Page 135 - Small Animal Internal Medicine, 6th Edition

CHAPTER 5 Congenital Cardiac Disease 107

echogenicity (probably from fibrosis) is common in animals recommended. Likewise, transvascular balloon dilation of
with severe obstruction; systolic anterior motion of the ante- the stenotic area can reduce the measured gradient in some
VetBooks.ir rior mitral leaflet and mid-systolic partial aortic valve closure dogs; however, significant survival benefit has not been doc-
umented with this procedure. More recently, a combined
suggest concurrent dynamic LV outflow obstruction.
Ascending aorta dilation, aortic valve thickening, and LA
plasty followed by high-pressure balloon dilation has been
enlargement with hypertrophy may also be seen. In mildly valvuloplasty procedure involving cutting balloon valvulo-
affected animals, 2-D and M-mode findings may be unre- tried, with the goal of “scoring” (disrupting) the fibrous ring
markable. Doppler echocardiography reveals systolic turbu- to make the subvalvular region more amenable to balloon
lence originating below the aortic valve and extending into dilation. This procedure can reduce LV pressure gradient
the aorta, as well as high peak systolic outflow velocity (see and appears to be safe, although serious complications can
Fig. 5.5). Some degree of aortic or mitral regurgitation is occur. However, some degree of re-stenosis is common by
common. Spectral Doppler studies are used to estimate the 6 to 12 months postprocedure. As with other interventions
stenosis severity by calculating the pressure gradient across for SAS, long-term survival benefit of this combined cutting
the LV outflow tract (between the LV and aorta). A pressure and high-pressure balloon dilation procedure has not been
gradient of <50 mm Hg is defined as mild stenosis, 50 to documented.
80 mm Hg is moderate stenosis, and >80 mm Hg indicates Medical therapy with a β-blocker has been advocated in
severe stenosis. The LV outflow tract should be interrogated patients with moderate to severe SAS, to reduce myocardial
from more than one position to achieve the best possible oxygen demand and minimize the frequency and severity of
alignment with blood flow. The subcostal (subxiphoid) posi- arrhythmias. Animals with a high pressure gradient, marked
tion usually yields the highest-velocity signals, although the ST-segment depression, frequent ventricular premature
left apical position is optimal in some animals. Doppler- beats, or a history of syncope might be more likely to benefit
estimated systolic pressure gradients in unanesthetized from this therapy. Whether β-blockers prolong survival is
animals are usually 40% to 50% higher than those recorded unclear. Exercise restriction is advised for animals with mod-
during cardiac catheterization under anesthesia. erate to severe SAS. Prophylactic antibiotic therapy is recom-
The Doppler-estimated aortic outflow velocity might be mended for animals with SAS before any procedures with
only equivocally high in animals with mild SAS, especially the potential to cause bacteremia (e.g., dentistry) are done,
with suboptimal Doppler beam alignment. With optimal although the efficacy of this practice in preventing endocar-
alignment, aortic root velocities of less than 1.9 m/sec are ditis is unclear.
typical in normal unsedated dogs; velocities over approxi- The prognosis in dogs and cats with severe stenosis (pres-
mately 2.25 m/sec are generally considered abnormal. Peak sure gradient > 80 mm Hg) is guarded. Median survival time
velocities in the equivocal range between these values could is approximately 4.5 years, with a bimodal age distribution
indicate the presence of mild SAS, especially if other evidence describing cause of death. Sudden death is more common in
of disease exists, such as a subaortic ridge, aortic regurgita- dogs younger than 3 years of age; overall, approximately 20%
tion, or disturbed flow in the outflow tract or ascending of dogs with SAS die suddenly. In contrast, infective endo-
aorta with an abrupt increase in velocity. This mainly is of carditis and CHF are more likely to develop later in life (8-10
concern when selecting animals for breeding. In some breeds years) in surviving dogs. Atrial and ventricular arrhythmias
(e.g., Boxer, Golden Retriever, Greyhound), outflow veloci- and worsened mitral regurgitation are complicating factors.
ties in this equivocal range (1.8-2.25 m/sec) are common in Dogs with mild stenosis (pressure gradient < 50 mm Hg)
normal dogs. This could reflect breed-specific variation in could live a normal life span without clinical signs.
LV outflow tract anatomy or response to sympathetic stimu-
lation, rather than SAS. A limitation of using the estimated PULMONIC STENOSIS
pressure gradient to assess outflow obstruction severity is the
dependence of this gradient on blood flow. Factors causing Etiology and Pathophysiology
sympathetic stimulation and increased cardiac output (e.g., PS is more common in small breeds of dogs. Some cases of
excitement, exercise, fever) will increase outflow velocities, valvular PS result from simple fusion of the valve cusps, but
whereas myocardial failure, cardiodepressant drugs, and valve dysplasia is more common. Dysplastic valve leaflets are
other causes of reduced stroke volume will decrease recorded variably thickened, asymmetric, and partially fused, with or
velocities. Cardiac catheterization and angiocardiography without a hypoplastic valve annulus. RV pressure overload
rarely are used now to diagnose or quantify SAS, except in leads to concentric hypertrophy, as well as secondary dila-
conjunction with balloon dilation of the stenotic area. tion of the RV. Severe ventricular hypertrophy promotes
myocardial ischemia and its sequelae. Excessive muscular
Treatment and Prognosis hypertrophy in the infundibular region below the valve can
Several palliative surgical techniques have been attempted create a dynamic subvalvular component to the stenosis.
in dogs with severe SAS. Although some have reduced the Other variants of PS, including supravalvular stenosis and
LV systolic pressure gradient and possibly improved exer- RV muscular partition (double-chamber RV), occur rarely.
cise ability, because of high complication rates, expense, Turbulence caused by high-velocity flow across the ste-
and lack of a long-term survival advantage, surgery is not notic orifice leads to poststenotic dilation in the main