Parallels Between Myometrium and Myocardium

Very little attention has been paid to the oxygen supply and demand balance of the myometrium, despite the fact that it is well known that hypoxic myometrium does not contract well. By contrast, the heart has received an immense amount of attention in this regard and one key therapeutic theme in critical care medicine and cardiology is reducing myocardial oxygen demand and increasing myocardial oxygen supply. Both the myometrium and the heart are perfused only when they are not contracting, and in both organs an increased rate of contraction increases oxygen consumption. Hence, both tachycardia and uterine tachysystole are dangerous. Also, Laplace’s law applies to both organs and both a large heart and a large (“overdistended”) uterus have precarious perfusion and oxygenation, compared to smaller organs. In both organs, elevated tissue pressure can reduce organ perfusion pressure. In the heart, left ventricular end-diastolic pressure serves as an index of subendocardial tissue pressure, and elevations are associated with myocardial infarction and heart failure. It may be possible to reduce tissue pressure in the uterus by having the patient assume a lateral position, and prevention of obstruction of uterine venous return might improve uterine perfusion pressure as well. Obstetric and midwifery practices which increase myometrial oxygen supply during gestation and labor by encouraging maternal mobility may improve pregnancy outcomes, just as improved oxygenation of heart muscle improves the lives of cardiac patients.