Download Prevalence of cardiomyopathy in apparently healthy cats

SMALL ANIMALS
Prevalence of cardiomyopathy
in apparently healthy cats
Christopher F. Paige, ms, dvm, dacvim; Jonathan A. Abbott, dvm, dacvim;
François Elvinger, dr med vet, phd, dacvpm; R. Lee Pyle, vmd, ms, dacvim
Objective—To determine the prevalence of cardiomyopathy and the relationship between
cardiomyopathy and heart murmurs in apparently healthy cats.
Design—Cross-sectional study.
Animals—103 privately owned, apparently healthy domestic cats.
Procedures—Cats were physically and echocardiographically examined by 2 investigators
independently. Left ventricular wall thickness was determined via 2-dimensional echocardiography in short-axis and long-axis planes. Left ventricular hypertrophy was identified
when end-diastolic measurements of the interventricular septum or posterior wall were
≥ 6 mm. Cats with left ventricular hypertrophy but without left ventricular dilatation were
considered to have hypertrophic cardiomyopathy (HCM). The associations between heart
murmurs and Doppler echocardiographic velocity profiles indicative of dynamic ventricular
outflow tract obstruction were evaluated.
Results—Heart murmurs were detected in 16 (15.5%; 95% confidence interval, 9.2% to
24.0%) cats; of these, 5 had cardiomyopathy. Cardiomyopathy was also identified in 16
(15.5%; 95% confidence interval, 9.2% to 24.0%) cats; 15 had HCM, and 1 had arrhythmogenic right ventricular cardiomyopathy. Of the cats with HCM, 11 had segmental left
ventricular hypertrophy, 3 had diffuse left ventricular hypertrophy, and 1 had borderline left
ventricular hypertrophy with marked systolic anterior motion of the mitral valve. Sensitivity
and specificity of auscultatory detection of a heart murmur for diagnosing cardiomyopathy
were 31% and 87%, respectively. Echocardiographic evidence of late systolic acceleration
within ventricular outflow tracts was associated with the existence of a heart murmur.
Conclusions and Clinical Relevance—Cardiomyopathy was common in the healthy cats
evaluated in this study. In apparently healthy cats, detection of a heart murmur is not a
reliable indicator of cardiomyopathy. (J Am Vet Med Assoc 2009;234:1398–1403)
S
ubclinical cardiomyopathy is sometimes identified after abnormalities are detected during auscultation of apparently healthy cats. However, little
is known about the prevalence of cardiomyopathy
in this population. Studies1–6 have been conducted
to determine the prevalence of cardiomyopathy and
heart murmurs in healthy cats, but these studies had
limitations attributable to referral bias, the retrospective nature of the analyses, and the inclusion criteria
used. Whereas many cats are referred for echocardiographic evaluation after a heart murmur is detected
by their veterinarian, the relationship between heart
murmurs and cardiomyopathy in healthy cats remains
unclear. The purpose of the study reported here was to
estimate the prevalence of cardiomyopathy and heart
From the Departments of Small Animal Clinical Sciences (Paige, Abbott, Pyle) and Large Animal Clinical Sciences (Elvinger), VirginiaMaryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute, Blacksburg, VA 24061. Dr. Paige’s present address
is VCA Animal Referral and Emergency Center of Arizona, 1648 N
Country Club Dr, Mesa, AZ 85201.
Supported by a grant from the Virginia Veterinary Medical Association Veterinary Memorial Fund.
Presented as a poster at the 24th Annual American College of Veterinary Internal Medicine Forum, Louisville, June 2006.
The authors thank Drs. Stephen Werre and Daniel Ward for statistical
analyses and Terry Lawrence for the schematic drawing.
Address correspondence to Dr. Abbott.
