Page 234 - Feline Cardiology

Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 239

P
P P
P +
T
T
T

QRS
QRS
QRS

A

P P P P Arrhythmias

B
Figure 18.20. Third-degree AV block in 2 cats. (A) A 16-year-old female spayed domestic long-haired cat with compensated hyper-
trophic cardiomyopathy (mild LV thickening) identified echocardiographically based on a heart murmur on routine physical exam. AV
block was an incidental finding. The P-P interval is regular (rate: 230/minute), but the P waves are not all followed by QRS complexes
at a consistent PR interval; this indicates AV dissociation, and specifically, third-degree AV block since the ventricular rate is slower (150
beats/minute) and the ventricular rhythm is regular, as escape rhythms usually are. This rapid escape rate explains the lack of clinical
signs. (B) Wide-complex tachycardia (first 1/3 of tracing) is suddenly blocked in a sustained fashion at the AV node, such that atrial
activity continues at the same rate (180/minute) but no ventricular activity is triggered. There is also failure of emergence of a ventricular
escape mechanism. This 13-year-old male domestic short-haired cat had multiple daily episodes of syncope and underlying hypertrophic
cardiomyopathy. This tracing was obtained from a portable event monitor the cat was wearing at the time of syncope. An episode like
this one occurred later the same day from which the cat could not be revived. 25 mm/sec, 1 cm = 1 mV.

electrical activity is a slow, regular, independent rhythm, usually constant, but no connection exists between the
called an escape rhythm (junctional or ventricular; see two. Despite signifying complete atrioventricular dis-
below) (Figure 18.20a). It is important to recognize the sociation, third-degree AV blocks may or may not
life-saving salvage function of a ventricular escape produce overt clinical signs such as exercise intolerance,
rhythm, because it prevents asystole. Therefore even weakness, or syncope in cats. The occurrence of such
though ventricular escape QRS complexes may be wide signs depends on the ventricular rate. If a cat’s ventricu-
and bizarre, ventricular antiarrhythmic therapy is abso- lar escape rate is sufficiently rapid (>100 beats/minute),
lutely contraindicated. Third-degree AV block with cardiac output may not be appreciably reduced com-
failure of escape mechanisms has been documented in pared to normal. Cats routinely appear to develop ven-
the cat and can be fatal (see Figure 18.20b). tricular escape rhythms that are suitably rapid: in a case
In third-degree AV block, electrical communication series of 21 cats with third-degree AV block, all cats had
between the atria and ventricles is nonexistent (com- a ventricular rate between 100 and 140 beats/minute
plete AV dissociation). Therefore the ECG diagnosis is (median 120 beats/minute) (Kellum and Stepien 2006),
based on the complete absence of P wave conduction (P which may explain the finding that only 3/21 cats (14%)
waves occur regularly and typically at a normal sinus had collapse as the presenting complaint.
rate of 140–260/minute, but are not followed immedi- Third-degree AV block represents 5.8% (10/170) of
ately and consistently by QRS complexes; there is no recorded ECG abnormalities in cats according to one
consistent PR interval) and the presence of a slow ven- retrospective analysis (Côté and Harpster 2009). Three
tricular rhythm, uniform, but wide and bizarre QRS retrospective studies and several case reports and text-
morphology (shape). The P-P interval is usually con- book chapters describe 37 cats with third-degree AV
stant, and the R-R interval (ventricular escape beats) is block and thus provide a useful compilation of clinical