![Graph depicting the relationship between PCDW and platelet count for 89 CKCSs (42 males [white squares] and 47 females [black circles]) with subclinical chronic valvular heart disease. Multiple regression analysis revealed that a model containing 2 variables (platelet count and sex) best explained the variation in PCDW (platelet count partial R 2 , 0.21; model R 2 , 0.25 [P = 0.03]).](https://www.researchgate.net/profile/Peter-Irwin/publication/305740142/figure/fig2/AS:11431281127494718@1679036412159/Graph-depicting-the-relationship-between-PCDW-and-platelet-count-for-89-CKCSs-42-males_Q320.jpg)
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860 AJVR • Vol 77 • No. 8 • August 2016
Chronic valvular heart disease, characterized by
progressive myxomatous degeneration and thick-
ening of the mitral valve leaflets, is the most common
heart disease in dogs.1 This disease is particularly
common in CKCSs, with echocardiographic evidence
of mitral valve prolapse reported in > 80% of CKCSs
that are < 3 years old.2
Platelet function and activation
in Cavalier King Charles Spaniels with subclinical
chronic valvular heart disease
Linda J. Tong bvsc
Giselle L. Hosgood bvsc, phd
Anne T. French mvb, phd
Peter J. Ir win bvetmed, phd
Robert E. Shiel mvb, phd
Received August 10, 2015.
Accepted October 19, 2015.
From the College of Veterinary Medicine, Murdoch
University, Murdoch, WA, Australia 6150 (Tong,
Hosgood, Irwin, Shiel); and the Department of Small
Animal Clinical Sciences, School of Veterinary Medi-
cine, College of Medical, Veterinar y and Life Sciences,
University of Glasgow, Glasgow, Scotland G61 1QH
(French). Dr. Tong’s present address is Western Aus-
tralian Veterinary Emergency and Specialty, 1/640
Beeliar Dr, Success, WA, Australia 6164. Dr. Shiel’s
present address is the School of Veterinary Medicine,
University College Dublin, Belfield, Dublin 4, Ireland.
Address correspondence to Dr. Tong (lindajtong@
gmail.com).
OBJECTIVE
To assess platelet closure time (CT), mean platelet component (MPC) con-
centration, and platelet component distribution width (PCDW) in dogs
with subclinical chronic valvular heart disease.
ANIMALS
89 Cavalier King Charles Spaniels (CKCSs) and 39 control dogs (not
CKC Ss) .
PROCEDURES
Platelet count, MPC concentration, PCDW, and Hct were measured by use
of a hematology analyzer, and CT was measured by use of a platelet func-
tion analyzer. Murmur grade and echocardiographic variables (mitral valve
regurgitant jet size relative to left atrial area, left atrial-to-aor tic diameter
ratio, and left ventricular internal dimensions) were recorded. Associations
between explanatory variables (sex, age, murmur grade, echocardiographic
variables, platelet count, and Hct) and outcomes (CT, MPC concentration,
and PCDW) were examined by use of multivariate regression models.
RE S U LT S
A model with 5 variables best explained variation in CT (R2, 0.74), with
> 60% of the variance of CT explained by mitral valve regurgitant jet size.
The model of best fit to explain variation in MPC concentration included
only platelet count ( R2, 0.24). The model of best fit to explain variation in
PCDW included platelet count and sex (R2, 0.25).