1398
Scientific Reports Abbreviations
2-D CI HCM IVSd LVIDd
LVOT LVPW LVPWd RVOT 2-dimensional
Confidence interval
Hypertrophic cardiomyopathy
End-diastolic thickness of the
interventricular septum
End-diastolic left ventricular internal
diameter
Left ventricular outflow tract
Left ventricular posterior wall
End-diastolic thickness of the left
ventricular posterior wall
Right ventricular outflow tract
murmurs in apparently healthy cats and, in so doing,
clarify the clinical relevance of heart murmurs and the
diagnostic usefulness of cardiac auscultation for detecting cardiomyopathy in this population.
Materials and Methods
Animals—To identify cats for the study, an e-mail
message was sent to a distribution list that included
veterinary students, staff, and faculty of the VirginiaMaryland Regional College of Veterinary Medicine. Pet
owners that were willing to enroll their cats were asked
to complete an electronic survey on the World Wide
JAVMA, Vol 234, No. 11, June 1, 2009
segments that extended from the papillary muscles to
the central point of the septum (Figure 1). The line
of measurement was parallel to a chord that extended
through the centroid of the ventricular lumen (Figure
2). End-diastolic basilar septal thickness was measured
in the long-axis plane; the greatest septal dimension between the aortic root and the point at which the anterior
mitral valve leaflet most closely approached the interventricular septum during diastole was recorded (Figure 3). Septal measurements included left ventricular
Study procedures—Physical examination, Doppler arterial blood pressure estimation, ECG, and then
echocardiography were performed in that sequence in 3
rooms. The physical and echocardiographic examinations were performed by 2 board-certified veterinary
cardiologists (RLP and JAA); the echocardiographer
(JAA) was unaware of the physical examination findings.
Echocardiographic images were digitally recorded for
subsequent quantitative and qualitative analysis. After
digital echocardiographic records were randomized and
cat identifiers were concealed, echocardiographic measurements were obtained by a third investigator (CFP).
The final echocardiographic diagnosis consisted of the
consensus opinion of the echocardiographer and the investigator (CFP) that performed the echocardiographic
measurements and was determined without knowledge
of the physical examination findings.
Physical examination—All cats underwent a systematic, dynamic cardiac auscultatory examination.
Auscultation was first performed when cats were at rest
and then after provocation (a maneuver in which the
examiner quickly lifted the cat in the air at least twice).
When identified, heart murmurs were described in
terms of intensity, which was graded on a 6-point scale
according to the recommendations of Levine, point of
maximal intensity, and timing.7 The existence or lack of
a gallop sound was recorded, as was a description of the
cardiac rhythm. Heart rate was recorded for all cats.
Echocardiography—Echocardiographic examinations
were performed without chemical restraint. A sonographa
with a 7.5-MHz transducer was used to perform transthoracic echocardiography as described elsewhere.8 Two-dimensional short-axis and long-axis right parasternal images of the left ventricle were used to measure end-diastolic
thickness of the interventricular septum and posterior wall.
In the short-axis plane, end of diastole was defined by the
onset of the QRS complex in a simultaneously recorded
ECG. However, some cats did not tolerate ECG monitoring,
and in other situations, motion artifact obscured the onset
of the QRS complex. For those examinations, the maximal
diastolic excursion of the ventricle was used to define end
of diastole. In the long-axis plane, the end of diastole was
defined as the first frame in which mitral valve closure was
visible. The dimensions LVIDd, IVSd, and LVPWd were obtained from the short-axis image.
From the short-axis image, the IVSd was also measured in 2 additional sites. Specifically, the maximal
thickness of the septum was measured in the 2 septal
JAVMA, Vol 234, No. 11, June 1, 2009
Figure 1—Schematic diagram of the regions measured at the
end of diastole during the echocardiographic examination of apparently healthy cats. For each of segments 1 and 2, 1 linear
dimension was measured to represent the maximal thickness of
each segment. The top solid portion of the vertical line represents
the IVSd, the middle dashed portion represents the LVIDd, and
the bottom solid portion represents the LVPWd.
