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Chapter 18: Arrhythmias and Other Electrocardiographic Abnormalities 237

a period of at least 15 minutes if no benefit is observed, form of excessive restriction of conduction to the ven-
and then followed with esmolol or propranolol if 2 tricles, called AV block. AV blocks are defined as delays
doses of lidocaine were ineffective. Refractory VT is the or stoppages of conduction between the atria and the
diagnosis of exclusion if no improvement is observed ventricles.
with this approach, and while magnesium sulfate or Recall that first-degree AV block simply is a delay in AV
additional treatment may be considered in such a situ- conduction. Conduction through the AV node is slower
ation, logistic failure (misdiagnosis of right bundle than normal, but every impulse crosses the node suc-
branch block or right axis deviation as VT [Figure cessfully (Figure 18.18). It may be permanent or tran-
18.16]; IV catheter is no longer the vein; hypokalemia sient and may arise from a structural lesion or simply be
is present rendering lidocaine ineffective; etc.) must functional. The ECG diagnosis is based on normal,
be addressed first and such problems corrected if sinus-appearing QRS complexes and a prolonged PR
present. interval. The clinical manifestations are nil; first-degree
AV block is an ECG finding only and neither produces
Ventricular Flutter, Ventricular Fibrillation, overt clinical signs nor warrants treatment. Its impor-
Pulseless Electrical Activity, and Asystole tance is at best diagnostic, drawing the clinician’s atten-
Each of these four entities represents cardiac arrest, and tion to possible causes of delayed conduction, which
they are discussed in Chapter 5. include AV nodal damage due to heart disease and iat-
rogenic problems including excessive administration of
Atrioventricular Block such cardiac glycosides as digitalis or other antiarrhyth- Arrhythmias
Disorders arising from faulty intracardiac electrical con- mic therapy. First-degree AV block is uncommon in the
duction (as opposed to abnormal spontaneous impulse cat, likely because it often arises from high vagal tone,
formation) are simply referred to as blocks. Blocks are which in turn is rare in this species in the clinical setting.
grouped according to anatomic and functional criteria. First-degree AV block is not known to progress to
Anatomic criteria separate them depending on their second- or third-degree AV block except in cases of drug
level of physical location: SA blocks, AV blocks, bundle toxicity (e.g., digoxin).
branch blocks (BBBs), and fascicular blocks. Functional Second-degree AV blocks involve a true interruption of
criteria characterize blocks according to their severity. AV conduction that occurs intermittently. Therefore, a
First-degree block produces a delay in conduction P wave exists for every QRS complex, but a QRS complex
without any instance of complete block; second-degree does not exist for every P wave. Two important subtypes
block causes complete but intermittent block; and third- of second-degree AV block exist. The first, Mobitz type I
degree block causes complete, sustained block. second-degree AV block, is characterized by a progres-
In a healthy heart, the AV node serves an important sive lengthening of the PR interval until ultimately a P
gatekeeper function that prevents excessive supraven- wave is blocked (P wave without QRS complex), an
tricular electrical impulses from reaching the ventricles entity known as the Wenckebach phenomenon. This
(see the section “Premature Atrial Complexes,” above). gradual lengthening until block typically repeats such
This normal and vital function can fail, either in the that every 3rd, 5th, or any other multiple of P waves is
form of insufficient filtering, which is rare (see blocked after PR interval lengthening. Anatomically,
“Preexcitation,” below), or at the opposite extreme in the Mobitz type I second-degree AV block originates high

QRS
P
T

Figure 18.18. First-degree AV block in a cat. The PR interval (red bar) is 0.12 sec., which is excessively long; the upper limit of normal
for the PR interval in the cat is 0.09 sec. (blue bar). This abnormality was an incidental finding in a 12-year-old male domestic short-
haired cat with unexplained right atrial enlargement and no overt clinical signs. Heart rate = 160 beats/min. 50 mm/sec, 1 cm = 1 mV.