later. Biaxial mechanics, hydrothermal isometric tension (HIT) testing, crosslink analysis, histology and biochemical tests for water and collagen contents were conducted. There were no changes in the collagen and water contents, or in the crimp length between pericardia of pregnant and non-pregnant animals. In late pregnancy, pericardial thickness increased with only slight decreases in biomechanical stretch ratios, mainly in the base-to-apex direction. Pericardium from pregnant cattle had a 14.8% decrease in cell density, 1. 3° lower collagen denaturation temperature, and a surprising 32.0% lower index of immature crosslinking. Indeed, hydrothermal studies of collagen from the pregnant animals' pericardia showed greater mature and total crosslinking (40.9 and 22.4% increases in isothermal load decay half-times) respectively. Overall, then, there was evidence on the molecular level of remodeling and growth in the pericardium in early pregnancy (<13 weeks gestation). This is the first study to demonstrate that pericardium remodels and grows to adapt to changes in pregnancy, perhaps as a means to maintaining its functional properties.