CONCLUSIONS AND CLINICAL RELEVANCE
In this study, a significant effect of mitral valve regurgitant jet size on CT
was consistent with platelet dysfunction. However, platelet activation, as
assessed on the basis of the MPC concentration and PCDW, was not a
feature of subclinical chronic valvular heart disease in CKCSs. (Am J Vet Res
2016;77:860 –868)
In all species, valvular heart disease has the po-
tential to affect platelet activation or function as a
result of turbulent high-velocity blood flow and fluid
shear stress.3 Increased platelet activation and reactiv-
ity would be expected initially; however, continuous
stimulation and stress may lead to structural and bio-
chemical changes associated with decreased platelet
function.4 Alteration of platelet function in humans
with heart disease contributes to the development of
vascular remodelling, thromboembolic events, and
fatalities.5
There are conflicting reports regarding platelet
activation and function in dogs with chronic valvu-
lar heart disease. Significantly longer CTs have been
described in CKCSs with moderate to severe mitral
valve regurgitation than for those with minimal or
mild regurgitation or healthy control dogs.6–8 A signif-
icant inverse relationship has also been identified be-
tween plasma concentration of von Willebrand factor
and regurgitant jet size as well as a relative absence
of high-molecular-weight von Willebrand factor mul-
ABBREVIATIONS
CI Confidence interval
CKCS Cavalier King Charles Spaniel
CT Closure time
IQR Interquartile range
LA:Ao Left atrial-to-aor tic ratio
LVDd Left ventricular diameter during diastole
LVDdn Left ventricular diameter during diastole
normalized on the basis of body weight
LVDs Left ventricular diameter during systole
LVDsn Left ventricular diameter during systole
normalized on the basis of body weight
MPC Mean platelet component
MPV Mean platelet volume
PCDW Platelet component distribution width
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AJVR • Vol 77 • No. 8 • August 2016 861
timers in samples obtained from dogs with moderate
to severe mitral valve regurgitation.6 Consequent-
ly, the prolonged CT was thought to be a result of
these quantitative and qualitative changes in von
Willebrand factor rather than an alteration of intrin-
sic platelet function. However, concurrent alteration
of platelet function or platelet activation could not be
excluded.
Enhanced platelet-leukocyte aggregation, which
is suggestive of platelet activation, has been identi-
fied in dogs with congestive heart failure, compared
with results for age-matched control dogs.a In addi-
tion, decreased platelet lifespan has been reported
in dogs following experimental induction of mitral
valve regurgitation, which would support the pres-
ence of younger platelets with increased activity.9
However, plasma concentration of thromboxane B2,
platelet surface P-selectin expression, and thrombo-
elastographic variables are not significantly different
between CKCSs with chronic valvular heart disease
and moderate to severe mitral valve regurgitation,
compared with results for affected dogs with absent
or minimal mitral valve regurgitation or healthy con-
trol dogs of other breeds.8,b
Aggregometric studies4, 8,10 have also yielded
conflicting results, with increased, decreased, and
unchanged responses described in CKCSs with ad-
vanced disease, compared with results for those with
mild or no disease or healthy dogs of other breeds.
Reasons for these discrepancies are unclear, but they
may be explained by differences in laboratory meth-
ods, lack of standardization of laboratory techniques,
and differences in criteria for the classification of dis-
ease severity and control dogs. In addition, marked
interbreed variability has been observed for the maxi-
mal aggregation response,11–13 and breed composition
of control groups has differed among studies. There-
fore, in contrast to information on mechanisms for
humans, the extent of platelet activation in dogs with
chronic valvular heart disease remains uncertain.
Mean platelet component concentration, which
is measured by use of an optical-based hematology
analyzer, has emerged as a marker of platelet acti-
vation.14 The MPC concentration is an estimate of
platelet density, and decreased values are indicative
of platelet activation.15 In dogs, decreased MPC con-
centrations have been detected after exercise, in pa-
tients with inflammatory disease, and in association
with immune-mediated thrombocytopenia.14,16 –18 It
has been suggested that decreased MPC concentra-
tions are a more sensitive marker of activation than
is P-selectin expression, in part because of the persis-
tence of decreased MPC concentrations despite the
loss of cell-surface P-selectin.14,19 The SD of the plate-
let component concentration, known as the PCDW,
can also be calculated. This index is increased if both
degranulated and nondegranulated circulating plate-
lets are present, and it is considered another marker
of platelet activation.14 Therefore, MPC concentration
and PCDW could provide additional information re-
garding the activation of platelets in dogs with heart
disease.
Therefore, the primary objective of the study
reported here was to measure MPC concentration,
PCDW, and CT in CKCSs with subclinical mitral valve
regurgitation. Another objective was to investigate
possible associations of these variables with markers
of heart disease severity (murmur grade, mitral valve
regurgitant jet size, left atrial size, and left ventricular
dimensions).
Materials and Methods
Animals
Client-owned CKCSs were prospectively recruit-
ed from the Murdoch University Veterinary Hospi-
tal, primary care veterinary practices, and CKCS
breeders. Dogs were recruited from September 2011
through June 2013. Dogs were eligible for inclusion
if they were ≥ 6 months old and apparently healthy
(defined as the absence of systemic or organ-related
disease, other than the presence of a left apical sys-
tolic murmur or dental disease) as determined by
an interview with the owner and results of a health
questionnaire and physical examination. Dogs that
received any drug (other than routine prophylactic
antiparasitic drugs or vaccinations) within 8 weeks
prior to participation were excluded. Age, sex, and
body weight of each dog were recorded.