Figure 2—Two-dimensional, short-axis echocardiographic image
of the left ventricle of an echocardiographically normal, apparently healthy cat that was evaluated for cardiomyopathy. Measurements to determine the thickness of 2 segments of the interventricular septum (bars) were made perpendicular to the left
ventricular endocardial border and directed toward the ventricular
centroid (C). IVS1 = Interventricular septum segment 1. IVS2 =
Interventricular septum segment 2.
Scientific Reports
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SMALL ANIMALS
Web that was used to identify cats meeting the inclusion criteria and collect zoographic data. Cats were considered healthy when they had not already undergone
an echocardiographic examination, were not receiving treatment for cardiovascular disease, and did not
have a history of chronic illness such as inflammatory
bowel disease, hyperthyroidism, renal disease, systemic
hypertension, or diabetes mellitus. Cats were excluded
when veterinary care had been sought for a systemic illness
in the previous 3 months. Cats that had a history of a heart
murmur but had not been examined echocardiographically were included. The study protocol was approved by the
Animal Care and Use Committee and Institutional Review
Board of Virginia Polytechnic Institute.
SMALL ANIMALS
when the mean of 3 consecutive arterial blood pressure
measurements was ≥ 180 mm Hg. After systemic arterial blood pressure was measured, cats were restrained
in right lateral recumbency and a 6-lead ECG was recorded. Mean ECG heart rate was determined from the
6-lead ECG only, whereas the existence of arrhythmias
was determined from the 6-lead ECG or the ECG that
was recorded during echocardiography.
Figure 3—Right parasternal long-axis echocardiographic image
of the left ventricle of an echocardiographically normal, apparently healthy cat that was evaluated for cardiomyopathy. Basilar
septal thickness was recorded as the maximal septal dimension
between the aortic root and the point at which the anterior mitral
valve leaflet most closely approached the IVS during diastole. IVS
= Interventricular septum. IVSb = Location of measurement of
the basilar septum. LA = Left atrium. Ao = Aorta.
endocardial echoes, but excluded echoes arising from
the right ventricular endocardium.9 Measurements of
the LVPW included endocardial echoes but did not include the pericardium.
The left atrial and aortic root diameters were
determined from M-mode images, with the M-mode
beam directed by use of right parasternal short-axis
2-D images. A ratio of these 2 values was calculated, and left atrial enlargement was defined as a ratio
> 1.54.10 The cats were also evaluated by means of
conventional color flow, pulsed-wave, and sometimes
continuous spectral Doppler echocardiographic examination. Left and right ventricular outflow tract
velocities were recorded. Echocardiographic dimensions and spectral Doppler echocardiographic velocities were calculated as the mean of 3, usually consecutive, cardiac cycles.
Hypertrophic cardiomyopathy was echocardiographically defined by an end-diastolic wall thickness
≥ 6 mm that affected > 50% of any region of the interventricular septum or LVPW.11 The existence or lack
of systolic anterior motion of the mitral valve was also
detected. Diffuse left ventricular hypertrophy was diagnosed when abnormal wall thickness was identified
in at least 3 wall segments. Other types of cardiomyopathy in cats were classified as described elsewhere.1,12
Dynamic RVOT obstruction was defined as detection
of a systolic jet that originated proximal to the pulmonary valve and had spectral Doppler echocardiographic
characteristics that indicated late-systolic acceleration.13
Dynamic LVOT obstruction was similarly defined by
late-systolic acceleration.
Arterial blood pressure measurement and electrocardiography—Systemic arterial blood pressure was estimated for all cats by use of the Doppler cuff–flow meter method.14 All estimates were obtained from the left
or right forelimb. Cats were considered hypertensive
1400
Scientific Reports Thyroid gland function—After completion of the
cardiovascular examination, a blood sample was obtained via jugular venipuncture from all cats that were
≥ 6 years old. Blood samples were centrifuged, and serum was harvested and refrigerated at 4°C until analyzed. Serum thyroxine concentration was determined
by use of a commercial assayb and an automated chemistry analyzer.c Cats with a serum thyroxine concentration that exceeded the upper limit of the laboratory
reference range were considered hyperthyroid and excluded from additional analysis.
