Page 136 - Problem-Based Feline Medicine

128 PART 3 CAT WITH SIGNS OF HEART DISEASE

Diagnosis is based on history, physical examination, trodorsal view or dorsoventral view, the maximum
radiography, electrocardiography and echocardiography. width of the normal heart occupies up to 50% of the
width of the chest cavity at the same level.
Normal echocardiographic values for the interven-
tricular septum and left ventricular posterior wall
in diastole are up to 5 mm in thickness. The dimension DISEASES CAUSING ABNORMAL
between 5–6 mm represents questionable hypertrophy. HEART SOUNDS AND/OR AN
ENLARGED HEART
Normal echocardiographic values of the left atrium are
up to 13 mm or a left atrium to aorta ratio less than 1.5.
HYPERTROPHIC CARDIOMYOPATHY***
Normal left ventricular dimension in diastole is up
to 17 mm. Increased dimensions are consistent with
Classical signs
left ventricular eccentric hypertrophy (dilation).
● Heart murmur (in some cases it may not be
Fractional shortening (percent change between the
present).
left ventricular diastolic and systolic dimensions)
● Gallop rhythm.
ranges from 35–55%.
● Arrhythmia.
The above values can be obtained from the two- ● Dyspnea.
dimensional image or the M mode. ● Collapse.
● Lameness, paralysis or paresis from
Cardiomegaly on radiographs is an indicator of heart
systemic thromboembolism.
disease although a normal-size heart does not rule
out heart disease. Cardiac enlargement on radiographs
Pathogenesis
can be determined by conventional methods or the ver-
tebral heart scale. Hypertrophic cardiomyopathy is defined as the pres-
ence of left ventricular hypertrophy in the absence of
The vertebral heart scale (VHS) compares cardiac
systemic disease causing secondary hypertrophy.
dimensions to the length of the thoracic vertebrae,
Hypertrophy may be generalized or focal.
which is an indicator of body size.
Focal hypertrophy may be recognized in the interven-
The VHS method is as follows: The long axis and short
tricular septum, left ventricular free wall or papillary
axis dimensions of the heart are transposed onto the
muscles.
vertebral column and recorded as the corresponding
number of vertebrae as measured caudally from the cra- Any combination of hypertrophic changes is possible.
nial edge of T4. These values are added to obtain the
Hypertrophic cardiomyopathy (HCM) is recognized to
vertebral heart size (VHS).
be genetically determined in selected families of cats:
The depth of the thorax is measured from the dorso- Persian, Maine Coon, American Shorthair and possibly
caudal border of the seventh sternebra to the closest in Ragdolls. Transmission is suspected to be autoso-
edge of the vertebral column. mal dominant.
Normal cats’ VHS are: mean VHS in lateral radi- The cause for hypertrophy is probably related to
ographs, 7.5 v (upper limit 8.0 v), mean cardiac short genetic alteration of the β-myosin heavy chain.
axis in lateral radiographs, 3.2 v (upper limit 3.5; mean
An association with increased levels of growth hor-
cardiac short axis in VD or DV radiographs, 3.4 v
mone concentration has also been found.
(upper limit 4 v); The cardiac long axis in the lateral
view approximates the length of three sternebrae, Echocardiographically there are two distinct varieties,
measured from S2 to S4. a non-obstructive form and an obstructive form.
Conventional methods of evaluating heart size in cats The non-obstructive form results primarily in dias-
are on the lateral view with the normal heart width tolic dysfunction due to delayed or decreased myocar-
approximately 2–2.5 intercostal spaces. On the ven- dial relaxation.