A control group of healthy dogs of breeds other
than CKCS was included for comparison of MPC con-
centration and PCDW. Client-owned dogs that were
not CKCSs were retrospectively recruited on the ba-
sis of review of the Murdoch University Veterinary
Hospital database for the period of November 2011
through May 2013. Dogs were included if they were
≥ 6 months old, were apparently healthy (defined
as the absence of systemic or organ-related disease,
other than the presence of dental disease, cutaneous
masses < 1 cm diameter that did not appear to cause
pain to the dog, or orthopedic disease), and had a
hematologic analysis performed by use of an optical-
based hematology analyzer.c The reason for admis-
sion, signalment (age, sex, and breed), body weight,
platelet count, MPC concentration, PCDW, and Hct
were recorded.
Signed owner consent was obtained for all dogs
participating in the study. The study was approved
by the Murdoch University Animal Ethics Committee.
Assessment of cardiac murmurs
Cardiac auscultation of each CKCS was per-
formed prior to echocardiographic assessment. Pres-
ence or absence, intensity (grades 1 through 6), loca-
tion, and character of each murmur were recorded.20
Echocardiography
Echocardiography was performed with an ultra-
sonography system.d Each CKCS was examined while
positioned in right and left lateral recumbency. Ex-
aminations were performed by 1 investigator (LJT),
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862 AJVR • Vol 77 • No. 8 • August 2016
and all recorded images were reviewed by a veteri-
nary cardiologist (ATF). Examinations were per-
formed both with and without color flow mapping by
use of a 4- to 10-Hz and 1- to 4-Hz electronic sector
transducer, respectively.
The LVDd and LVDs were measured by use of
M-mode echocardiography at the level of the chordae
tendinae as guided by a 2-dimensional right paraster-
nal short axis view.21 The reported measurement for
each dog was the mean of 3 measurements normal-
ized on the basis of body weight by use of the follow-
ing equations22:
LVDdn = LVDd/(body weight in kg)0.294
LVDsn = LVds/(body weight in kg)0.315
Reference intervals for LVDdn and LVDsn were
1.27 to 1.85 and 0.71 to 1.26, respectively.22 Left atrial
and aortic root diameters were measured by use of
a 2-dimensional right parasternal short-axis view at
the level of the aortic valve when the left atrium was
maximally dilated during diastole.21 These diameter
measurements were used to calculate LA:Ao. The
reported LA:Ao was the mean of 3 ratios calculated
from 3 paired measurements. The criterion for left
atrial enlargement was LA:Ao > 1.5.
Presence of mitral valve regurgitation and estima-
tion of the severity was made with color flow Doppler
echocardiography by use of the left apical 4-chamber
view with the dogs positioned in left lateral recum-
benc y.21 Images were analyzed frame by frame to es-
timate the percentage of the left atrium occupied by
the largest mitral jet (jet size) as previously reported.7
Mitral valve regurgitation was subsequently classified
as jet size < 15%, jet size 15% to 50%, or jet size > 50%,
as reported elsewhere.6,7
Collection and preparation of blood
samples
A blood sample (10 mL) was collected from each
CKCS via jugular venipuncture with a 21-gauge nee-
dle and 10-mL syringe. Two milliliters of blood was
transferred into a tube containing K3EDTA,e which
was used for automated hematologic and blood film
assessment. Four milliliters was transferred into two
2-mL tubes containing sodium citrate,f which were
used for CT assessment. The remaining 4 mL of blood
was stored for use in another study.
All tubes were filled to the manufacturer’s speci-
fied volume. Blood was mixed with the anticoagulant
by gentle inversion of tubes immediately after sample
collection. Blood tubes were mechanically rotated
with a blood tube rotatorg until processing. Blood
was stored at room temperature (approx 25°C).
Hematologic analysis
Blood samples were analyzed with an optical-based
hematology analyzerc and manufacturer-developed mul-
tispecies software.h All samples were processed within
40 minutes after collection. Platelet count, MPC concen-
tration, PCDW, and Hct were recorded.