Statistical analysis—Prevalence of murmurs and
cardiomyopathy and the corresponding 95% CIs were
calculated by use of statistical software.d Associations
between detection of a heart murmur and dynamic ventricular outflow tract obstruction were assessed with the
χ2 test, and prevalence odds ratios were calculated. Sensitivity, specificity, positive and negative predictive values, positive and negative likelihood ratios, and respective 95% CIs were calculated by use of other softwaree to
determine the diagnostic usefulness of cardiac auscultation for identifying cardiomyopathy. Echocardiographic
detection of cardiomyopathy was considered the gold
standard with which auscultation was compared, and
auscultatory detection of an unprovoked heart murmur
was considered a positive test result. Continuous data
are reported as mean ± SD. A value of P < 0.05 was considered significant for all analyses.
Results
Animals—One hundred sixty-five people responded to the electronic survey, and 145 cats were identified
as meeting the initial inclusion criteria. On 6 nonconsecutive days from late August 2005 to November 2005,
132 cats were examined. Twenty-nine of these cats were
later excluded because they resisted manual restraint
(n = 22), were hyperthyroid (4), or had an incomplete
echocardiographic examination (3). Therefore, 103 apparently healthy cats were included in the study. Fortythree cats were female. Mean ± SD body weight was
5.01 ± 1.13 kg (11.02 ± 2.50 lb). Precise ages were not
always available, so the following discrete age categories
were used instead: < 1 year (n = 6), 1 to 5 years (62), 6
to 10 years (27), 11 to 15 years (7), and ≥ 16 years (1).
Most cats were of mixed breed (75 domestic shorthair,
9 domestic medium hair, and 10 domestic longhair).
Purebred cats included Himalayan (n = 3), Siamese (3),
Ocicat (2), and Maine Coon (1).
Physical examination—Heart murmurs were
detected in 16 (15.5%; 95% CI, 9.2% to 24.0%) cats.
Grades of heart murmur intensities were distributed as
follows: grade 1/6 (n = 5), grade 2/6 (9), and grade 3/6
(2). After the provocative maneuver, heart murmurs
JAVMA, Vol 234, No. 11, June 1, 2009
Arterial blood pressure and ECG—All cats were
normotensive, and systolic arterial blood pressure in 87
echocardiographically normal cats was 131.1 ± 17.8 mm
Hg. In 15 cats with HCM, systolic arterial blood pressure
was 136.4 ± 19.6 mm Hg. Based on results of electrocardiography, the heart rate of 80 cats without cardiomyopathy was 189.4 ± 23.8 beats/min and that for cats
with HCM was 188.7 ± 27.2 beats/min. Electrocardiography or echocardiography revealed ECG abnormalities
in 7 cats without cardiomyopathy. Two of these cats had
ventricular preexcitation, 4 had ventricular premature
complexes, and 1 had ventricular tachycardia. Of the
cats with cardiomyopathy, 2 with HCM had ventricular
premature complexes. In addition, a cat with arrhythmogenic right ventricular cardiomyopathy also had ventricular tachycardia.
Echocardiography—Cardiomyopathy was identified in 16 (15.5%; 95% CI, 9.2% to 24.0%) cats; 15
had HCM, and 1 had arrhythmogenic right ventricular
cardiomyopathy. Of the 15 cats with HCM, 11 had segmental left ventricular hypertrophy, and in 3 of these
cats, hypertrophy was localized to the basilar septum.
The magnitude and distribution of ventricular hypertrophy in the cats with HCM varied among cats (Table
1). Of the cats with basilar septal hypertrophy, 1 cat was
between 1 and 5 years of age and 2 were > 10 years of
age. Three cats had diffuse left ventricular hypertrophy.
