Page 62 - Feline Cardiology

Chapter 7: Echocardiography 57

the echocardiogram. If the rhythm is irregular, measure- to M-mode measurements in order to better identify
ments from several beats should be averaged, typically 5 cats with asymmetric disease (Paige et al. 2009). The
beats occurring at a rate that would be expected to mimic substantial improvement conferred by this technique is
the rate of a healthy cat in the same context (e.g., 160–240 the inclusion of regional abnormalities like focal LV
beats/minute). The E point to septal separation (EPSS) is thickening that are otherwise missed on M-mode
a measurement that is obtained from the M-mode image studies. Measurements should be made on an image that
derived at the level of the mitral valve annulus. The EPSS is of excellent quality. Using a poor quality image pre-
is the distance from the E point of the anterior mitral valve disposes to diagnostic error whether the sonographer Diagnostic Testing
leaflet to the interventricular septum measured at the uses an M-mode or a 2D image. Recall that the leading
valve’s largest excursion in early diastole (Figure 7.4B). edge is the part of the line produce by an acoustic inter-
M-mode studies are traditionally obtained for mea- face (i.e., the interface between blood and tissue), which
surements at the midventricular (see Figure 7.5A), AV is closest to the transducer, whereas the trailing edge is
valve annulus (see Figure 7.5B), and heart base levels the part of the line farthest from the transducer.
(see Figure 7.5C). Although an M-mode study is rou- Historically, the American Society of Echocardiography
tinely obtained at the heart base, the aortic and left atrial recommended measuring structures from leading edge
measurements are optimally obtained using the 2D to leading edge in M-mode and 2D images because this
image at this level because M-mode alignment may border represented the location of the specific interface.
include the left atrial appendage rather than the left Therefore, the leading edge to leading edge measure-
atrial body or may cross the chamber tangentially, ment technique resulted in the most accurate measure-
resulting in an inaccurate measurement (Oyama 2004; ment of a given structure. However, improvements in
Abbott and MacLean 2006). In the short axis, the left image processing have led to echocardiographic
atrium:aorta ratio should be <1.5 during diastole. Left machines capable of substantially improved structure
atrial size is commonly expressed as a ratio of the atrial resolution so that it is now possible to visualize the
and aortic dimensions, a value that is then independent actual tissue-blood interface (Lang et al. 2005).
of body size. Atrial size reflects long-standing ventricular Additionally, in some cats it is difficult to center the
filling pressures and is therefore a good measure of M-mode line between the papillary muscles, resulting in
hemodynamic stress and a critical echocardiographic erroneous wall thickness measurements. LV papillary
variable to measure. If the left atrial size is normal, dia- muscles are larger in cats with hypertrophic cardiomy-
stolic function is likely to be fairly normal suggesting opathy than normal cats and a normal range for papil-
that any concurrently measured LV wall thickening is lary muscle measurements exists (Adin and Diley-Poston
artifactual or disease such as hypertrophic cardiomyopa- 2007). Papillary muscle hypertrophy would be missed
thy is early/mild. Moreover, increased left atrial size has with standard M-mode measurements alone. Although
been associated with poor prognosis in cats affected with there is overlap between normal and diseased groups,
hypertrophic cardiomyopathy (Abbott and MacLean left ventricular papillary muscle measurement may
2006). In this study comparing aortic ratio to left atrial provide additional information for interpretation of the
size using 2D- or M-mode–based measurements, there feline echocardiogram, particularly since papillary
was relatively poor agreement between left atrial dimen- muscle hypertrophy may be the first indicator of disease.
sions obtained from M-mode and 2D studies; however, Left ventricular wall measurements are critical in cats,
results were similar and a ratio greater than 1.5 using where myocardial diseases, particularly hypertrophic
either technique suggested left atrial enlargement cardiomyopathy, are a common cause of heart disease.
(Abbott and MacLean 2006), suggesting the most Hypertrophic cardiomyopathy may result in segmental
important factor is probably consistency of measure- or diffuse concentric ventricular hypertrophy. Less com-
ment technique, rather than the technique chosen. monly, regional thinning of the ventricular wall may
Although the standard view to measure the left ven- occur secondary to chronic infarction or aneurysm.
tricular (LV) chamber and walls is by M-mode at the Diastolic wall measurements must be made at end-
level of the chordae tendineae, left ventricular measure- diastole, when the walls are at their thinnest, to avoid
ments should optimally also be obtained on 2D. 2D overdiagnosis of hypertrophic cardiomyopathy. Diastolic
imaging is superior for assessing regional, asymmetric measurements are made at the beginning of the QRS.
hypertrophy, which may not be obvious on the M-mode The measurement may also be overestimated using
(Wagner et al. 2010). One group has developed a method machines with slow frame rates. Normal echocardio-
using 3 interventricular septal measurements (2 in the graphic values have been reported for the cat by multiple
short axis [left: from 9 to 12 o’clock; right: from 12 to 3 investigators (Table 7.1). A left ventricular wall thickness
o’clock] and 1 in the long axis [midseptum]) in addition of greater than or equal to 0.6 cm is abnormal in a