Blood films were prepared from each CKCS sam-
ple and stained with modified Wright staini by use of
an automated staining instrument.j One investigator
(LJT), who was not aware of the platelet count, ex-
amined each blood film. Number and size of platelet
aggregates were estimated in 5 consecutive hpfs (40X
objective with a 10X ocular) at the feathered edge of
each blood film. This protocol was a modification of
that described in a study23 in which the number and
size of platelet aggregates were estimated in fields
by use of oil immersion (100X objective). Number
of platelet aggregates was judged on a scale of 0 to
3 as follows: 0 = absent, 1 = sparse with 1 or 2 ag-
gregates/5 hpfs, 2 = moderately frequent with 3 to
5 aggregates/5 hpfs, and 3 = numerous with > 5 ag-
gregates/5 hpfs. Size of aggregates was assessed on a
scale of 1 to 3 as follows: 1 = small (3 to 8 platelets),
2 = medium (9 to 20 platelets), and 3 = large (> 20
platelets).
CT
Closure time was determined by use of a plate-
let function analyzerk and collagen-ADP cartridges in
accordance with the manufacturer’s instructions. All
samples were processed within 60 minutes after col-
lection. Each sample was assayed in duplicate. When
the coefficient of variation was > 15%, the results
were rejected, and a duplicate assay was performed.
Results for dogs with a PCV < 35% were excluded.24
Statistical analysis
Descriptive variables with continuous data (age,
body weight, platelet count, MPC concentration,
PCDW, and Hct) were tested for normality by use of
the Shapiro-Wilk test. A normal distribution was de-
termined when there was failure to reject the null
hypothesis of normality at P < 0.05. Normally dis-
tributed data were summarized as mean and 95% CI.
Nonnormally distributed data were summarized as
median and IQR. Comparisons were made between
groups by use of Student t tests for parametric vari-
ables and the Mann-Whitney U test for nonparametric
variables. Estimated frequencies of select variables
were summarized as proportion and 95% CI. The dis-
tribution of sex for each group was compared by use
of a Fisher exact test. Comparisons among 3 or more
categories within CKCS cohorts were performed
by use of an ANOVA for parametric responses or a
Kruskall-Wallis test for nonparametric responses.
Pairwise post hoc comparisons were performed by
use of the least squares mean or the Mann-Whitney
U test, respectively, with a Bonferonni correction for
overall type I error. Significance was set at P < 0.05
for all comparisons.
Measured variables for the CKCS group were
summarized as described previously. Associations be-
tween possible explanatory variables (sex, age, mur-
mur, jet size, LA:Ao, LVDdn, LVDsn, platelet count,
and Hct) and outcomes (CT, MPC concentration, and
PCDW) were assessed with multivariate regression
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AJVR • Vol 77 • No. 8 • August 2016 863
models. The model of best fit was selected on the
basis of the Cp statistic and consideration of the R2.
All possible subsets were evaluated. The best models
for subsets were selected on the basis of the small-
est Cp statistic with the least bias (ie, the Cp statistic
closest to p, where p is the number of variables in
the model).25 The simplest subsets that explained the
outcome were chosen with the selected Cp statistic
and R2. The proportion of variance explained by the
explanatory variable or variables, partial R2, and cor-
responding P value were reported.
Results
Animals
The study included 89 CKCSs and 39 control
dogs ( Table 1 ) . There was no significant (P = 0.150;
Fisher exact test) difference in the frequency of sex
between groups.
Hematologic analysis
Thrombocytopenia, defined as a platelet count
< 100 X 109 cells/L, was identified in 18 of 89 (20%;
95% CI, 13% to 30%) CKCSs but was not identified in
any of the control dogs. The relationship between MPV
and platelet count was graphed (Figure 1). Platelet ag-
gregates were identified in 44 of 89 samples. Number of
platelet aggregates was judged as sparse, moderatly fre-
quent, and numerous in 26, 9, and 9 blood films, respec-
tively. Aggregate size was classified as small, medium,
or large in 14, 16, and 14 samples, respectively. In the
18 CKCSs with thrombocytopenia, sparse, moderatly
frequent, and numerous numbers of aggregates were
identified in 3, 3, and 5 samples, respectively. Small,
medium, and large aggregates were identified in 7, 2,
and 2 samples, respectively. No platelet aggregates were
identified in the remaining 7 CKCSs. There was no sig-
nificant difference in CT, MPC concentration, or PCDW
between CKCSs with and without platelet aggregates (P
= 0.095; Student t test) or between categories for num-
ber of aggregates (P = 0.134; ANOVA) or size of platelet
aggregates (P = 0.06; ANOVA).
