Page 69 - Basic Monitoring in Canine and Feline Emergency Patients

to accessory pathways. Isolated SVPCs may not Once the wave of depolarization reaches the AV
require treatment, particularly if overall average node, conduction to the ventricles occurs normally
VetBooks.ir heart rate remains relatively normal. through the specialized conduction system (as out-
lined in Fig. 3.1), unless concurrent conduction
Atrial fibrillation block is present. In dogs and cats, AF is typically a
permanent (rather than intermittent) rhythm,
Atrial fibrillation (AF), as distinguished from other meaning that patients do not alternate between AF
supraventricular tachyarrhythmias, is caused by mul- and normal sinus rhythm.
tiple disorganized wavefronts of electrical activation Since dogs and cats with AF usually have signifi-
within the atria. This chaotic electrical activity pre- cant cardiac disease, diagnosis and treatment of the
vents organized atrial depolarization and contraction underlying structural heart disease (including conges-
and bombards the AV node with rapid stimulation, tive heart failure, if present) is a priority. For directed
leading to an irregular tachycardia. As a rule, AF treatment of AF, rate control (decreasing ventricular
occurs in patients with large atria. Dogs and cats response rate) is accomplished using drugs that slow
with AF typically have significant structural heart AV nodal conduction. Options include calcium chan-
disease causing severe left atrial enlargement. Large nel blockers (diltiazem), β-blockers (atenolol), or
animals and some giant breed dogs can develop AF digoxin (a positive inotrope with parasympathomi-
in the absence of structural heart disease. metic properties; see Table 3.6). For patients with
AF is readily recognized on ECG as an irregular severe structural heart disease or CHF, β-blockers are
tachycardia (see Fig. 3.13). Since the irregularity in generally avoided due to their negative inotropic
rhythm can be more difficult to discern at higher effects. Diltiazem and digoxin are often used in com-
heart rates, use of calipers or a ruler can facilitate bination, and the combination of both drugs been
comparison of R–R intervals. Normal P waves are shown to offer superior long-term rate control in AF
not present. Variable baseline undulations can be compared to either drug alone. In patients with AF
seen, which depending on their level of organiza- and CHF, the initial goal is to reduce heart rate to
tion may appear as irregular fibrillation waves approximately 160 bpm in the hospital (a rate that
(‘f-waves’). In atrial flutter, which can be conceptu- maximizes cardiac output). For chronic manage-
alized as a variant of AF, the baseline appears as ment, the goal is to reduce heart rate to more physi-
rapidly oscillating positive and negative deflections ologic rates (~100–110 bpm at home). Rhythm
(‘sawtooth’ waves). The QRS complexes in AF are control (electrical cardioversion to normal sinus
the same as sinus complexes for that patient, since rhythm) can be considered for patients with ‘lone’ AF
the abnormal impulses in AF originate in the atria. (no evidence of structural heart disease).

Fig. 3.13. Lead II ECG (25 mm/s, 10 mm/mV) showing atrial fibrillation. The heart rate is approximately 160 bpm.
The rhythm is irregularly irregular (beat-to-beat variation in R–R interval that does not follow a clear pattern). QRS
complexes are normal (narrow and upright in lead II). There is baseline undulation with no clear P waves visible.
T waves are visible as positive deflections at a consistent interval after each QRS complex, and should not be
confused with P waves.

Electrocardiography 61