Page 131 - Feline Cardiology

130 Section D: Cardiomyopathies

highest prevalence rate of 50% for LV hypertrophy by left ventricle, and a limited number of acoustic windows
2D measurements, and prevalence was lower (28%) in the cat. In other words, the ability to visualize the
when using M-mode measurement of 5.5 mm or greater entire left ventricular myocardium often limits the use
wall thickness (Wagner et al. 2010). There is no valida- of Simpson’s rule by echocardiography to correctly
tion of which criterion (5.5 mm or greater, 6 mm or measure LV mass using echocardiography in the cat.
greater, or >50% wall segment 6 mm or greater), is the Papillary muscle hypertrophy is sometimes the only
most appropriate for diagnosing presence of LV hyper- abnormality present in cats with early HCM, and con-
trophy and HCM, although standard practice typically centric hypertrophy often develops 6–12 months later.
utilizes a maximal thickness of 6 mm or greater in any Papillary muscle hypertrophy is often a subjective assess-
part of the wall segment. Further prospective evaluation ment. Normal cat papillary muscles are slender, and
Cardiomyopathies Maine coon cats and Ragdoll cats, and histopathologic base, larger, and on the short-axis echocardiographic
hypertrophied papillary muscles become wider at the
of these patients over time, including genotypic data for
view of the left ventricle may have a more triangular
evaluation may help define which criterion is the most
shape (i.e., moundlike) rather than a slender elliptical
appropriate to diagnose HCM.
shape (i.e., like the tip of a pinkie finger) (Figure 11.12).
Left ventricular mass measurements allow the most
accurate assessment of extent of left ventricular hyper-
muscle, creating a disproportionate, distorted shape.
trophy, but are not typically done for clinical patients. Papillary hypertrophy may occur in just one papillary
Left ventricular mass may be measured by echocardiog- This must be differentiated from oblique imaging of a
raphy using several methods that require geometric normal papillary muscle due to poor technique, because
assumption of an elliptical left ventricular shape, and the left ventricle should appear round (and its lumen
most cardiac ultrasound machines can calculate LV mass should appear mushroom-shaped) in an appropriate
from routine left ventricular echocardiographic mea- short-axis view rather than oblong. Hypertrophied pap-
surements (Devereux et al. 1986; Stack et al. 1987; illary muscles often appear hyperechoic, either patchy or
Schiller et al. 1983). However, when using routine LV diffusely, which is attributed to myocardial fibrosis. The
measurements and M-mode–derived calculations for LV papillary muscles are the most vulnerable to myocardial
mass that include cubing the wall thickness followed by ischemia, since they are further removed from the (epi-
various volume-correcting formulae, these techniques cardial) coronary arterial blood supply than the rest of
have been shown to overestimate or underestimate left the LV myocardium. Quantitative methods of measur-
ventricular mass by 6–30% depending on the formula ing papillary muscle size have been described in cats
used (Devereux et al. 1986). The truncated ellipse model with LV hypertrophy and compared to normal cats
and the cylinder ellipse area-length model have been (Adin and Diley-Poston 2007); such measurements are
validated in dogs and in cats, and reasonably quantify not yet performed systematically in clinical cases.
left ventricular mass (Schiller et al. 1983; MacDonald Papillary muscle size can be quantified on routine 2D
et al. 2004). The modified truncated ellipse model was studies, using several techniques: the area subtraction
validated in domestic short hair cats that were normal method (subtracting the LV myocardial area including
or had mild left ventricular hypertrophy, and was mod- the papillary muscles from the LV myocardial area
erately correlated with actual left ventricular mass mea- excluding papillary muscles), the direct area trace
sured at necropsy (R = 0.85, P = 0.015) but consistently method (directly tracing the area of the papillary muscle
underestimated LV mass (actual LV mass = 3.07 + and extrapolating the ventral surface as the continuation
0.76(echo-derived LV mass) (MacDonald et al. 2004). of the endocardial surface of the LV free wall), and the
All formulae described above are still plagued with papillary diameter method (measuring the vertical and
assuming uniform geometric shape of the left ventricle, horizontal diameters) (Adin and Diley-Poston 2007).
which is invalid in HCM. The most accurate method of Although there was some overlap in papillary size com-
left ventricular mass quantification is the method of pared to normal cats, cats with LV hypertrophy had
discs (Simpson’s rule), where the myocardial volume larger papillary size (120–155% greater than normal
(volume of the ventricle including walls and lumen, papillary measurements) quantified by methods
minus the volume of the LV lumen) is summated in a described above (Adin and Diley-Poston 2007).
series of parallel discs from the annulus to the apex End-systolic cavity obliteration is diagnosed when
(Reichek 1994). This method allows measurement of there is no left ventricular chamber lumen remaining at
irregular areas and has fewer geometric assumptions. the end of systole, and it is caused by severe concentric
However, echocardiography is often inadequate due to hypertrophy of the papillary muscles and left ventricular
poor visualization of the cardiac apex, inadequate and walls (see Figure 11.9C). End-systolic cavity obliteration
incomplete delineation of the epicardial surface of the is a common feature of HCM, but may also be present