Page 156 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 155

tricular outflow tract obstruction (LV:Ao pressure gradi- further define the cardiac disease by echocardiography
ent >40 mm Hg). This decision must include the impact due to an owner’s financial constraint or lack of per-
of chronic oral medications for the client and cat; patient ceived need for further workup, which makes it difficult
temperament; the owner’s motivation for administering to appropriately assess whether treatment is justified.
lifelong oral therapy; and whether clinical signs are Without further assessment with an echocardiogram,
present (associated with moderate to severe SAM, such many asymptomatic cats with murmurs and possible
as reluctance to play or exercise, exercise-induced tachy- radiographic evidence of cardiomegaly may not actually
pnea or weakness, lethargy) (Table 11.1). require any therapy and may be overtreated by empirical
Exertional dyspnea, chest pain, dizziness, presyncope therapy.
or syncope, and reduced exercise capacity occur in some A common misconception based on the wide margin
people with HCM and diastolic dysfunction, often with of safety of beta blockers and aspirin is the concept that
concurrent significant left ventricular outflow tract neither will do any harm. Such a belief fails to take into
obstruction (Maron et al. 2003). These symptoms are account the logistics of medication administration and Cardiomyopathies
caused by increased left ventricular pressure during lack of conclusive studies demonstrating benefit to disease
exertion (since the left ventricular outflow tract obstruc- progression or survival. Adverse responses of chronic
tion worsens with increased contractility), leading to oral therapy in cats are not necessary based on deleteri-
myocardial ischemia, and elevated diastolic filling pres- ous effects of the actual medication, but effects of
sure and left atrial pressure with exertion (Maron et al. administering any medication chronically to cats that may
2003). These patients are aggressively treated with be notoriously resistant to this procedure (see Table 11.1).
various medications (beta blockers, calcium channel It is critical to remember that not all heart murmurs
blockers, disopyramide) or medical interventions are caused by HCM, and there are many other causes of
(ethanol septal ablation, septal myectomy) to dramati- heart murmurs that do not require treatment. A common
cally reduce the severity of left ventricular outflow tract cause of murmurs in cats is dynamic right ventricular
obstruction and relieve symptoms (Maron et al. 2003). outflow tract obstruction, which is not associated with
It is challenging to identify cats with intermittent symp- any structural cardiac abnormality, yet cannot be distin-
toms that occur with moderate to severe hypertrophic guished from pathologic murmurs based on ausculta-
obstructive cardiomyopathy with moderate to severe tion alone (Rishniw et al. 2002). In fact, 69% of cats
SAM, because cats, unlike people, are sedentary and tend (11/16) with murmurs have no echocardiographic evi-
to mask intermittent or mild symptoms. Among veteri- dence of cardiac disease (Paige et al. 2009). Misdiagnosis
nary cardiologists, there is great controversy regarding can logically lead to mistreatment. For these reasons,
whether the benefits of medicating an asymptomatic cat treatment of a heart murmur alone, without documen-
outweigh the potential negative impact of lifelong oral tation of underlying structural heart disease, is not war-
medication administration cats. ranted and may be detrimental. An exception is the cat
Presence of left atrial dilation is considered to indicate with a heart murmur that also has a tachyarrhythmia
a more severe disease state with an elevated diastolic (specifically ventricular tachycardia, atrial tachycardia,
filling pressure, which may prompt more serious con- or atrial fibrillation) demonstrated on ECG, in which
sideration for treatment. Still, there are many cats in a case beta blockade may be indicated for treatment of the
“gray zone” of mild or moderate left atrial dilation arrhythmia. Similarly, a cat with a conclusive physical
wherein some deteriorate more rapidly and others diagnosis of aortic thromboembolism in its past medical
remain well compensated for extended periods of time. history should be given an anticoagulant, even if an
Conversely, cats with mild left ventricular hypertrophy echocardiogram is not possible.
(6– 6.5 mm), normal left atrial size, and mild or no SAM
of the mitral valve do not require therapy, but should be Drug selection for treating asymptomatic
monitored for echocardiographic evidence of disease HCM in cats
progression. HCM does not represent a single, uniform disease, but
it has wide clinical variations, likely due to different
Suspected but unconfirmed HCM: Whether to causative mutations and potential predisposing or exac-
treat asymptomatic cats with a murmur but erbating factors. Due to the void in data on clinical
no echocardiogram effects on disease progression, an intense debate exists
Empirical treatment of an asymptomatic cat that has an over whether calcium channel blockers or beta blockers
incidental murmur with beta blockers, calcium channel are superior for treatment of patients with HCM.
blockers, or ACE inhibitors is not warranted (see Chapter Although beta blockers worsen early ventricular relax-
1). Often veterinarians are challenged by the inability to ation, there is symptomatic benefit in people with HCM