Page 126 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 125

enlargement pattern (QRS amplitude of >0.9 mv ± QRS Thoracic radiographs are a readily available, important
duration of >0.04 s) on ECG (Ferasin et al. 2003). diagnostic tool to evaluate cats with suspected heart
Increased QRS amplitude is not always specific for left disease. They are essential for evaluation of congestive
ventricular hypertrophy either, because some cats with heart failure, and may play a role in monitoring progres-
hyperthyroidism and normal left ventricular size and sion of disease and assessing response to heart failure
wall thickness have tall R waves on ECG (Moïse 1986c). treatment. However, radiographs are often normal in
Increased P wave duration is extremely insensitive cats with mild or moderate HCM and normal left atrial
(ranging from 0–30%) for detection of left atrial dila- size. Normal cardiac size on thoracic radiographs does
tion in cats (Schober et al. 2007). P wave amplitude and not rule out HCM as a diagnosis. In two large scale
PR intervals are also insensitive for detection of left studies of HCM, cardiomegaly was evident on thoracic
atrial dilation in cats (12% and 24%, respectively), but radiographs in 60–90% of cases (see Figure 11.7)
are highly specific for left atrial dilation (90–100%) (Ferasin et al. 2003; Rush and Freeman 2002). Left atrial
(Schober et al. 2007); therefore, left atrial enlargement dilation is more difficult to diagnose on lateral thoracic Cardiomyopathies
rarely alters the ECG in cats, but when a tall P wave or radiographs in cats than in dogs, but is more often iden-
prolonged PR interval does occur, left atrial enlargement tified using the ventrodorsal or dorsoventral views (see
is highly likely. Figure 11.7). In a study of 24 normal cats with normal
Additional electrocardiographic modalities, such as a left atrial size measured by echocardiography, 26% of
24-hour ambulatory electrocardiogram (i.e., Holter cats were misdiagnosed with left atrial dilation by sub-
monitor) or an event monitor, may be helpful to identify jective assessment on thoracic radiographs (Schober
significant arrhythmias in cats with symptoms of syncope et al. 2007). Sometimes presence of intrapericardial fat
or episodic weakness that have a normal resting electro- can lead to a misdiagnosis of cardiomegaly in cats.
cardiogram (see Chapter 9). For example, a Holter Seventy-two percent of cats with HCM and left atrial
monitor revealed severe ventricular tachycardia in a cat dilation diagnosed by echocardiography have radio-
with syncope, dyspnea, and severe HCM, despite a graphic evidence of left atrial dilation by subjective
normal resting electrocardiogram (see Chapter 9) assessment (Schober et al. 2007). In this study, subjective
(Goodwin et al. 1992). Diagnosis of episodic but symp- assessment of left atrial size was moderately sensitive
tomatic arrhythmias is important because these patients (72%) and specific (74%) for left atrial dilation.
may benefit from antiarrhythmic therapy. Calculation of the vertebral heart size may be useful to
quantify cardiac size and confirm presence of cardio-
Thoracic radiographs (see also megaly (see Chapter 6). Using the right lateral projec-
Chapters 6 and 19) tion, the long- and short-axis measurements of the
heart, expressed in vertebral lengths starting at the
fourth thoracic vertebrae, are added to yield vertebral
Key Points heart size. Normal vertebral heart size is 7.5 ± 0.3 in cats
(Litster and Buchanan 2000). Normal VHS cannot be
• Thoracic radiographs may be normal in cats with mild used for ruling out HCM, because concentric hypertro-
HCM that have concentric hypertrophy but no atrial phy causes left ventricular hypertrophy with a normal
dilation.
• Cardiomegaly and left atrial dilation may be seen in VHS and normal LV chamber size (Mattoon et al. 2002).
asymptomatic cats with HCM prior to development of It is impossible to predict the specific cardiac disease
heart failure. present in cats with radiographic evidence of cardio-
• Thoracic radiographs are essential to evaluate for heart megaly, but identification of cardiomegaly on radio-
failure in cats with HCM and left atrial dilation, or in cats graphs warrants further evaluation, first by assessing the
with presenting complaint of respiratory abnormalities. radiographs for artifacts that mimic cardiomegaly (expi-
• Pulmonary edema and/or pleural effusion may be ratory films, improper patient positioning), and if arti-
present in cats with heart failure secondary to severe facts are ruled out, echocardiography is indicated.
HCM. Pulmonary edema is more common than pleural Congestive heart failure may be evidenced by pulmo-
effusion in cats with heart failure caused by HCM nary edema, pleural effusion, or a combination of both
(22–66% vs 7–34%, respectively). (Figure 11.8). Pulmonary edema is more common (23–
• Classic radiographic abnormalities of feline heart failure 66%) than pleural effusion (7–34%) in cats with heart
include cardiomegaly, left atrial dilation, pulmonary venous failure secondary to severe HCM (Ferasin et al. 2003;
distension, and patchy interstitial to alveolar pulmonary
infiltrates that are often variably distributed and not Rush and Freeman 2002). Dyspnea is attributed to pul-
necessarily perihilar to caudodorsal as seen in dogs. monary edema in 80% of cats with HCM, versus only
14% due to severe pleural effusion (Rush and Freeman