Page 239 - Feline Cardiology

244 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

conduction system disease exclusively, as in “true” sinus Bypass tracts can often conduct impulses in either direc-
node dysfunction. tion, such that the ventricular impulse conducts retro-
grade through the bypass tract to the atria, and then
Ventricular Preexcitation and Macroreentry again through the AV node in the normal direction,
In preexcitation, the normal impulse originating from initiating an endless loop of conduction that activates
the SA node is split at the end of atrial depolarization, the ventricles with each turn of the loop. This type of
with part of the impulse traveling normally through the self-perpetuating circuit is a macroreentry circuit, and it
AV node and another part of the impulse traveling may produce a potentially very rapid and clinically overt
simultaneously through an abnormal shaft of rapidly (apparent discomfort, exercise intolerance, lethargy,
conductive fibers that links the atria and the ventricles, syncope) tachycardia called orthodromic (the impulse
called an accessory pathway or bypass tract, thus bypass- travels in a normal, “normograde” direction through the
ing the AV node. The result is partial, premature, imme- AV node) AV reentrant tachycardia (OAVRT) (see Figure
diate activation of the ventricles through the bypass 18.27) (Roland and Estrada 2006). There are various
tract, without the benefit of a pause in the AV node; types of macroreentry tachycardias, named according to
hence the term preexcitation. With one major exception the location of the bypass tract fibers. The two most
(see below), the effect of this abnormal pattern of activa- common are the Wolff-Parkinson-White syndrome,
Arrhythmias asynchrony means only a part of the atrial contribution fibers, and the Lown-Ganong-Levine syndrome, anec-
where atria are directly linked to ventricles via the Kent
tion is minimal, because the minor interventricular
to ventricular filling is lost. The ECG (see Figure 18.26,
dotally the most common form on the cat, where the
later in this chapter) demonstrates that the normal delay
through the AV node was preempted by conduction atrioventricular bypass tract consists of the James fibers.
Initial treatment can involve vagal maneuvers that,
through the bypass tract (i.e., the PR interval is reduced, through slowing of AV conduction (i.e., negative drom-
often with the P wave apposed directly against the QRS otropic action), break the cycle of reentry. Beta blockers
complex) and that conduction through the bypass tract such as atenolol also are useful for reducing the
caused early activation of the ventricles (the bypass tract likelihood of recurrent tachycardia, but digoxin is con-
and the normal AV nodal conduction ultimately each traindicated because it shortens the refractory period of
activate their share of the ventricles), resulting in a the bypass tract, facilitating extreme tachycardia.
notched QRS complex. The size and location of the QRS Preexcitation is well-documented in the cat, although its
complex’s notch, the delta wave, depends on the distance prevalence in the feline population is low as evidenced
that separates the bypass tract and the AV node in the by identification of 3 cases in 118 cases of feline HCM
individual’s heart (i.e., on the amount of myocardium (2.5%) (Harpster 1987) and 1 case in 65 cumulative
that the His bundle and the bypass tract can each depo- cases of hyperthyroidism (0.8%) (Peterson et al. 1982;
larize before the impulses collide with each other). These Moïse et al. 1986). At least 29 cases of ventricular preex-
are the ECG findings noted in the vast majority of cats citation have been reported in the cat (Fox and Harpster
with preexcitation (28/29 feline cases; 97%) (Fox and 1999; Paige et al. 2009; Peterson et al. 1982; Rishniw
Harpster 1999; Paige et al. 2009; Peterson et al. 1982; 2000; Harpster 1992; Tilley et al. 1977; Ogburn 1977;
Rishniw 2000; Harpster 1992; Tilley et al. 1977; Ogburn Riesen and Lombard 2005; Roland and Estrada 2006;
1977; Riesen and Lombard 2005; Roland and Estrada Meurs and Miller 1993; Goodwin 1990; Berry and
2006; Meurs and Miller 1993; Goodwin 1990; Berry Lombard 1986; Hill and Tilley 1985; Flecknell et al.
and Lombard 1986; Hill and Tilley 1985; Flecknell et al. 1979). From these, various data are available for some
1979). cases but not others, and the following observations can
When cats with preexcitation are examined, a sinus be compiled: preexcitation is recognized in adult cats
rhythm with the features noted above is usually appar- (median age 4.5 years; range 1–12 years) with no clear
ent on ECG. However, in individuals with preexcitation, gender predominance (44% male, 56% female) and a
a premature atrial complex (PAC) may initiate a type of varied breed distribution (domestic short-haired
reentry cycle that can produce extreme tachycardias. [n = 12], Persian [n = 2], Siamese, domestic long-haired,
Although bypass tracts conduct impulses rapidly, their Himalayan [n = 1 each]). In these reports, ventricular
refractory period typically is longer than that of the AV preexcitation was often an incidental ECG finding
node. Therefore the timing of a PAC may fail to conduct (11/20; 55%) although several cases presented with clin-
through the bypass tract but be able to conduct through ical signs consistent with reciprocating tachycardia
the AV node, depolarizing the ventricles normally. As the (9/20; 45%). In two cases, (one hypovolemia due to GI
impulse completes the depolarization of the ventricles, disease, one hyperthyroidism) preexcitation was noted
the bypass tract has repolarized and is able to conduct. at presentation during severe illness, and supportive care