Page 70 - Basic Monitoring in Canine and Feline Emergency Patients

Basic algorithm for differentiating to a situation where the AV node is able to conduct
tachyarrhythmias some impulses but blocks others; this results in a
VetBooks.ir In an emergency setting, tachyarrhythmias are the situation where some P waves are conducted nor-
mally (resulting in P–QRS–T complexes), but other
most common rhythms to cause hemodynamic com-
promise and require urgent treatment. The following P waves are not conducted (see Fig. 3.15). Third-
algorithm can help to differentiate the four common degree AV block occurs when the AV node is inca-
tachycardic rhythms (sinus tachycardia, AF, VT, and pable of conducting any electrical impulses; all
SVT) using ECG findings (see Fig. 3.14). P waves are blocked at the level of the AV node,
and the patient is sustained by a junctional or ven-
1. Is the tachycardia regular (all R–R intervals the tricular escape rhythm (see Fig. 3.16). Third-degree
same) or irregular (variation in R–R interval)? As AV block necessarily involves AV dissociation
a rule, tachycardias are perfectly regular EXCEPT because there is no relationship between P waves
for AF. Exceptions include polymorphic ventricular and QRS complexes.
tachycardia or paroxysmal SVT or VT alternating There are two potential causes of AV block: (i)
with sinus rhythm. fibrosis or other structural disease of the AV node
2. Is QRS complex morphology ‘normal’? QRS itself; or (ii) increased vagal tone causing physio-
complexes in VT are wide and bizarre. In all other logic delay AV nodal conduction. In general, low-
tachycardias, the impulse originates above the grade AV block (1st degree and low-grade 2nd
ventricles, so QRS complexes should be identical degree, where over half of P waves are conducted)
to normal sinus complexes (narrow and upright is caused by high vagal tone. In contrast, high-grade
in lead II, unless there is a concurrent bundle AV block (3rd degree and high-grade 2nd degree,
branch block or other intraventricular conduction where less than half of P waves are conducted) is
abnormality). caused by structural disease of the AV node. An
3. What is the morphology and timing of the P atropine response test (administration of atropine
waves? In ST, P waves of normal morphology occur subcutaneously, with repeat ECG performed 20–30
consistently before every QRS complex at a normal minutes later; see Table 3.6) can differentiate
P–R interval. In AF, P waves are absent, and instead whether a bradyarrhythmia such as AV block is
baseline undulation or f waves may occur. In VT, caused by high vagal tone versus structural disease
normal P waves may be variably seen, but have no of the conduction system. A vagally mediated brad-
consistent relationship to QRS complexes. In SVT, yarrhythmia will be atropine-responsive, meaning
atypical P′ waves can occur before, during, or after that the resulting rhythm will be normal sinus
the QRS; however, P′ morphology will be consistent rhythm or sinus tachycardia at a rate of 140–
throughout the tachycardia. 160 bpm with normal P–R intervals and no non-
conducted P waves. In contrast, AV block caused by
structural AV nodal disease will be non-atropine-
Common bradyarrhythmias in small animals responsive, meaning that administration of atro-
pine will not change the rate, rhythm, or degree of
Atrioventricular block
AV block.
AV block is not truly a distinct cardiac ‘rhythm,’ Low-grade or atropine-responsive AV block
but instead describes ECG manifestations of typically does not require any directed treatment;
delayed or interrupted AV nodal conduction that similar to sinus bradycardia, this rhythm is either
usually occur within the context of an underlying a manifestation of either increased vagal tone (see
sinus rhythm. That is, the SA node depolarizes and Table 3.2) or drug administration (especially seda-
the impulse is conducted through the atria to cause tives such as opioids or α2-agonists). In contrast,
a P wave as usual, but the impulse is either delayed high-grade AV block (which is generally non-
or completely blocked at the level of the AV node. atropine- responsive) is a medical emergency in
st
AV block is classically categorized by ‘degrees’ (1 -, dogs. Dogs with high-grade AV block are at high
2nd-, and 3rd-degree AV block). First-degree AV risk for their ventricular escape rhythm degenerat-
block is essentially a sinus rhythm characterized by ing into ventricular fibrillation, causing sudden
a prolonged P–R interval; each P wave is still even- cardiac death; permanent pacemaker implanta-
tually conducted through the AV node and results tion (transvenous or epicardial) is indicated as
in a QRS complex. Second-degree AV block refers soon as feasible.

62 J.L. Ward