One cat with marked systolic anterior motion of the mitral valve and a maximal ventricular wall thickness of
5.9 mm was classified as having HCM. The cat with a diagnosis of arrhythmogenic right ventricular cardiomyop-
athy had mild right ventricular enlargement, abnormal
septal motion, an end-systolic left ventricular diameter
of 11.9 mm, tricuspid valve regurgitation, ventricular
tachycardia, and an intermittent gallop sound.
Dynamic RVOT obstruction with a peak velocity
≥ 1.7 m/s was identified in 5 cats, and 3 other cats had
lower velocities with evidence of late systolic acceleration.
Dynamic LVOT obstruction was identified in 8 cats. Two
cats with HCM had systolic anterior motion of the mitral
valve with supraphysiologic LVOT velocities (4.2 and 2.1
m/s) and mitral valve regurgitation and 2 had mid left ventricular obstruction and supraphysiologic velocities (1.8
and 1.7 m/s), whereas the remaining 4 had late systolic
acceleration with lower velocities (1.0 m/s [n = 2], 1.1 m/s
[1], and 1.6 m/s [1]). Of the 6 cats with dynamic LVOT
obstruction in which systolic anterior motion of the mitral valve was not detected, 2 had HCM and 4 did not.
Two cats, 1 with and 1 without HCM, had right and left
dynamic ventricular outflow tract obstructions. Trace or
mild tricuspid valve regurgitation was detected in 7 cats,
and 1 of these also had slight mitral valve regurgitation. In
1 cat that was classified as free of cardiomyopathy, echocardiography revealed an equivocally enlarged right ventricle, slight tricuspid value regurgitation, and brief paroxysm of nonsustained ventricular tachycardia.
All evaluated cats had normal left atrial size. In 1
cat without cardiomyopathy, the M-mode image did not
include 3 measurable cardiac cycles, but size of the left
atrium was unremarkable in 2-D short-axis and long-axis
views. In 3 additional cats also free of cardiomyopathy,
the M-mode ratio of the left atrial and aortic root diameters marginally exceeded 1.54. However, the M-mode–
derived end-systolic dimension of the left atrium did not
exceed 1.45 cm, and left atrial size was unremarkable in
the 2-D short-axis and long-axis views.10
Association between heart murmurs and dynamic
outflow tract obstruction—Of the 16 cats with heart
murmurs, 5 had HCM. Six of 16 cats with unprovoked
Table 1—Left ventricular chamber dimensions and wall thickness of cats with HCM, reflecting the variability in the magnitude and distribution of the disease among cats.
Cat
Wt (kg)
1
2
3
4
5
6
7†
8
9
10
11
12
13
14
15
6.03
7.59
4.31
4.46
6.96
9.80
6.15
4.06
4.23
4.28
4.29
5.10
7.03
4.82
4.54
LVIDd (cm) LVES (cm) IVSd (cm)
1.25
1.68
1.49
1.68
1.36
1.45
1.15
1.62
1.04
1.26
1.18
0.88
1.35
1.14
1.25
0.24
0.62
0.61
0.59
0.56
0.62
0.2
0.38
0.27
0.31
0.44
0.11
0.55
0.36
0.38
0.44
0.57
0.46
0.57
0.53
0.52
0.59
0.37
0.40
0.64*
0.39
0.50
0.54
0.48
0.69*
LVPWd (cm) IVS1 (cm)
0.61*
0.68*
0.38
0.67*
0.45
0.61*
0.58
0.33
0.59
0.42
0.61*
0.60*
0.61*
0.50
0.61*
0.42
0.47
0.60*
0.76*
0.52
0.45
0.56
0.47
0.48
0.54
0.53
0.53
0.68*
0.61*
0.53
IVS2 (cm)
IVSb (cm)
0.61*
0.37
0.44
0.49
0.51
0.44
0.56
0.56
0.41
0.51
0.38
0.51
0.57
0.52
0.37
0.69*
0.47
0.64*
0.74*
0.64*
0.42
0.58
0.71*
0.61*
0.71*
0.37
0.71*
0.59
0.48
0.61*
Ao (cm) LA (cm)
1.07
1.14
0.84
1.33
0.92
1.04
1.03
0.95
1.07
0.92
0.89
0.92
0.89
0.93
0.79
1.45
1.32
1.14
1.57
1.39
1.45
1.24
1.36
1.10
1.14
1.08
1.25
1.36
1.38
1.14
LA:Ao
1.35
1.15
1.36
1.18
1.52
1.40
1.21
1.43
1.03
1.24
1.21
1.35
1.53
1.48
1.45
*Indicated value is  6 mm, indicating the existence of HCM. †This cat had a diagnosis of HCM on the basis of values that were close to the
6-mm cutoff and detection of marked systolic anterior motion of the mitral valve.
