Page 54 - Small Animal Internal Medicine, 6th Edition

26 PART I Cardiovascular System Disorders

TABLE 2.2
VetBooks.ir Echocardiographic Measurement Guidelines for Cats* IVS D (mm) IVS S (mm) LA (mm) AO (mm)

LVW S (mm)
LVID S (mm)
LVW D (mm)
LVID D (mm)
†
12-18 5-10 ≤5.5 ≤9 ≤5.5 ≤9 7-14 8-11
FS 35%-65%.
EPSS ≤ 4 mm.
AO, Aortic root; EPSS, mitral E-point septal separation; FS, fractional shortening; IVS D, interventricular septum at end diastole; IVS S ,
interventricular septum at end systole; LA, left atrium (systole); LVID D , left ventricular internal diameter at end diastole; LVID S , left ventricular
internal diameter at end systole; LVW D, left ventricular wall at end diastole; LVW S , left ventricular wall at end systole.
*These values are based on the author’s experience and compilation of published studies. Measurements can be higher in large cats.
Ketamine increases heart rate and decreases LVID d. See Suggested Readings for additional references.
† Orientation of M-mode cursor across the LA is variable among animals; maximum LA dimension is best assessed by two-dimensional imaging.

The FS commonly is used to estimate LV systolic function (point F) at the end of rapid ventricular filling. Atrial con-
in dogs and cats. FS is the percent change in LV dimension traction causes the valve to open again (point A). At rapid
from diastole to systole ([LVIDd − LVIDs]/LVIDd × 100). heart rates, the E and A points often merge. The mitral valve
Most normal dogs have an FS between (25%-) 27% and 40 closes (point C) at the onset of ventricular contraction. In
(-47)%; FS in most cats is between 35% and 65%, although normal animals, the mitral E point is close to the interven-
there is some variability. It is important to remember that tricular septum. Increased E point-to-septal separation
this index, like others taken during the cardiac ejection usually is associated with reduced myocardial contractility,
phase, has the important limitation of being dependent on although aortic insufficiency also can cause this. In animals
ventricular loading conditions. For example, reduced LV with dynamic LV outflow obstruction, hemodynamic forces
afterload (as occurs with mitral insufficiency, ventricular during ejection pull the anterior mitral leaflet toward the
septal defect, or peripheral vasodilation) facilitates blood septum, causing so-called systolic anterior motion (SAM).
flow out of the LV during systole, and therefore produces With SAM, some of the normally straight mitral echoes
a smaller end-systolic dimension and greater FS. This (between points C and D) bend toward the septum during
“enhancement” of FS occurs even though intrinsic myocar- systole (see Fig. 8.5, p. 164). Diastolic flutter of the anterior
dial contractility is not increased. The exaggerated FS mitral leaflet sometimes can be seen when an aortic insuf-
common in patients with severe mitral regurgitation creates ficiency jet causes the leaflet to vibrate (Figs. 2.11 and 2.12).
the appearance of increased contractility in those with The diameter of the aortic root and sometimes its motion
normal myocardial function and can mask deteriorating are measured with M-mode. The parallel walls of the aortic
contractile function. Regional wall motion abnormalities root shift rightward (upward on the screen) in systole. During
and arrhythmias also can affect the FS. diastole, one or two aortic valve cusps may be visualized as a
The use of the calculated end-systolic volume index straight line parallel to and centered between the aortic wall
(ESVI) has been suggested as a more accurate way to assess echoes. At the onset of ejection, the cusps separate toward
myocardial contractility in the presence of mitral regurgita- the walls of the aortic root and then quickly come together
2
tion in dogs. This index (ESV/m body surface area) com- again at the end of ejection. The shape of these echoes (two
pares ventricular size after ejection with body size rather cusps) has been described as a train of boxcars or little rect-
than with the dilated end-diastolic ventricular size. LV angular boxes attached together by a string. Aortic diameter
volumes should be estimated from 2-D rather than M-mode is measured at the level of the valve annulus in end diastole.
images. Extrapolation from human studies to dogs suggests The amplitude of posterior-to-anterior motion of the aortic
an ESVI less than 30 mL/m is normal, 30 to 60 mL/m indi- root is often decreased in animals with poor cardiac output.
2
2
cates mild LV systolic dysfunction, 60 to 90 mL/m repre- The LA dimension (caudal to the aortic root) is measured
2
2
sents moderate LV dysfunction, and greater than 90 mL/m at maximal systolic excursion. In normal cats and dogs, the
indicates severe LV dysfunction. Other methods also can be (M-mode) ratio of LA to aortic root diameters is about 1 : 1.
used to assess LV function. However, LA size is underestimated with this M-mode view
Mitral valve motion is evaluated with M-mode also. The because (especially in dogs) the M-mode cursor usually
anterior (septal) leaflet is most prominent; its motion has an transects the LA close to the left auricle, not at its maximal
“M” configuration. The posterior (parietal) leaflet is smaller; dimension. In cats, the M-mode beam is more likely to cross
its motion mirrors the anterior leaflet, appearing as a “W.” the body of the LA, but its orientation can be inconsistent.
Tricuspid valve motion is similar. The mitral valve motion Echo beam placement may be difficult in some animals,
pattern is identified by letters (Fig. 2.10C). Point E occurs at and the pulmonary artery can be inadvertently imaged
maximal opening of the valve during the rapid ventricular instead. Therefore LA size assessment is best done from
filling phase. The valve drifts into a more closed position 2-D images.