5.10.2 Clinical signs
Clinical signs will depend on the presence of other congenital lesions and the size of the ductus. Animals with a large shunt or complex defects are likely to present with CHF at an early age. Horses with small shunts may present with exercise intolerance later in life. On examination, a continuous waxing and waning 'machinery' murmur is heard, with a PMI over the left base. The murmur is continuous because a continuous pressure gradient exists between the high-pressure aorta and the relatively low-pressure PA. The murmur waxes and wanes in relation to the change in pressure gradient and volume of flow throughout systole and diastole. It may radiate widely, but the diastolic element may be heard over a localised region only. In some cases, if pulmonary hyperdevelops, the murmur may loose its diastolic element so that the continuous sound may be replaced by a predominantly systolic murmur. Bounding water-hammer arterial pulses may be palpated because of the large stroke volume which results from volume overload and the 'let-off valve' effect of the PDA.