Page 147 - Feline Cardiology

146 Section D: Cardiomyopathies

DE that correspond to histopathologic regions of (ESV and EDV) are defined by the smallest and largest
replacement and interstitial fibrosis, and inflammation volumes on the plot, respectively. Stroke volume (SV) is
(Nanjo et al. 2002). Assessment of myocardial fibrosis calculated by subtracting ESV from EDV, and cardiac
may yield important clinical information such as risk of output (CO) is the product of SV and heart rate. Cardiac
2
development of CHF or sudden cardiac death in patients index equals CO/body surface area (m ). The first deriva-
with HCM. Regional heterogeneity of diastolic function tive of vol/time is then calculated, which produces the
has been diagnosed by cMRI, and occurs in regions of graph dV/dt (see Figure 11.24). From this plot, cMRI
extensive fibrosis. The extent of DE is highly correlated indices of diastolic function are obtained and include
with extent of diastolic dysfunction in people with HCM. peak early filling rate within the first half of diastole
(ml/s), peak late filling rate within the last half of dias-
Cardiomyopathies with HCM percentage (volume increase from end-systole to mid-
Delayed enhancement cardiac MRI in cats
tole, peak filling rate, average filling rate, early filling
point of diastole/SV × 100), time to peak early filling,
Currently, there are no noninvasive methods to assess the
extent of myocardial fibrosis in cats. Circulating bio-
and early to late filling rate ratio.
Gradient echo cMRI was insensitive to detect diastolic
markers of collagen synthesis and degradation (collagen
type I terminus (ICTP), procollagen type I (PINP), and
HCM without heart failure (MacDonald et al. 2006a).
procollagen type III) were measured using a radioimmu- dysfunction in 7 Maine coon cats with moderate to severe
noassay (Orion Diagnostica, Finland), and were not sig- PW TDI identified diastolic dysfunction in all cats with
nificantly elevated in asymptomatic Maine coon cats with HCM, but there was no difference in the cardiac MRI
mild to severe HCM compared to normal cats (author’s indices of diastolic function in normal and HCM cats. It
unpublished data). Assessment of myocardial fibrosis is possible that other techniques of diastolic function
using DE-MRI and calculation of myocardial contrast quantification by cardiac MRI such as velocity encoded
enhancement has been performed in 26 Maine coon cats cine would be able to identify diastolic dysfunction in
with mild to severe HCM without heart failure and 10 cats with HCM, but no studies have evaluated other
normal control cats (MacDonald et al. 2005). Only one cMRI techniques in cats. There are few cMRI studies
cat with HCM had obvious evidence of DE (see Figure assessing diastolic function in humans with HCM
11.23), and there was no difference in myocardial contrast (Jarvinen et al. 1996). One study examined 24 patients
enhancement between normal cats and cats with HCM. and 10 normal volunteers and identified that there was
Therefore, contrast cMRI appears to be minimally useful regional early diastolic dysfunction in people with HCM,
to evaluate for evidence of myocardial fibrosis in asymp- with greater diastolic impairment in patients with greater
tomatic Maine coon cats with familial HCM, since pres- LV mass (Yamanari et al. 1996). Left atrial volume was
ence of delayed enhancement is rarely seen in these cats. increased in mildly symptomatic patients with HCM,
and both left and right atrial filling and emptying rates
Assessment of diastolic function by were reduced (Jarvinen et al. 1996). Increased LV stiff-
cardiac MRI ness may reduce atrial emptying rates, and increased
cMRI is an accurate noninvasive tool for quantifying LV atrial stiffness may reduce atrial filling rates.
mass and assessing left and right ventricular function in One disadvantage of the gradient echo sequence,
normal people and people with cardiac diseases includ- which is often used for cMRI, is that it employs prospec-
ing dilated cardiomyopathy, HCM, hypertensive cardiac tive ECG gating, where no images are acquired during
disease, and aortic stenosis (Kudelka et al. 1997). Diastolic the brief trigger delay at the R wave and a short trigger
function may be assessed using cine or gradient echo window preceding the R wave. Prospective gating may
cMRI (Figure 11.24), velocity encoded cine cMRI, and result in a loss of early systolic and late diastolic images,
myocardial tagging techniques. For the cine or gradient which may impact functional analysis. Velocity encoded
echo assessment of cardiac function, short-axis images cine cMRI is an attractive alternative to gradient echo
are acquired extending from the mitral annulus to the cMRI for cardiac function studies, since it employs ret-
apex over multiple phases of the cardiac cycle (18–25 rospective gating throughout the cardiac cycle. Velocity
phases, depending on heart rate). The endocardial border encoded cine is a novel technique that encodes the veloc-
is manually or semiautomatically traced using a special- ity of moving blood cells to the phase of the MR signal.
ized program (MASSPlus®, Netherlands). Papillary Red blood cells undergo a shift in magnetic spin as they
muscles are included within the tracing. LV volume is move along the magnetic field gradient relative to the
the sum of the endocardial volumes of all slices. LV myocardium. Black and white correspond to the direc-
volume versus time in cardiac cycle is plotted, interpo- tionality of the flow. Flow is summated throughout the
lated, and smoothed between measurement points (see cardiac cycle. The amount of time required for image
Figure 11.24). End-systolic and end-diastolic volume analysis is reduced compared to cine MRI since images