Wt = Body weight. LVES = Left ventricular wall thickness at end-systole. IVS1 = Thickness of the interventricular septum segment 1. IVS2 =
Thickness of the interventricular septum segment 2. IVSb = Thickness of the basilar septum. Ao = Aortic diameter at end-diastole. LA = Left atrial
diameter at end-systole. LA:Ao = Ratio of left atrial diameter to aortic diameter.
Hypertrophic cardiomyopathy was echocardiographically defined by an end-diastolic wall thickness $ 6 mm that affected . 50% of any
region of the interventricular septum or LVPW.
JAVMA, Vol 234, No. 11, June 1, 2009
Scientific Reports
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SMALL ANIMALS
were detected in 28 cats, 13 of which did not have a
heart murmur at rest. One cat had a heart murmur at
rest but not after the provocative maneuver. Based on
results of auscultation, the mean heart rate of the 103
cats was 173.9 ± 16.4 beats/min. Additional cardiac abnormalities identified through auscultation were a gallop rhythm in 2 cats and bradycardia in another.
SMALL ANIMALS
heart murmurs had a dynamic ventricular outflow tract
obstruction: 3 cats without HCM had RVOT obstruction
only, 1 cat with HCM had RVOT and LVOT obstruction,
and 2 cats with HCM had LVOT obstruction and mitral
regurgitation associated with systolic anterior motion
of the mitral valve. The other 10 cats had no evidence
of a dynamic ventricular outflow tract obstruction; two
of these cats had HCM. On the other hand, 10 of 85
(12%) cats without an unprovoked heart murmur had a
dynamic ventricular outflow tract obstruction, whereas
the other 75 (88%) did not.
Ten of the 28 (36%) cats with heart murmurs detected after provocation had Doppler echocardiographic
evidence of dynamic ventricular outflow tract obstruction.
The other 18 (64%) cats with provoked heart murmurs
had no dynamic ventricular outflow tract obstruction. Of
the cats with heart murmurs detected after provocation, 5
without HCM had RVOT obstruction only, 1 with HCM
and 1 without had LVOT only, 1 with HCM had RVOT and
LVOT obstructions, and 2 with HCM had LVOT obstruction and mitral valve regurgitation associated with systolic
anterior motion of the mitral valve. The 6 cats that had
a heart murmur at rest and dynamic ventricular outflow
tract obstruction also had a heart murmur after provocation. On the other hand, 6 of 72 (8%) cats without a provoked heart murmur had a dynamic ventricular outflow
tract obstruction, whereas the other 66 (92%) did not.
The odds that a cat with an unprovoked heart murmur had a dynamic ventricular outflow tract obstruction
as detected via Doppler echocardiography were 4.5 times
as high as the odds that a cat without an unprovoked
heart murmur had the same problem (95% CI, 1.3 to 15.1;
P = 0.01; n = 101). The odds of a cat with a provoked
heart murmur having a dynamic ventricular outflow
tract obstruction were 6.1 times as high as the odds of a
cat without a provoked heart murmur having the same
problem (95% CI, 2.0 to 19.1; P < 0.001; n = 100).