Cardiac murmur and echocardiographic
findings
A left apical systolic murmur was detected in 51
of 89 (57%; 95% CI, 46% to 66%) CKCSs ( Ta b l e 2 ) .
Of the 51 CKCSs with a murmur, 24 (47%) were ≤ 6
years old. Mitral valve regurgitation was detected by
use of color flow Doppler echocardiography in 86 of
89 (97%; 95% CI, 90% to 99%) CKCSs (Ta b l e 3 ) . The
3 dogs that had no evidence of mitral valve regurgita-
tion were 8 months old, 3 years old, and 4 years old,
respectively. Mean LA:Ao, LVDdn, and LVDsn values
of 1.401 (95% CI, 1.353 to 1.449), 1.395 (95% CI, 1.349
to 1.442), and 0.863 (95% CI, 0.832 to 0.894) were
detected in the 89 CKCSs, respectively. Left atrial en-
largement was evident in 22 of 89 CKCSs. Four of 89
CKCSs had left ventricular enlargement during dias-
tole. Left ventricular enlargement during systole was
not detected in any CKCS.
CT
Closure time was recorded for 76 of 89 CKCSs.
Closure time was unavailable for 12 CKCSs because
of failure to perform duplicate assays with the plate-
let function analyzer and for 1 CKCS because it had
an Hct < 0.35 L/L. The median CT was 73 seconds
(IQR, 62 to 102 seconds; Table 3). Closure time was
greater than the upper limit of the reference interval
for 27 CKCSs.
Multivariate regression analysis
Complete data were available for 76 CKCSs (Ta -
ble 4). A model with 5 variables best explained the
variation in CT, with regurgitant jet size exerting the
largest effect. The LA:Ao, age, sex, body weight, and
Hct had no significant effect on CT in the multiple
linear regression analyses. Closure times differed sig-
nificantly (P < 0.001; ANOVA) among regurgitant jet
Variable CKCSs Control dogs P value*
No. of dogs 89 39 —
Males 42 19 0.150
Females 47 20 —
Age (y)† 4.0 (2.0–6.0) 9.8 (5.5–12.3) < 0.001
Body weight (kg)† 10.1 (8.2–11.6) 20.5 (6.4–29.0) 0.010
Platelet count (X 109 cells/L)† 213 (143–306) 315 (252–409) < 0.001
Platelet distribution width (%)† 59.7 (52.4–68.5) 57.4 (53.2–59.3) 0.044
MPC concentration (g/L)‡ 209 (207–211) 204 (199–208) 0.017
PCDW (g/L)‡ 41 (39–43) 54 (50–57) < 0.001
MPV (fL)† 11.2 (8.8–16.1) 8.6 (8.0–9.1) < 0.001
Mean platelet mass (pg)† 2.10 (1.66–2.62) 1.59 (1.51–1.67) < 0.001
Plateletcrit (%)† 24 (19–29) 27 (23–36) 0.011
Large platelets (%)† 9.78 (2.60–24.10) 2.55 (1.53–3.87) < 0.001
Hct (L/L)‡ 0.41 (0.40–0.42) 0.50 (0.48–0.52) < 0.001
*Values were considered significant at P < 0.05. †Nonnormally distributed data; value reported is median (IQR). ‡Normally distributed data; value
reported is mean (95% CI).
— = Not applicable.
Table 1—Comparison of cohort descriptions and hematologic findings for 89 CKCSs with subclinical chronic valvular heart disease
and 39 control dogs that were not CKCSs.
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864 AJVR • Vol 77 • No. 8 • August 2016
sizes; CKCSs with a jet size > 50% had a significantly
(P < 0.001; least squares mean) longer CT than did
CKCSs with a jet size < 15% or CKCSs with a jet size
of 15% to 50% (Table 3; Figure 2).
MPC concentration and PCDW
Complete data were available for 89 CKCSs (Ta-
ble 4). The model that best explained the variation
in MPC concentration included only platelet count.
The model that best explained the variation in PCDW
included platelet count and sex. The relationship be-
tween MPC concentration and platelet count (Figure
3) and between PCDW and platelet count (Figure 4)
was graphed.
Discussion
The objective of the study reported here was to
investigate associations between markers of heart
disease severity and platelet function or activation.
Although 5 variables significantly affected CT, the
regurgitant jet size exerted by far the largest effect.
