ECG reading session—cardiac arrhythmias (Proceedings)
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.