Diagnostic usefulness of auscultation for detecting cardiomyopathy—Comparison of results of auscultation with those of echocardiography indicated
that auscultation of an unprovoked heart murmur had
poor sensitivity but moderate specificity in the detection of cardiomyopathy (Table 2). The P values for the
likelihood ratios were not significant. Given the prevalence of cardiomyopathy in the sample as determined
via echocardiography (15.5%), the probability that a cat
with an unprovoked heart murmur would have cardiomyopathy (ie, the positive predictive value of the test)
was only 31% (95% CI, 9% to 54%). In addition, 13%
of cats without unprovoked heart murmurs would have
cardiomyopathy (negative predictive value, 87%; 95%
CI, 80% to 94%).
Table 2—Results for the evaluation of auscultation of unprovoked
heart murmurs as a means of detecting cardiomyopathy in 103
apparently healthy cats, with echocardiography used as the gold
standard.
Characteristic
Value
95% CI
Prevalence of cardiomyopathy (%)
Sensitivity (%)
Specificity (%)
Positive likelihood ratio
Negative likelihood ratio
15.5
31.3
87.4
2.5
0.8
9.2–24.0
8.5–54.0
80.4–94.3
1.0–6.2
0.6–1.1
1402
Scientific Reports Discussion
In the present study, the relationship between auscultatory abnormalities and echocardiographic findings
was evaluated in a sample of apparently healthy cats that
were prospectively enrolled. Results indicated that the
prevalence of subclinical cardiomyopathy in the population of nonreferred cats in the area (Southwest Virginia)
was approximately 16%; most affected cats had HCM, of
which approximately a third had heart murmurs. Only 5
of 16 cats with heart murmurs had cardiomyopathy.
Other researchers have evaluated epidemiologic characteristics of cardiomyopathy in cats, but many of these
studies involved referral-based populations or excluded cats
with heart murmurs. One group reported a 21% prevalence
of heart murmurs in 103 overtly healthy cats.4 However,
cats that had a heart murmur or history of cardiac disease
were excluded from that study; therefore, that prevalence
estimate cannot be directly compared with our findings.
Because we included cats on the basis of apparent health
status, we believe we obtained a better estimate of the true
prevalence within the target population (apparently healthy
cats). Importantly, our study design removed the bias that
would have been introduced by an echocardiographer attempting to identify the source of a heart murmur.
Results of the present study indicated that HCM in cats
is more prevalent than it is in humans (estimated prevalence,
0.2%15). This prevalence of HCM in apparently healthy cats
was high, but to our knowledge, there are no similar data
with which to compare our findings. It is important to mention that M-mode echocardiography, which has been used
in many studies of HCM in cats, is likely to be less sensitive
than 2-D echocardiography for detection of hypertrophy,
as is true in people with HCM.16 Most (11/15) cats with
HCM had segmental left ventricular hypertrophy. The cats
identified with HCM were apparently mildly affected. The
degree of hypertrophy was not marked and was generally
segmental; furthermore, none of the cats had echocardiographic evidence of atrial enlargement. The high prevalence
of HCM in apparently healthy cats was consistent with the
diversity of this disorder in humans. In the past, the clinical
impact of HCM in humans and perhaps in cats has been
exaggerated because data have typically been obtained from
referral facilities. In humans, HCM is a genetic disorder
that is associated with diverse phenotypic expression and
a variable clinical course.15,16 In Maine Coon and Ragdoll
breeds of cats, HCM is heritable and the associated genetic
mutations have been reported.17,18 Familial HCM has been
detected in other breeds of cats; therefore, it is possible that
HCM is generally a genetic disorder in cats.
