Page 134 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 133

cusps, and the first diastolic frame of aortic valve closure The degree of left atrial dilation is usually a subjective
is selected (see Figure 11.13) (see also Chapter 7). The finding. While exact cutoff values for mild, moderate,
aortic diameter is measured by a line parallel to the com- and severe left atrial dilation have not been formalized,
missures of the right coronary and noncoronary cusps, suggested parameters include LA:Ao = 1.51–1.79, 1.8–
using the inner edge method. The left atrial diameter is 1.99, 2.1, and ≥2.01, respectively (Box 11.2, see also
measured by a line parallel to the commissure of the left Figure 11.13).
coronary and noncoronary cusp, with care to avoid the Identification of left atrial dilation in a cat with HCM
pulmonary vein insertion into the left atrium (which confirms that there is significant diastolic dysfunction
would overestimate left atrial diameter) (see Figure leading to increased diastolic filling pressures. Mitral
11.13). In a study of 17 healthy cats, normal 2D-derived regurgitation secondary to SAM of the mitral valve may
LA:Ao was 1.18 ± 0.11 (Abbott and MacLean 2006). also contribute to left atrial dilation. In cats with severely
LA:Ao ratio of >1.5 is consistent with left atrial dilation elevated left atrial pressure, there is often echocardio-
(Abbott and MacLean 2006). Using the same right para- graphic evidence of pulmonary venous distension seen Cardiomyopathies
sternal short-axis view, M-mode echocardiographic at the insertion of the anterior and posterior pulmonary
measurement of the LA:Ao involves directing the cursor veins into the roof of the left atrium (see Figure 11.13).
through the body of the left atrium as well as perpen- In cats presenting with pulmonary infiltrates or pleural
dicular to the short axis of the aorta at the level of the effusion, assessment of left atrial size is crucial for deter-
aortic cusps. The aortic diameter is measured at end- mining whether there is left-sided congestive heart
diastole just before the aortic cusps open, and the left failure versus primary respiratory disease. Identification
atrial diameter is measured at end-systole just when the of normal left atrial size in cats diagnosed with HCM
aortic cusps close, using a leading edge to leading edge that also have pulmonary infiltrates or pleural effusion
technique (Figure 11.14). Normal M-mode–derived and no known inciting trigger such as intravenous fluid
LA:Ao in cats is 1.25 ± .18 with 95% confidence interval administration virtually eliminates left-sided congestive
of 1.21– 1.29 (Drourr et al. 2005). heart failure from the diagnosis.
Spontaneous echocardiographic contrast (i.e., smoke)
may be seen throughout a dilated left atrium or only
within the left auricle and it occurs when there is red cell
aggregation. Spontaneous contrast is viewed as the pre-
cursor to a clot and suggests that there is a high risk for
development of arterial thromboembolism. In these
cases, it is particularly important to carefully evaluate
for a thrombus, especially within the stagnant blood of
the left auricle in a dilated left atrium, because cats with
intracardiac thrombi have higher mortality rates and are
Ao at great risk for massive thromboembolic event or
sudden death (Figure 11.15). The left auricle has the
slowest blood flow velocity within the left atrium and
LA
also has small branching pectinate muscles, both favor-
ing thrombus formation in the auricle. Cats with spon-
taneous contrast or an intracardiac thrombus should be
placed on an anticoagulant (see Chapter 20).
Figure 11.14. M-mode echocardiogram showing left atrial dila-
tion in a cat with severe hypertrophic cardiomyopathy. The M-
mode cursor is placed through the aortic root at the level of the BOX 11.2. Echocardiographic Assessment of Left Atrial Size
aortic cusps and through the body of the left atrium. This image is
obtained from the same cat as depicted in Figure 11.13C. The aor- Left atrial diameter to Left atrial
ta is measured at end-diastole just before the aortic cusps open, aortic diameter (LA:Ao) diameter (mm)
and the left atrial diameter is measured at end-systole when the
aortic cusps close, using a leading edge to leading edge tech- Normal: <1.5 <16
nique. The left atrial diameter is indexed to the aortic diameter Mild LAE: 1.51–1.79 16–19.9
(LA:Ao), and in this cat is moderately increased at 1.7. Care must Moderate LAE: 1.8–1.99 20–24
be taken to avoid measuring the left auricle rather than the body Severe LAE: >2 >24
of the left atrium, as this would grossly underestimate left atrial LAE = left atrial enlargement
size.