Page 1009 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

984 CHAPTER 8

VetBooks.ir from cyanosis at birth and performance reduction, velocities of >4 m/s. Congestive heart failure often
The clinical signs associated with VSD may vary
occurs before 5 years of age in those animals with
to no haemodynamic effect at all. The severity of
clinical signs is associated with the location and peak shunt velocity of <3 m/s (lower velocity is sug-
gestive of a lower pressure differential across the
size of the VSD. Large defects are more commonly defect, and therefore indicates increased right heart
associated with severe clinical signs, which result pressures). Horses with a VSD are considered to be
from volume overload and congestive heart failure. at higher risk for bacterial endocarditis because of
Pulmonary oedema may result in dyspnoea. Small disturbed blood flow and increased probability of
defects may have no haemodynamic effects. Cardiac endocardial damage.
arrhythmia, most commonly AF, may be present
when atrial enlargement has occurred. ATRIAL SEPTAL DEFECT

Diagnosis Definition/overview
Radiographic changes in cardiac silhouette are An atrial septal defect (ASD) is a communication
uncommon with VSDs in the horse. Radiographs are between the left and right atria. This may be due
useful, however, in determining the haemodynamic to abnormal septation, true ASD, or due to failure
effects (based on pulmonary oedema). No character- of closure of the foramen ovale at birth (persistent
istic electrocardiographic changes have been associ- foramen ovale). ASDs are rare.
ated with VSDs in the horse.
Definitive diagnosis can be reached in many Aetiology/pathophysiology
cases with echocardiography. Small defects may be No familial or breed predispositions have been iden-
difficult to see unless careful examination of the tified. Most commonly, ASD occurs in combination
septum in both short-and long-axis views is per- with other defects in complex congenital anomalies.
formed. Infundibular defects are particularly diffi- The cause is unknown.
cult to evaluate. They may be slit-like and therefore In ASDs, blood flow shunts from left to right
present on only one view. Colour-flow Doppler can due to the pressure gradient between the atria. The
assist in identifying a defect and in demonstrating pressure gradient is much smaller than that for the
the direction of flow. Velocity measures assist in ventricles. Left-to-right flow results in increased vol-
evaluating haemodynamic effects. Contrast echo- ume in the right heart; however, unless the defect
cardiography with agitated saline may be of benefit, is large, the haemodynamic consequences are mini-
especially when Doppler echocardiography is not mal. With large defects, volume overload of the right
available. heart may occur. Over time, right-to-left shunting
may develop, resulting in hypoxaemia.
Management/prognosis The foramen ovale is essential for fetal blood
The prognosis with VSD is variable. With small circulation. With the increased left-heart pressures
defects the only abnormality detected may be the present after birth, the foramen ovale normally
heart murmur and the animal may be able to lead a closes. Functional closure occurs, followed by adhe-
normal productive career. A poor prognosis is asso- sion of the valve to the crista dividens, such that
ciated with large defects and those in which the aor- the structure cannot be reopened. This anatomical
tic valve is disrupted. With time, volume overload closure takes several days to occur, and therefore
may lead to congestive heart failure. No treatment persistent foramen ovale would be expected in any
exists for VSD in the horse. Supportive treatment is foal that dies soon after birth from any cause.
indicated if signs of congestive heart failure are pres-
ent. Repeated echocardiographic examination allows Clinical presentation
assessment of progression. The severity of clinical signs varies with the size of
Horses have been reported to be able to race the defect and the presence of additional congeni-
with defects of <25 mm in diameter and peak shunt tal defects. In most cases the defect is small, and