6.10.3 Rupture of the pulmonary artery
Rupture of the PA is an event which usually results in sudden death and may occur in animals with pulmonary hypertension. Usually the degree of pulmonary hypertension required to result in rupture is caused by severe left-sided cardiac disease. Animals with severe MR are most at risk.
No clinical signs may be attributed to the dilation of the PA which precedes rupture; however, clinical evidence of the primary disease is likely to be present. However, the PA can be dangerously dilated in some horses in which cardiac disease has not been recognised by the owners. Usually animals will be showing signs of CHF such as dependent oedema, but this may not be evident in all animals in which the artery is dilated.
Diagnosis of rupture of the PA is usually made at PM, although it may be missed because the condition does not figure prominently in the literature. Occasionally animals will survive long enough to be presented for echocardiographic examin which case, flailing of the pulmonary valve and dissection of blood into the surrounding tissue may be seen. Prior to rupture, dilation of the PA will be seen on echocardiography. Although measurement of the PA diameter cannot be made with absolute precision, adult Thoroughbred horses with PA diameters
>8 cm at the valve base should be considered to be at risk of rupture if accompanying signs of severe left-sided heart disease are present. In general terms the PA diameter should not exceed the aortic diameter.
Owners of animals with moderate to severe MR should be encouraged to present them for periodic repeat echocardiographic examinations (approxievery 3-12 months), so that signs of dilation of the artery can be detected before the risk of rupture becomes too great to allow the animal to be ridden.
The prognosis for animals with dilated pulmonary arteries is poor and they are unsafe to ride. The prognosis for animals with a rupture of the artery which has not resulted in sudden death is hopeless and euthanasia is the only course of action.