Page 146 - Feline Cardiology

Chapter 11: Hypertrophic Cardiomyopathy 145

to accurately quantify LV mass in animal models of LV cellular (interstitial) space and has a slow washout period
hypertrophy including experimental canine aortic ste- (6–20 minutes). In cardiac fibrosis, the extracellular space
nosis and a transgenic mouse model of LV hypertrophy is expanded due to the loss of myocytes and the presence
(Franco et al. 1998; Wang et al. 1990). Results of cMRI of collagen. The expanded extracellular space results in
are also highly reproducible, with much lower inter- and pooling of the gadolinium and slow washout in this
intraobserver variability than echocardiography (7.8 region. Using T1-weighted images, delayed enhancement
grams vs 19.2 grams mean weighted interstudy standard (DE) is a hyperintense (bright) appearance of the myo-
deviation) in normal people and patients with LV hyper- cardium where contrast medium has accumulated.
trophy (Grothues et al. 2002). Studies that quantified LV DE-cMRI has been used in human patients as well as
mass in dogs and normal people have found similar low cats with HCM to noninvasively assess the presence of
intraobserver error (0.4% and 4%, respectively) and myocardial fibrosis (Figure 11.23) (Moon et al. 2004;
interobserver error (5.8% and 4%, respectively) (Fieno Choudhury et al. 2002; Wilson et al. 2002). A majority
et al. 2002). Similarly, there was extremely low intraob- of asymptomatic to mildly symptomatic human patients Cardiomyopathies
server and interobserver variability of 2% for cMRI (80%) have evidence of DE of the hypertrophied regions
quantification of left ventricular mass in cats (MacDonald of the LV, with a patchy to multifocal distribution, pre-
et al. 2004). With the higher accuracy and reproducibility dominantly involving the middle third of the LV wall
of cMRI, the number of subjects needed to obtain mean- (Choudhury et al. 2002). Histologic studies have con-
ingful data is dramatically reduced. For example, in one firmed that regions of DE correspond to regions of
study the sample size necessary to demonstrate a 10 gram myocardial fibrosis in people with HCM (Moon et al.
change in LV mass in people with LV hypertrophy was 2004). Given the spatial resolution of contrast cMRI,
15 patients with cMRI versus 152 patients with echocar- replacement fibrosis is the most likely type of fibrosis
diography (Grothues et al. 2002). detected. Detection of diffuse interstitial fibrosis by DE
An unexplored area of research is the use of cMRI to is more limited since the technique is sensitive to regional
evaluate LV variables over time to assess whether phar- differences in gadolinium accumulation. DE is seen in
maceutical interventions cause significant changes in fewer than 50% of patients with dilated cardiomyopathy
patients with cardiomyopathy (Hoffmann et al. 2001). where there is diffuse interstitial fibrosis (McCrohon
cMRI has been proposed as the gold standard for mea- et al. 2003). DE-MRI has also been performed in Bio
suring regression of LV mass in people (Hoffmann et al. 14.6 cardiomyopathic hamsters, which have regions of
2001; Rajappan et al. 2003). Regression of LV mass fol-
lowing aortic valve replacement in patients with aortic
stenosis was evaluated in one study by cMRI and by
echocardiography using M-mode measurements. cMRI
identified a regression of LV mass in all patients 1 year
after valve replacement, while M-mode echocardiogra- RV
phy failed to accurately identify the regression in these
individuals. Using cMRI for LV mass quantification, LV
there was no regression in concentric hypertrophy in 26
asymptomatic Maine coon cats with familial HCM that
were treated with ramipril for 12 months (MacDonald
et al. 2006b). The use of cMRI provides more accurate
assessment of LV mass with less variability in measure-
ments than echocardiography and allows for fewer
patients needed while maintaining a powerful study.
Figure 11.23. Delayed enhancement seen on cardiac magnetic
Assessment of myocardial fibrosis with resonance imaging in a cat with severe hypertrophic cardiomyopathy.
cardiac MRI Approximately 7–10 minutes after administering intravenous
gadolinium, postcontrast cross-sectional images are obtained of
Contrast enhancement cMRI is a noninvasive technique
used to identify myocardial fibrosis or necrosis in various the left ventricle to evaluate for delayed enhancement (DE). There
is a large region of DE (i.e., a bright, hyperintense region) of the
cardiac diseases. A low molecular weight paramagnetic anterior left ventricular free wall, indicating myocardial fibrosis
contrast agent is administered intravenously, distributes (arrow). The DE occurred at the most severely hypertrophied re-
to the myocardium, and is reabsorbed by the capillary bed gion of the left ventricle (LV) in this cat with severe HCM. The
to be excreted by the kidneys. In regions of abnormal hyperintense circle to the right is a reference marker. RV = right
myocardium, the contrast agent accumulates in the extra- ventricle.