This may have reflected quantitative and qualitative
changes in von Willebrand factor rather than an al-
teration of intrinsic platelet function, as has been de-
scribed previously.6
By contrast, none of the indicators of heart dis-
ease severity significantly affected MPC concentra-
tion or PCDW. This suggested that platelet activation
was not a feature of valvular heart disease in CKCSs,
which supports results of thromboelastographyb and
evaluation of P-selectin expression and thromboxane
concentrations.8 In humans with cardiac disease,
valvular changes or associated alterations in blood
flow are believed to initiate platelet adherence and
aggregation, which potentially leads to thromboem-
bolic complications.26,27 Significantly increased plate-
let aggregation responses and plasma concentrations
of platelet factor 4 and β-thromboglobulin have been
described for humans with combined mitral valve
prolapse and mitral regurgitation, compared with re-
sults for healthy control subjects.28,29 Similarly, an in-
crease in MPV, which is suggestive of platelet activa-
tion, has been observed in a variety of human cardiac
diseases, including mitral valve prolapse.30,31 Reasons
for the apparent differences in platelet activation be-
tween heart disease in dogs and humans remain un-
clear. Presumably, platelet reactivities or pathogenic
mechanisms that incite activation differ between the
species.
In the present study, 86 of 89 CKCSs had echo-
cardiographic evidence of mitral valve regurgitation.
Because of the high proportion of dogs with mitral
valve regurgitation, lack of a significant association
between indicators of heart disease severity and
indices of platelet activation could also have been
explained by the activation of platelets in almost
all dogs. This was considered unlikely because the
MPC concentration was higher in CKCSs, compared
with concentrations in healthy control dogs, which
suggested a lack of platelet activation. It was also
possible that activated platelets were removed from
circulation, leaving a residual population of platelets
with a relatively higher MPC concentration. However,
platelet activation in humans is proportional to the
severity of mitral valve regurgitation,28,30 and in the
present study, a relationship between indicators of
heart disease severity and MPC concentration would
have been expected, regardless of the direction of
change.
Figure 1—Graph depicting the relationship between MPV
and platelet count for 89 CKCSs with subclinical chronic val-
vular heart disease. The dotted line indicates a platelet count
of 100 X 109 cells /L. Higher MPV values are evident in several
dogs with lower platelet counts, which is consistent with the
presence of macrothrombocytopenia within these animals.
Murmur grade No. of dogs
0 38
1 20
2 13
3 9
4 7
5 2
6 0
Table 2—Frequency of murmur grades in a group of 89 CKCSs
with subclinical chronic valvular heart disease.
Table 3—Regurgitant jet size and corresponding CT values
(mean [95% CI]) in a group of CKCSs with subclinical chronic
valvular heart disease.
Regurgitant jet size (%)* n CT (s)
< 15 43 67 (64–70)
15 to 50 12 78 (65–92)
> 50 21 138 (122–155)†
*Regurgitant jet size was < 15%, 15% to 50%, and > 50% for 52, 14,
and 23 CKCSs, respectively; CT was not determined for all CKCSs.
†Value differs significantly (P < 0.001) from the values for the other
regurgitant jet sizes.
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AJVR • Vol 77 • No. 8 • August 2016 865
Platelet aggregates were identified in 44 of 89
(49%) CKCSs. In 1 previous study,32 26 blood samples
were collected by experienced phlebotomists from
a jugular vein of cooperative dogs and expediently
transferred to anticoagulant. Despite use of this tech-
nique, at least mild platelet aggregation was observed
in 14 (54%) samples. In theory, platelet aggregation
during or after blood collection has the potential to
activate platelets and alter MPC concentration. A re-
sultant lower MPC concentration would be expected
with a decreasing platelet count, rather than the in-
verse relationship that was observed in the study re-
ported here. Alternatively, a lower platelet count and
higher MPC concentration could be expected if acti-
vated platelets preferentially clumped, which would
result in a residual platelet population with a higher
MPC concentration. However, MPC concentrations
were not significantly different between dogs with
and without platelet aggregation.
Table 4—Results of multiple regression analysis of data for CKCSs with subclinical chronic valvular heart disease.
