Page 128 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 127

trophy is defined as end-diastolic left ventricular free
Continuous-wave Doppler measurement of the aortic wall or interventricular septal thickness of ≥6 mm, with
blood flow velocity from the left apical parasternal an equivocal zone from 5.5–5.9 mm. Although there is
5-chamber view is necessary to quantify the severity of not a formalized definition of mild, moderate, and
the obstruction. severe concentric hypertrophy, the authors typically
• Diastolic function can be assessed using pulsed- classify 6– 6.5 mm as mild, 6.6–7.5 mm as moderate, and
wave (PW) Doppler measurement of the mitral inflow
velocity, measurement of pulmonary venous inflow >7.5 as severe concentric hypertrophy in euvolemic/
velocity profiles, and by tissue Doppler imaging and hydrated cats. Some cats may have concurrent HCM and
measurement of myocardial or mitral annulus velocities. other secondary causes of concentric hypertrophy. For
• A delayed relaxation pattern is commonly seen on PW example, a hyperthyroid cat that has been treated with
Doppler measurement of mitral inflow velocities and antithyroid medications for 2 or more months but
is defined as prolonged isovolumic relaxation time, whose echocardiogram reveals persistent LV hypertro-
decreased early mitral inflow velocity (E), increased late phy may have poor control of the hyperthyroid state or Cardiomyopathies
atrial systolic velocity (A), prolonged deceleration of the concurrent HCM. If the total thyroxine concentration is
early diastolic inflow wave, and E:A reversal with E:A in the low- to midnormal range following several months
ratio <1. of hyperthyroid treatment, the cat likely is also affected
• A restrictive filling pattern is seen when left atrial with HCM. Identification of SAM of the mitral valve
pressure is significantly increased and is identified as supports the diagnosis of HCM, although uncommonly
shortened isovolumic relaxation time, increased early
diastolic filling (E), shortened early diastolic filling it may be seen in other secondary causes of concentric
deceleration, decreased late atrial systolic wave (A), and hypertrophy (Luckie and Khattar 2008). Assessment of
E:A ratio >2. left atrial size is crucial for staging of disease severity,
• Tissue Doppler imaging echocardiography (TDI) is a because cats with left atrial dilation are at risk for con-
more sensitive test to evaluate for diastolic dysfunction gestive heart failure or arterial thromboembolism.
and is less influenced by preload than is PW Doppler Thoracic radiographs should be taken in cats with sig-
interrogation of mitral inflow velocity. PW TDI and color nificant left atrial dilation to evaluate for presence of
TDI are two techniques to assess diastolic and systolic heart failure. Assessment of diastolic function is not
myocardial function, which may identify feline heart essential to achieve a diagnosis of HCM but may be
diseases at earlier stages, but which do not have specific informative to follow disease progression or response to
treatment implications at this time. therapy.
• Longitudinal left ventricular myocardial function
is assessed by measurement of the mitral annulus Left ventricular hypertrophy
velocities from the left apical 4-chamber view, and
circumferential fiber function is measured from the left Left ventricular concentric hypertrophy is standardly
ventricular short-axis view from the right parasternal defined as an end-diastolic ventricular wall or septal
window. thickness of 6 mm or greater in cats. Left ventricular wall
• Diastolic dysfunction is identified on tissue Doppler thickness has historically been determined using
imaging when there is decreased early diastolic mitral M-mode echocardiography, since at that time the
annular velocity (E’), which is inversely proportional to 2-dimensional imaging was limited by slower frame rate
the severity of diastolic dysfunction. and resolution (see Chapter 7) (Moïse 1986a,c; Jacobs
• Using PW TDI of the lateral mitral annulus, E’ > and Knight 1985). M-mode is still an acceptable method
7.2 cm/sec is highly sensitive (92%) and specific (87%) of measurement of left ventricular chamber dimensions
for discriminating between normal cats and cats with during diastole and systole, but it has several limitations
left ventricular concentric hypertrophy due to HCM.
that are overcome by using 2-dimensional echocardiog-
raphy with current machines (see below).
Measurement of interventricular septal and left ven-
tricular free wall thickness is preferably done using
Overview 2-dimensional echocardiography in the right paraster-
Echocardiography is the most important diagnostic tool nal short-axis view at the level of the papillary muscles
to diagnose HCM. In order for HCM to be confirmed, just below the chordae tendineae (Figure 11.9). This
there must be concentric symmetric or asymmetric left allows global assessment of wall thickness, which is
ventricular concentric hypertrophy (i.e., thick left ven- important in cats with regional or asymmetrical hyper-
tricular walls) in the absence of other causes of concen- trophy (i.e., segmental hypertrophy). Using an ECG, the
tric hypertrophy such as aortic stenosis, systemic end-diastolic wall thickness is measured during the R
hypertension, or hyperthyroidism). Concentric hyper- wave, and the thickest regions of the interventricular