5.7.1 Haemodynamic effects
ASDs allow blood to be shunted from the high pressure LA to the low pressure RA, or from right to left if RA pressure is high. In fact, the left to right flow is seldom of great haemodynamic significance except if the defect is large. Blood flow through a large ASD increases flow through the pulmonary valve and may result in an ejection-type murmur of 'relative pulmonary stenosis', with a PMI at the left base. More often, the haemodynamic significance of ASDs is to allow right to left shunting in animals with a right-sided obstruction (e.g. tricuspid atresia). In some animals, the shunt may only open up during exercise, when right-sided pressures rise significantly.