Variable Explanatory variable Partial R2 Model R2 Cp statistic P value*
CT (n = 76) Regurgitant jet size 0.61 0.61 35.2 < 0.001
LVDdn 0.06 0.67 23.2 0.001
Murmur grade 0.03 0.70 16.7 0.009
Platelet count 0.02 0.72 13.0 0.026
LVDsn 0.02 0.74 8.6 0.015
Model — 0.74 8.6 0.015
MPC (n = 89) Platelet count 0.24 0.24 –1.8 < 0.001
Model — 0.24 –1.8 < 0.001
PCDW (n = 89) Platelet count 0.21 0.21 1.25 < 0.001
Sex 0.04 0.25 4.86 0.030
Model — 0.25 4.86 0.030
*Values were considered significant at P < 0.05.
— = Not applicable.
Figure 2— Graph depicting CT for 76 CKCSs with subclini-
cal chronic valvular heart disease that had regurgitant jet
size < 15% (n = 43), 15% to 50% (12), and > 50 % (21). The
CT was significantly (P < 0.001; least squares mean) longer
for CKCSs with regurgitant jet size > 50% , compared with
CT for dogs with regurgitant jet size < 15% or 15% to 50%.
Figure 3—Graph of the relationship between MPC con-
centration and platelet count for 89 CKCSs with subclinical
chronic valvular heart disease. Multiple regression analysis
revealed that a model containing only 1 variable (platelet
count) best explained the variation in MPC concentration
(model R2, 0.24; P < 0.001).
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866 AJVR • Vol 77 • No. 8 • August 2016
Several differences were identified between
platelet indices in the CKCS and control groups. Un-
surprisingly, platelet count was significantly lower
and MPV significantly higher in the CKCS group,
which likely reflected the presence of macrothrom-
bocytopenia in several CKCSs. This was supported
by higher MPV values in CKCSs with lower platelet
counts (Figure 1). Macrothrombocytopenia in CKCSs
is associated with a nonsynonymous single nucleo-
tide polymorphism in the gene encoding β1-tubul in,
which is believed to lead to altered proplatelet pro-
duction by megakaryocytes.33 Although the β-1 iso-
late of β-tubulin is considered to be megakaryocyte
specific, it may be upregulated in other tissues under
pathological conditions. Although densification of
microtubular networks within cardiomyocytes con-
tributes to contractile dysfunction in subjects with
experimentally induced heart disease, it is unclear
whether this altered β-tubulin structure plays a role
in the pathogenesis of naturally occurring chronic
valvular heart disease in CKCSs.34,35
The plateletcrit was significantly lower for the
CKCS group. However, the plateletcrit for the opti-
cal-based hematology analyzerc was determined in-
directly by use of the equation plateletcrit = platelet
count X MPV/10,000. The optical-based hematology
analyzer used light-scatter signals acquired at 2 angles
that were converted into volume and refractive indi-
ces via calculation with the Mie light-scatter theory.36
A graphic representation of the 2 light-scatter mea-
surements was created, and platelets were identi-
fied in the region corresponding to a volume of 1 to
60 fL and refractive index of 1.35 to 1.40. The platelet-
scatter cytogram displayed cells with volumes of 0
to 30 fL. The large platelet area of the RBC-scatter
cytogram displayed large platelets with volumes be-
tween 31 and 60 fL. The reported platelet count was
the sum of platelets and large platelets identified in
the platelet- and RBC-scatter cytograms, respectively.
It has been suggested that the platelet algorithm for
the optical-based hematology analyzerc may deliber-
ately exclude large platelets from analysis if the hypo-
chromic-macrocytic flag results in misclassification
of large platelets as small RBCs, with a resultant lower
platelet count and derived plateletcrit.37 Such misclas-
sification of platelets as a result of macrothrombocy-
topenia could explain the lower plateletcrit derived
for the CKCS group.
A greater MPC concentration within CKCSs was
consistent with interbreed variation. To the authors’
knowledge, this has not been previously described in
dogs. However, lower platelet aggregation responses
have been reported in CKCSs relative to healthy Bea-
gles, and higher responses have been reported rela-
tive to a group of Cairn Terriers, Labrador Retrievers,
and Boxers, which suggests that breed factors should
be considered when interpreting results of platelet
function tests.11,13
Platelet count had a small but significant effect on
MPC concentration and PCDW in the multiple regres-
sion models. The MPC concentration was lower and
PCDW higher with increasing platelet counts. Both
of these findings suggested greater platelet activation
at higher counts. Investigators of a previous study10
found higher maximal platelet aggregation responses
for CKCSs with platelet counts > 100 X 109 cells/L,
compared with responses for healthy control dogs
and CKCSs with < 100 X 109 cells/L. The reasons for
these associations are not known. Additional studies
are necessary to compare the effects of platelet count
and macrothrombocytopenia on markers of platelet
activation.
