Page 215 - Feline Cardiology

220 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

P
T

QRS

Figure 18.5. Respiratory sinus arrhythmia. This cat displayed severely exaggerated inspiratory and expiratory efforts due to near-
complete upper airway obstruction by a very large nasopharyngeal polyp. Note that the rhythm accelerates and decelerates cyclically, a
pattern that was repeated continuously. Blue = expiration; red = inspiration. 25 mm/sec, 1 cm = 1 mV.

centers in the medulla (Rishniw and Bruskiewicz 1996). abnormal impulse originates prior to emergence of the
Arrhythmias et al. 1985) and in the home environment in general occurred prematurely and a true PAC exists. The abnor-
It occurs commonly when cats are asleep (Schechtman
normal heartbeat from the SA node, the impulse has
mal impulse takes control of the atria for that beat and
(Hanås et al. 2009; Ware 2009). As described above, cats
in a clinical setting generally have a dominantly sympa-
with the entire heartbeat therefore occurring sooner
thetic influence on the cardiovascular system, and feline depolarizes them and then conducts to the ventricles,
RSA is therefore very rarely observed in the exam room. than expected: a premature atrial complex. Note that if
Pathologically high elevations in vagal tone, such as the abnormal impulse were to arise from the atrial tissue
those observed with intoxications (e.g., digoxin, organo- later, after the normal heartbeat has already emerged
phosphate), central nervous system disorders, or gastro- from the SA node, then the normal wavefront controls
intestinal disturbances, can cause RSA in cats (Rishniw the atrial and the abnormal impulse fails to conduct; the
and Bruskiewicz 1996; Ware 1992), as can upper airway normal heartbeat carries through to the AV node and a
obstructions that force the cat to create abnormally normal heartbeat occurs instead of a PAC. Thus, the
elevated and decreased intrathoracic pressures on expi- term “premature atrial complex” nicely explains the fea-
ration and inspiration, respectively (see Figure 18.5). tures of this arrhythmia: atrial impulses that are ectopic
These same mechanisms are also responsible for the (originating outside the SA node) trigger a complete
wandering pacemaker, which, when it occurs, often heartbeat and are apparent on ECG if they occur sooner
coexists with RSA. Occasionally, RSA is observed in than a normal heartbeat should (i.e., prematurely;
normal cats in the clinical environment (Rishniw and Figure 18.6). A temporary pause in recording the ECG
Bruskiewicz 1996; Ware 1992). Respiratory sinus may give the false impression of a PAC, and this type of
arrhythmia does not warrant antiarrhythmic treatment, misinterpretation must be avoided (Figure 18.7). The
and no clinical signs are expected due to RSA itself. ECG representation of atrial activity in a PAC is termed
Identification of this arrhythmia in cats simply justifies a P’ wave, and it may be of mildly or markedly different
investigation of possible underlying causes. The high morphology (shape) than the patient’s normal sinus P
degree of vagal tone required to overcome sympathetic waves depending on the distance of the ectopic focus of
influences in the hospital setting likely explains both the origin from the SA node: how different the P’ wave looks
rarity of RSA in the cat, and the observation that the depends on how different the path of atrial depolariza-
nature of the inciting cause is usually apparent on physi- tion was compared to that seen in a normal sinus beat.
cal exam alone—such as severe upper respiratory dis- In cats, PACs are associated with structural atrial
tress or signs of marked CNS dysfunction. abnormalities (Boyden et al. 1984). As a general concept,
atrial lesions may be responsible for one of 3 types of
Premature Atrial Complexes functional disorders, giving rise to PACs: spontaneous
As described above, every normal heartbeat begins as a impulse formation in atrial myocardium where no
wavefront of organized electrical activity that originates impulse should normally begin (“abnormal automatic-
in the SA node and activates the atria in a wavelike ity”), abnormal perpetuation of an existing impulse that
sequence. A premature atrial complex (PAC; synonyms: should normally have been transient (“reentry”), or
atrial premature complex/contraction/depolarization) spontaneous depolarization during repolarization
preempts this rhythm. A PAC begins as a solitary impulse (“afterdepolarizations,” causing “triggered activity”)
originating from atrial tissue outside the SA node. If the (Blömstrom-Lundqvist et al. 2003; Nattel 2002). These