Abnormal auscultatory findings often prompt an
echocardiographic examination, which is the next logical diagnostic step when cardiomyopathy is suspected
in cats. However, retrospective studies5,12 have revealed
that some cats with cardiomyopathy do not have heart
murmurs. In the present study, 11 of 16 cats with cardiomyopathy did not have heart murmurs. Sensitivity
and specificity as relevant to the present study characterized the proportion of cats with or without cardiomyopathy that were identified by the presence or absence a
heart murmur, respectively.19 Interestingly, only 31% of
cats with cardiomyopathy had heart murmurs, although
it may be relevant that both cats that had hypertrophic
obstructive cardiomyopathy associated with systolic anJAVMA, Vol 234, No. 11, June 1, 2009
a.
b.
c.
d.
Vingmed System FiVe, General Electric Medical Systems, Waukesha, Wis.
DRI thyroxine assay, Microgenics Corp, Fremont, Calif.
Olympus AV 400, Olympus America Inc, Melville, NY.
SAS, version 8.02, SAS Institute Inc, Cary, NC.
JAVMA, Vol 234, No. 11, June 1, 2009
e.
SISA, Hilversum, The Netherlands. Available at: home.clara.net/
sisa/binomial.htm. Accessed Dec 15, 2006.
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Scientific Reports
1403
SMALL ANIMALS
terior motion of the mitral valve also had heart murmurs.
Because the sensitivity was low, use of auscultation as a
diagnostic screening test for cardiomyopathy would likely yield a high proportion of false-negative test results.20
The calculation of likelihood ratios is another approach to evaluate the usefulness of a diagnostic test.21
In the present study, neither likelihood ratio was significant. Considered with the other measurements of
diagnostic accuracy, auscultation of heart murmurs in
healthy cats does not usefully discriminate between
those with cardiomyopathy and those without.
In general, diagnostic tests that effectively screen
populations for disease have high sensitivity. When detection of a heart murmur was considered a positive test
result, cardiac auscultation had low sensitivity and moderate specificity for detection of cardiomyopathy. In this
regard, auscultation had limitations as a diagnostic test
for cardiomyopathy.
The association between abnormal ventricular outflow tract velocities and heart murmurs was evaluated
through the calculation of prevalence odds ratios.22 In
our study, cats with Doppler echocardiographic evidence
of dynamic ventricular outflow tract obstruction were
4.5 times as likely to have a heart murmur at rest and 6
times as likely to have a heart murmur after provocation,
compared with cats that did not have obstruction. Because
the study was cross-sectional, we could not infer a causal
association between echocardiographic findings and the
existence of a heart murmur. However, we could conclude
that cats with Doppler echocardiographic evidence of abnormal ventricular outflow velocity were more likely to
have a heart murmur at rest and after provocation. Heart
murmurs in cats are reportedly labile and vary in intensity
with heart rate.13 Because auscultation was not performed
during the echocardiogram, it is plausible that some, if not
all, cats with dynamic ventricular outflow tract obstruction could develop a heart murmur in certain conditions.
Results of the present study must be interpreted in the
context of the study limitations. The CIs for the descriptive
statistics were broad, and this was a reflection of the sample
size, which was chosen on the basis of practicality. The cats
enrolled in the study were pets; therefore, it was not possible to confirm the diagnosis of cardiomyopathy by use
of postmortem examination. Several (n = 22) cats resisted
manual restraint, and exclusion of these cats may have influenced the prevalence results. Furthermore, although we
attempted to echocardiographically examine each cat in a
consistent and systematic manner, some cats did not tolerate ECG monitoring or, in other situations, motion artifact
obscured the onset of the QRS complex in the ECG. For
those examinations, the maximal diastolic excursion of the
left ventricle and not the onset of the QRS complex was
used to define the end of diastole. Although the magnitude
of the difference between the 2 methods was likely small,
this inconsistency constitutes a minor limitation of our
work. Finally, a few cats had isolated basilar septal hypertrophy, and a change in aorticoseptal angle (ie, the development of a so-called sigmoid septum) could have resulted in
an artifactual appearance of septal hypertrophy.23