maladie de Roger		Eponymous designation for a small restrictive ventricular septal defect that is not associated with significant left ventricular volume overload or elevated pulmonary artery pressure. There is a loud VSD murmur due to the high velocity turbulent left-to-right shunt across the defect.
malposition		An abnormality of cardiac position. see cardiac position.
MAPCA		Major Aorto-Pulmonary Collateral Artery. A large abnormal arterial vessel arising from the aorta, connecting to a pulmonary artery (usually in the pulmonary hilum) and providing blood supply to the lungs. Found in complex pulmonary atresia and other complex CHD associated with severe reduction or absence of antegrade pulmonary blood flow from the ventricle(s).
Marfan syndrome		A connective tissue disorder with autosomal dominant inheritance caused by a defect in the fibrillin gene on chromosome 15. The phenotypic expression varies. Patients may have tall stature, abnormal body proportions, ocular abnormalities, dural ectasia, protrusio acetabulae, and present with skeletal and cardiovascular abnormalities. Mitral valve prolapse with mitral regurgitation, ascending aortic dilation/aneurysm with subsequent aortic regurgitation, and aortic dissection are the most common cardiovascular abnormalities. see also Ghent criteria.
mesocardia		Cardiac apex directed to mid-chest. see cardiac position.
mesoposition		Shift of the heart toward the midline. see cardiac position.
mitral arcade		Chordae of the mitral valve are shortened or absent and the thickened mitral valve leaflets insert directly into the papillary muscle ('hammock valve'). Mitral valve excursion is limited and results in mitral stenosis.
moderator band		A prominent muscular structure traversing the right ventricle from the base of the anterior papillary muscle to the septum near the apex.
muscular VSD		see ventricular septal defect.
Mustard procedure (operation)		An operation for complete transposition of the great arteries, in which venous return is directed to the contralateral ventricle by means of an atrial baffle made from autologous pericardial tissue or (rarely) synthetic material, after resection of most of the atrial septum. As a consequence, the right ventricle supports the systemic circulation. A type of "atrial switch" operation (see also Senning procedure). (Mustard WT. Successful two-stage correction of transposition of the great vessels. Surgery 1964;55:469-472.)