Arrhythmias can be classified based on ECG analysis based on the heart rate (normal, bradyarrhythmias, tachyarrhythmias);
anatomic origin of the rhythm disturbance (SA, atrial, atrioventricular, or ventricular); or electrophysiologic mechanism
when evident. Keys to recognizing cardiac arrhythmias include an analysis of rate, regularity, patterns, P-QRS relationship,
waveform morphology, and conduction intervals. In terms of a methodological approach to rhythm diagnosis, it is recommended
that one begin as follows: 1) Identify the patient, lead(s), paper speed, calibration signals, and artefacts; 2) Decide if
the rate is slow, normal, or fast for the species; 3) Identify regularity or lack thereof and search for repetitive patterns
in irregular rhythms; 4) Identify P and QRS complexes and the relationship between these waveforms; 5) Scrutinize the morphology
and consistency of the P-waves and the QRS complexes; 6) Consider the conduction intervals across the atria (P-wave duration),
atrioventricular conduction system (P-R interval), ventricles (QRS duration), and overall repolarization time (Q-T interval);
7) Identify the frontal axis as normal, left, or right; 8) Evaluate the QRS morphology for conduction disturbances, obvious
bundle branch or fascicular blocks, and for cardiomegaly pattern(s); 9) Assess the ST-T for repolarization abnormalities;
and 10) interpret the ECG with consideration of the entire clinical and laboratory picture.
Physiologic rhythms during routine exam include normal (regular) sinus rhythm and sinus arrhythmia. Sinus rhythm disorders
are often due to high vagal or sympathetic tone; any patient with sinus bradycardia or tachycardia should be evaluated with
this in mind. Additionally, drugs, anesthetics, temperature, and endocrine status (thyroid or adrenal) can affect sinus node
rate. Dogs with respiratory disease can show pronounced sinus arrhythmia with wandering pacemaker; the short cycles can resemble
premature atrial complexes. Management of sinus rhythm disturbances is focused first on treating any underlying conditions.
Occasionally inappropriate sinus tachycardia is treated with a beta-blocker. Sinus bradycardia can be treated in the hospital
with atropine or glycopyrrolate. Chronic, progressive, sinus node dysfunction is common in miniature Schnauzers, West Highland
white terriers, and cocker spaniels. Insufficient escape activity may result in collapse or syncope ("sick sinus syndrome").
The best long-term therapy for this syndrome is not drugs like anticholinergics or terbutaline but permanent transvenous pacing.
Pacemaker programming is critical for optimal system performance (e.g. VVIR mode) and long-term outcomes are generally excellent.