Page 237 - Feline Cardiology

242 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

I II III aVR aVL aVF V2

Figure 18.22. Left anterior fascicular (LAF) block pattern in a 13-year-old male domestic short-haired cat with mild symmetrical left
ventricular concentric hypertrophy. The marked left axis shift seen here (−75°) suggests LAF block, which has been associated with hy-
pertrophic cardiomyopathy in cats. Sinus tachycardia; heart rate = 230 beats/minute. 25 mm/sec, 1 cm = 1 mV.

diagnosis of BBBs is based on the abnormal shape of the clinically, and indeed this ECG finding is rather used
QRS complexes, which become widened due to the for increasing the index of suspicion of underlying car-
Arrhythmias considered arrhythmias because they do not alter the leads I and aVL, and a deep S wave in leads II, III, and
diomyopathy. LAF block produces a tall R wave in
desynchronization of the two ventricles. BBBs cannot be
aVF, and therefore a left-axis deviation (Figure 18.22).
rhythm of the heartbeat; therefore, the ECG diagnosis
should be stated as “rhythm” (e.g., normal sinus rhythm)
thy, notably HCM, is a histologically proven but clini-
“with [right or left] bundle branch block” (or bundle The association between LAF block and cardiomyopa-
branch block pattern). In cats with BBB, the duration of cally ambiguous one. Histologic observation of fibrosis
the QRS complexes is greater than 0.04 second, and the involving especially the left parts of the His-Purkinje
polarity is positive in lead II for left BBBs and negative system in cats with HCM supports the link between
in lead II for RBB blocks. If BBBs occur during sinus HCM and LAF block (Kaneshige et al. 2006); a survey
rhythm, the ECG diagnosis is straightforward, because of 63 cardiomyopathic feline hearts revealed degenera-
other than the abnormal appearance of the QRS com- tion, fibrosis, osseous metaplasia, and other lesions in 58
plexes, the P-QRS-T sequence throughout the ECG is (92%) of cases, with 54 (86%) of left bundles affected
normal: a P wave occurs before each QRS and the PR compared to only 20 (32%) of right bundles (Liu et al.
interval is fixed and normal. Still, care must be taken to 1975).
properly identify the rhythm as normal sinus rhythm It should be noted that an important artifact could
and avoid misdiagnosis of VT based on wide, bizarre mimic LAF block and it is not routinely addressed in
QRS complexes alone. If the block occurs concurrently retrospective studies of ECG in cats. Simply altering
with a nonsinus rhythm, such as AF, establishing a diag- the position of a cat’s limbs may deviate the mean
nosis in BBB can be much more challenging. Important electrical axis in a way that could mimic LAF block
characteristics that allow differentiation are that AF + on the ECG. Therefore, in some cases, LAF block
BBB produce an irregularly irregular rhythm and the could be misdiagnosed (see Chapter 9). From a
heart may slow with application of a vagal maneuver, practical standpoint, LAF block may be addressed as
whereas VT tends to be regular (constant R-R interval) follows: if good ECG technique was used when obtain-
when monomorphic (when the wide, bizarre QRS com- ing the tracing and the tracing is consistent with LAF
plexes all look alike) and VT tends to not respond to block, then further investigation is warranted (e.g.,
vagal maneuvers. echocardiogram). If the patient’s position at the time
the ECG was done is not known or was known to
Left Anterior Fascicular (LAF) Block be incorrect, then ECG findings consistent with LAF
Cats with heart disease, especially cardiomyopathy, are block should be disregarded until the ECG can be
classically described as being prone to developing block repeated.
in a subdivision of the LBB known as the left anterior The causes of BBB are many, because BBBs may
fascicle (LAF). The LAF, also called the left anterior be due to a variety of pathologic changes, including
hemibundle or left anterior arborization, is described as concentric hypertrophy (as seen in hypertrophic cardio-
covering the dorsalmost part of the left ventricle, but myopathy, above), dilation (as seen in dilated cardiomy-
such anatomic distribution is extrapolated from humans opathy), and inflammation (endocarditis, traumatic
(where it is highly variable). Blockage of the LAF is not myocarditis). In many feline cases, RBB block is often
appreciably harmful: no hemodynamic impact is noted a completely normal, unnecessarily worrisome ECG