Page 225 - Feline Cardiology

230 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

signalment information rather is confined to managing to be audible, and only the pause that follows the PVC
the rest of the case. is heard. Thus, an important physical exam differential
The history and physical examination confirm that diagnosis for PVCs is second-degree AV block, and any
premature ventricular complexes by definition create instance of a “dropped beat” during a cat’s physical
a premature heartbeat, which is audible as such on examination warrants an ECG to differentiate between
cardiac auscultation. The arrhythmia also may be pal- the two.
pable on the chest surface (apex beat). The exact sound Other historical and physical examination findings in
of the premature heartbeat is often softer, and only 1 cats with PVCs are nonspecific and generally no differ-
heart sound for the PVC may be heard, rather than the ent from the findings of patients with the same underly-
normal 2. The rhythm may be regularly irregular, if the ing disorder but no PVCs. That is, PVCs alone do not
premature beats are occurring in a repetitive sequence produce telltale historical or physical examination
(e.g., every 4th heartbeat is a PVC), or irregularly irregu- abnormalities outside an abnormal auscultation and
lar, if the PVCs occur with no discernable pattern of pulse, unless they occur in sustained fashion (see the
repetition, as they often do when more than one focus section “Ventricular Tachycardia,” below).
in the ventricles is generating PVCs (polymorphic or The confirmatory diagnostic test of choice in a patient
multiform PVCs are seen on the ECG, meaning PVCs of suspected of having PVCs is an ECG. There are three
Arrhythmias beats, pairs, or various combinations of premature and occurrence, 2) QRS complex of different morphology
electrocardiographic features of PVCs: 1) premature
different shapes). PVCs may consist of single premature
normal beats or other arrhythmias such as PACs. An
than sinus derived beats, and 3) T wave of different
alternating sequence of 1 PVC following 1 sinus beat,
repetitively, is referred to as ventricular bigeminy, and an morphology (see Figures 18.12, 18.13). Premature
occurrence refers to a shorter-than-normal coupling
alternating sequence of 1 PVC following 2 sinus beats or interval: the PVC occurs sooner than a normal sinus
2 PVCs following 1 sinus beat is known as ventricular QRS would have occurred for that beat. For a PVC to
trigeminy. When this type of regularity is noted in an occur, spontaneous firing within the ventricles takes
arrhythmia on physical examination initially, correla- control of ventricular depolarization for that beat,
tion to respiration should be sought (as would be meaning the normal depolarization from the SA node
expected with respiratory sinus arrhythmia), but if no courses normally through the atria (and a P wave is
such correlation is clear, or if the heart rate is elevated generated) but the impulse is denied access to the ven-
regardless (since respiratory sinus arrhythmia is vagally tricles because they are either still in the midst of the
mediated and unlikely to occur at a rate >160 beats/ PVC’s depolarization, or refractory while repolarizing
minute in the cat), then an ECG is indicated to identify post-PVC. Thus, a P wave exists for a PVC but is not
the exact nature of the rhythm. If a PVC occurs with a associated with the PVC: the normal PR interval is not
near-normal coupling interval (i.e., is barely premature), seen between the PVC and a preceding P wave. Rather,
the PVC may be missed on physical examination because P waves are often hidden by the PVC (and not seen), or
the heart rhythm may seem to be regular and pulse they can be identified adjacent to the PVC but separated
strength, maintained by adequate ventricular diastolic from it by a shorter distance than the normal PR interval.
filling, is essentially normal. The more prematurely a A QRS complex of different morphology is expected in
PVC occurs, the more likely it will be perceived as such, a PVC because the pattern of ventricular depolarization
especially when diastolic filling is cut short so that the spreads in a different orientation if it originates from a
stroke volume ejected during the PVC fails to generate focus within the ventricles than if it spreads from the
a palpable pulse. The lack of a single pulse when a pre- His bundle during a normal sinus beat. The electrocar-
mature heartbeat is audible on auscultation is referred diograph “perceives” the overall pattern of electrical acti-
to as a pulse deficit, a term that should not be confused vation of the ventricles as being different from normal
with a hypotension-induced pulse quality that is consis- and plots a QRS of different shape accordingly. The QRS
tently poor or simply not palpable. These pulse qualities complex of a PVC is always wider than a normal sinus
are termed weak or absent pulses, respectively. QRS complex because myocyte-to-myocyte electrical
In cats, PVCs often occur so prematurely, and the transmission is more time-consuming than the normal
normal rhythm (sinus tachycardia) resumes so quickly, rapid conduction through the ventricles’ His-Purkinje
that the most common auscultatory finding is not of the system. Finally, since the wave of ventricular repolariza-
premature beat itself but of a missing or “dropped” beat. tion follows exactly along the same course as the wave
That is, the PVC occurs but is not heard because heart of depolarization that preceded it, the T wave of a PVC
sounds that were generated by the PVC blend in with is of a different morphology compared to a sinus T wave.
the previous beat’s heart sounds or simply are too soft The degree of abnormality of these morphologies varies