Page 1010 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Cardiovascular system 985

VetBooks.ir no clinical signs are observed. With large defects, pressure and resultant decrease in pressure gradient
from left to right. The prognosis for complex con-
hypoxaemia and heart failure may develop. Volume
overload of the left heart then occurs. ASD is not
commonly associated with a cardiac murmur; how- genital heart disease is grave and neonatal death is
common.
ever, a holosystolic murmur has been described in
the pulmonic valve area. Murmurs may also result PATENT DUCTUS ARTERIOSUS
from tricuspid dysplasia or pulmonic stenosis.
Definition/overview
Differential diagnosis True PDA is a rare defect in the foal and is most
With ASD as a sole abnormality, clinical signs are often associated with complex congenital heart dis-
rare, and no murmur is usually detected. Differential ease. It is important to know that the ductus arte-
diagnoses include pulmonic stenosis and aortic steno- riosus does not close immediately at birth, and a
sis (both rare). Differential diagnoses for complex con- diagnosis of PDA should not be made in the early
genital anomalies (tricuspid or pulmonic atresia) are neonatal period (<4 days).
variable and depend on the cardiac structures involved.
Aetiology/pathophysiology
Diagnosis In the fetus the ductus arteriosus allows blood to
There are no radiographic or electrocardiographic pass from the pulmonary artery to the aorta, allow-
changes that are characteristic of ASD. AF has been ing oxygenated blood to access the systemic circu-
associated with ASD when congestive heart failure lation. At birth the pressure gradient reverses and
has developed. Increased pulmonary vascularity and the ductus arteriosus normally closes. With complex
cardiac enlargement are not specific. congenital cardiac deformities, abnormal pressure
ASD is diagnosed by echocardiography. Careful gradients often result in PDA.
examination of the interatrial septum is necessary. As part of the fetal circulation, the ductus arte-
Contrast or colour-flow Doppler echocardiography riosus usually narrows near birth and then constricts
is of value in determining the presence of shunting. rapidly as systemic vascular pressures increase and
It is important to evaluate closely and to differen- pulmonary vascular pressures decrease. If the ductus
tiate true ASD from echocardiographic drop-out. remains large, left-to-right flow may occur due to the
If an ASD is detected, careful examination for development of a pressure differential. This results
additional cardiac malformations is indicated. in volume overload, which may progress to pulmo-
Pulmonary artery dilatation is evidence of right- nary hypertension, right ventricular hypertrophy
heart overload. While uncommonly performed, and heart failure. If the pulmonary pressure increases
cardiac catheterisation may demonstrate increased sufficiently, right-to-left shunting may occur.
right heart/pulmonary pressure and increased oxy-
gen saturation in pulmonary and right-heart blood. Clinical presentation
Clinical signs are variable and range from none to
Management severe. The size of the PDA, as well as the pres-
There is no treatment for ASD and in most cases no ence of other congenital deformities, contribute to
therapy is required. There are no therapeutic options the severity of clinical signs. Cyanosis may occur if
for complex congenital deformities, and euthanasia the shunt reverses. Caudal cyanosis may be noted
is often indicated. if the PDA enters the aorta distal to the brachioce-
phalic trunk.
Prognosis
In cases of lone ASD of relatively small size, a rea- Differential diagnosis
sonable level of performance may be expected. The The typical ‘machinery murmur’ in the immedi-
degree of blood shunted across the defect decreases ate post-natal period may be detected in the normal
during exercise due to the increased right-heart foal and it usually disappears over the first 96 hours.