- If the ductus arteriosus fails to close at birth, it allows blood to pass between the systemic and pulmonary circulations.
- Since aortic pressure is greater than pulmonary artery pressure - the shunt is from arterial to venous side.
- Large volumes of blood pass through a relatively narrow ductus causing a loud murmur.
- The increased pressure in the pulmonary artery causes right ventricular hypertrophy and may eventually cause compensation breakdown.


Reference:
1. Anatomy & Physiology Manual. Butler L, Bouncir G, Burnett G. Lancashire & South Cumbria Cardiac Network. 2004.