Continuous (Machinery) Murmurs

1. Mechanism: High- or low-pressure shunts usually caused by abnormal communications between arteries.

2. Causes: Aortopulmonary shunts such as from patent ductus arteriosus, atrial septal defect (under specific conditions; related to tricuspid valve inflow and pulmonary valve outflow murmurs), as well as various arteriovenous malformations, fistulae, or aneurysms. Coarctation of the aorta may produce a biphasic, but systolic pre-dominant murmur). One type of continuous murmur is a physiologic venous hum originating from the internal jugular vein. It is frequently audible in children and young adults (most often pregnant women) or in patients with hyperdynamic circulation. A mammary flow murmur may also be heard. Other causes of continuous murmur are exceedingly rare.

3. Auscultation: Continuous murmurs are audible throughout the entire cardiac cycle without an interval between systole and diastole. They begin in systole, usually achieve their maximum intensity close to the S2, and persist throughout the entire or part of diastole. Some continuous or biphasic murmurs have different characters in systole and diastole. Venous hum is best audible between the clavicle and the sternocleidomastoid muscle, more often on the right side. It is loudest in diastole and increases in the upright position and upon turning the head to the opposite side; it decreases or disappears in the supine position and upon gentle compression of the internal jugular vein above the point where the stethoscope is placed. Obliterating the jugular vein with pressure abolishes the murmur.