Dogs in the study reported here were apparently
healthy; however, subclinical diseases could not be
excluded and may have affected platelet activation in-
dices in some cases. For example, increased platelet
activation has been described in dogs with malignant
neoplasia,38 and internal neoplasia could not be ex-
cluded in the dogs of the present study without ad-
ditional diagnostic imaging. Although such unrecog-
nized diseases could have affected platelet activation
indices in some dogs, it was thought unlikely to affect
results of the present study because of the expected
low prevalence of such diseases in dogs with no clini-
cal signs of illness.
Limitations of the present study included inclu-
sion of a small number of CKCSs without mitral valve
regurgitation and the fact there were no dogs with
congestive heart failure. Unaffected healthy CKCSs
would have been the ideal control group for assess-
ment of the effects of mitral valve regurgitation on
markers of platelet activation. However, given the
high prevalence of mitral valve regurgitation in this
breed, obtaining a large number of unaffected CKCSs
would have been difficult. The inclusion of dogs with
congestive heart failure might have confounded the
Figure 4—Graph depicting the relationship between PCDW
and platelet count for 89 CKCSs (42 males [white squares]
and 47 females [black circles]) with subclinical chronic valvu-
lar heart disease. Multiple regression analysis revealed that
a model containing 2 variables (platelet count and sex) best
explained the variation in PCDW (platelet count partial R2,
0.21; model R2, 0.25 [P = 0.03]).
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AJVR • Vol 77 • No. 8 • August 2016 867
interpretation of results because several commonly
used drugs, including diuretics and angiotensin-
converting enzyme inhibitors, can reportedly affect
platelet function in humans,39–41 and the same may
be true in dogs.
In the study reported here, platelet activation,
as assessed on the basis of MPC concentration and
PCDW, was not a feature of subclinical chronic val-
vular heart disease in CKCSs. Increased CT in CKCSs
with a regurgitant jet size > 50% may reflect quanti-
tative and qualitative changes in von Willebrand fac-
tor, as previously described.6 Significant differences
in several platelet variables, including platelet count,
MPV, analyzer-derived plateletcrit, MPC concentra-
tion, and PCDW, were detected between CKCSs and
dogs of other breeds. Such interbreed variation must
be considered when interpreting results. Additional
studies are needed to investigate potential platelet ac-
tivation in dogs with clinical heart disease.
Acknowledgments
Presented in abstract form at the Australian and New Zealand
College of Veterinary Scientists Science Week Scientific Confer-
ence, Gold Coast, QLD, Australia, July 2014.
Supported by Boehringer Ingelheim.
The authors thank Drs. Fleur James and Jennifer Mills for tech-
nical assistance and Dr. Caroline Mansfield for clinical assistance.
Footnotes
a. Tarnow I, Olsen LH, Andreasen S, et al. Congestive heart fail-
ure in dogs is associated with enhanced platelet-leukocyte
aggregates—a marker for platelet activation (abstr). J Vet In-
tern Med 2010 ;24:67 3.
b. Tarnow I, Olsen LH, Moesgaard SG, et al. Thromboelastog-
raphy in dogs with asymptomatic my xomatous mitral valve
disease (abstr). J Vet Intern Med 2 010 ;24 :673.
c. Advia 120, Siemens Healthcare Diagnostics, Tarr ytown, NY.
d. Acuson Sequoia 512, Siemens Healthcare Diagnostics, San
Jose, Cali f.
e. Vacuette, Greiner Bio- One GmbH, Kremsmünster, Austria.
f. Sarstedt, Nümbrecht, Germany.
g. Innovative Medical Systems Corp, Ivyland, Pa.
h. Version 3.1.8.0-MS, Siemens Healthcare Diagnostics, Tarry-
to wn, N Y.
i. Hematek stain pack, Siemens Healthcare Diagnostics, Tarr y-
to wn, N Y.
j. Hematek 1000, Siemens Healthcare Diagnostics, Tarr ytown,
NY.
k. PFA-100, Siemens Healthcare Diagnostics, GmbH, Marburg,
Ger m a ny.
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