Page 1008 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Cardiovascular system 983

VetBooks.ir ponies when compared with Thoroughbreds. No The significantly higher pressure in the systemic
circulation means the direction of blood flow
gender predilection has been identified.
Aetiology/pathophysiology across the VSD is usually from left to right dur-
ing systole. In most cases (the exception being
VSD is a result of failure of complete formation of the apical defects) the shunted blood flows directly
interventricular septum during embryogenesis. The from the left ventricle to the right heart outflow
interventricular septum forms by several processes, tract during systole, without increasing the right
a failure of any of which results in a VSD. Several ventricular volume. This results in overcircula-
possible locations of a VSD are therefore possible tion of the pulmonary vasculature. Consequently,
(Fig. 8.22). Membranous septal defects lie just below the left atrium and left ventricle become volume
the septal cusp of the tricuspid valve at the top of the overloaded. Over time, progressive pulmonary
septum. Those defects occurring just below this area vasculature hypertension results in pressure over-
result from failure of formation of the smooth sep- load of the right ventricle and progressive hyper-
tum. Large defects involving both the smooth and trophy occurs. Progressive instability of the aortic
membranous septa have been reported. Infundibular valve may be associated with infundibular defects,
defects involve the ridge of muscle that divides the resulting in aortic regurgitation. Thickening of
inflow from the outflow tracts of the right ventricle the septal cusp of the tricuspid valve has been
(crista supraventricularis). These defects may disrupt associated with repetitive trauma by shunted flow
the integrity of the base of the pulmonic and aortic across membranous defects.
valves. Those defects that occur in the lower sep- Right-to-left shunting may occur following pul-
tum are commonly referred to as apical or trabecular monary overload and hypertension, resulting in
defects and are rare in the horse. increased right-sided pressure and a loss of the left-
Most VSDs are located in the membranous to-right pressure gradient. This is referred to as
or smooth septum at the base of the ventricles. Eisenmenger complex. This has not been reported
in horses with smaller defects (<4 cm) or in animals
less than 2 years of age. Cyanosis is common with
8.22 right-to-left shunting.
Aorta
Clinical presentation
A coarse or harsh holosystolic to pansystolic plateau-
Left atrium shaped murmur, audible on both sides of the tho-
2
1 rax, is typical of a VSD. A palpable thrill may also
Right 3 be present. The characteristic of the murmur aids
atrium in determining the position of the defect within the
Right Interventricular septum. With membranous defects, the PMI is far
ventricle septum
Left ventricle forward on the right side of the thorax. The murmur
4 is often also audible well forward on the left side.
With infundibular defects, the PMI is loudest on the
left side at the area of the outflow tracts. The grade
Fig. 8.22 Representation of the equine heart of the murmur does not necessarily correlate well
opened to visualise the interventricular septum. with either the extent of the defect or its haemody-
Membranous defects (1) lie just below the septal cusp namic significance. Frequently, small defects gener-
of the right AV valve. Infundibular defects (2) occur ate louder murmurs. For example, a large defect with
just below the outflow tracts. Smooth septum defects pressure equalisation across the VSD would gener-
(3) occur in the muscular portion of the septum just ate a very soft murmur but would be haemodynami-
below the membranous septum. Apical defects (4) are cally significant.
rare in the horse.