Page 70 - Feline Cardiology

Chapter 7: Echocardiography 65

systemic hypertension when compared to normal cats suggesting age and heart rate should not be confounding
(Koffas et al. 2006; Carlos et al. 2006; Simpson et al. factors when using the technique for diagnostic pur-
2009), and systolic velocities are increased over normal poses in cats with heart disease. Pulmonary venous flow
in hyperthyroid cats (Simpson et al. 2009). However, patterns are a useful adjunctive way to interpret diastolic
fusion of the diastolic waves is common in cats because function, but similar to TDI indexes, this tool is in the
of high heart rates, making interpretation of TDI more early stages of its clinical use. See Table 7.3 for a summary
difficult than in dogs or humans. Also, from multiple of the echocardiographically obtained measurements of
planes, pulsed-wave Doppler TDI results are not always diastolic function from normal feline subjects. Diagnostic Testing
repeatable, which could lead to unreliable results. Finally, Left atrial appendage (LAA) flow velocity has been
there is substantial overlap between the normal and studied in cats with myocardial disease (Schober and
abnormal published feline TDI ranges, so interpretation Maerz 2006). Left atrial enlargement, left atrial func-
of TDI variables can be quite challenging (Simpson et tional abnormalities, left ventricular diastolic dysfunc-
al. 2007). Despite its limitations, this modality can tion, and left-sided congestive heart failure were all
provide useful information, especially when other results associated with decreased LAA flow velocities. Similar to
are ambiguous or unclear, to add to the entire echocar- pulmonary venous flow pattern assessment and TDI,
diographic picture of cardiac function. In general, alter- LAA flow assessment is in the early stages of clinical
ations in TDI indexes suggestive of diastolic dysfunction utility. However, these techniques allow the clinician to
support a greater frequency of reexamination in breed- gain insight into a patient’s diastolic function, an assess-
ing animals and/or caution when administering fluid ment that can be challenging (yet important in the feline
therapy or selecting medications that may cause intra- patient).
vascular volume expansion. However, treatment recom-
mendations should not be made based on tissue Doppler CONTRAST ECHOCARDIOGRAPHY
variables alone.
Myocardial velocity gradients can be calculated from Contrast echocardiography is a simple technique for
basic TDI measurements, and changes in these param- evaluating cardiovascular shunts, which can be very
eters have also been reported with various forms of car- helpful when Doppler echocardiography is not available
diomyopathy (MacDonald et al. 2006; MacDonald et al. or when the results remain ambiguous. The principle is
2007; Koffas et al. 2006). In normal cats, higher tissue to alter the echocardiographic appearance of the blood
velocities are measured at the valve annulus compared pool through the injection of agitated saline (which pro-
to the apex, and at the free wall compared to the septum duces microbubbles), iodinated contrast solution, or
(Chetboul 2005) and this gradient has been validated various other contrast agents (Bonagura 1994). The
(Koffas et al. 2003; Chetboul et al. 2004; Chetboul et al. microbubbles reflect the ultrasound beam, thereby alter-
2006). Interestingly, the myocardial gradient has also ing the acoustic impedance of blood, and are filtered by
been shown to be lower than normal in cats with HCM capillary beds. When agitated saline is injected into a
(Koffas et al. 2008). However, these evaluations of myo- large vein during echocardiographic examination (after
cardial velocity gradients (in contrast to pulsed-wave evacuating excess air from the syringe), the echogenic
TDI, described above) require specialized software, bubbles can be visualized on the echocardiogram enter-
which is less commonly available on basic echocardio- ing the right atrium and ventricle and exiting the pul-
graphic equipment. monary artery. Blood containing these bubbles will
Pulmonary venous flow is another Doppler-derived enter the right atrium from the cranial (if cephalic or
measurement, which yields information regarding jugular injection) or caudal vena cava (if saphenous
cardiac diastolic function. Pulmonary venous flow injection). If a right to left intracardiac shunt is present,
reflects changes in LV compliance and LV filling pressure bubbles can be visualized in the left heart (left atrium if
(Schober et al. 1998). Obtaining a high-quality pulmo- an atrial septal defect or left ventricle if a ventricular
nary venous flow tracing can be difficult and color septal defect). In animals with normal anatomy, the
Doppler is often helpful to optimize transducer angle bubbles will be retained by, and dissolve in, the pulmo-
from the apical 4-chamber view. The normal pulmonary nary capillary bed and will therefore not enter the left
venous flow pattern has been described in cats and heart. A negative contrast effect can be recorded by
results from several studies suggest that the pattern, and noncontrast-containing blood (anechoic) shunting into
LV diastolic function, change throughout a cat’s life a chamber filled with echogenic bubbles when a left to
(Santilli et al. 1998; Disatian et al. 2008). However, right shunt exists.
although age and heart rate were statistically related to Other adjunctive echocardiographic tests are also
pulmonary venous flow changes, the effect was minor, available at tertiary care facilities, though not widely