Introduction to electrocardiography and rhythm diagnosis Marc S. Kraus, DVM, Dip ACVIM (Cardiology, Internal Medicine), ECVIM PennVet Diagnosis I. Although a specific diagnosis may be suggested by auscultation and physical examination electrocardiography (ECG) is required for a definitive diagnosis. II. A systematic approach is also helpful for rhythm diagnosis (see Figures I, II, and III) A. Criteria used in assessing arrhythmias include: 1. Is the rate fast or slow (tachycardia or bradycardia)? 2. Is the rhythm regular or irregular? 3. If irregular, is the rate slow, fast, or are there premature beats? 4. Are there P waves and are they of normal morphology (upright in lead II, which suggest the rhythm is sinus in origin)? 5. Is there a P wave for every QRS complex and a QRS complex for every P wave? 6. Are the QRS complexes normal or abnormal in appearance? B. Supraventricular arrhythmias must be differentiated from ventricular arrhythmias (see Table 1). III. P-QRS-T Complex evaluation (see Normal values Table 1) 1. Is the height and with of the P wave normal? 2. How long is the P-R interval? 3. How wide is the QRS complex 4. How tall is the R wave? 5. How long is the Q-T interval? 6. Is there ST depression or elevation?

Normal values of time durations and intervals (seconds) Table I Species P PR(PQ)) QRS QT Dog .04 0.06-0.13 .06 0.250 Cat 0.35-0.04 .05-.09 .04 0.220

Table -2. Features that differentiate supraventricular from ventricular arrhythmias

Electrocardiographic Feature Supraventricular Ventricular Arrhythmia Arrhythmia Size of QRS complex Narrow Wide

Premature complex has similar appearance to a Yes No sinus beat

P waves are associated with the QRS complex Yes No

Fusion beats are present No Yes

Figures I,II, and III represent a systemic approach for rhythm diagnosis

Fig I Rhythm Analysis SVT Fig II Supraventricular Tachycardia (SVT) Normal ventricular Abnormal ventricular conduction conduction SVT with bundle SVT with 3rd branch blocks Irregularly irregular rhythm Regular rhythm AV block

Atrial fibrillation Junctional atrial flutter Atrial tachycardia or All atrial tachycardia or tachycardias

No P waves ± Blocked P waves ± Pre excitation

AV node independent Atrial fibrillation AVnode dependent VT Ectopic atrial tachycardia Orthodrom ic-Atrio- Ventricular Tachycarida (OAVRT)

These algorithms will provide a framework to diagnose the majority or ECG’s

Fig III

Specific ECG criteria (definition and characteristic morphology)) to diagnose the rhythm/conduction abnormality

Sinus Arrhythmia Definition I. Sinus arrhythmia is a physiologic, autonomically mediated cyclical change in sinus rate. The PP intervals are “regularly irregular” due to fluctuations of the autonomic tone that result in phasic changes of the sinus node discharge rate. II. ECG characteristics A. Heart rate is normal to slow (usually < 140 BPM in the dog) B. Variability in PP interval is greater than 10% C. Normal P wave morphology and every P wave is followed by a QRS complex in a 1:1 ratio. D. A wandering pacemaker is often seen in sinus arrhythmia, where the P wave height changes depending on whether the heart rate is slow or fast. E. QRS complexes are normal in appearance

Atrial premature complexes Definition I. Atrial premature complexes (APC) originate in the atria but in a location other than the sinus node. II. ECG characteristics A. QRS morphology looks similar to normal sinus impulse B. The QRS complex of the APC occurs earlier than expected compared to the normal sinus rhythm. C. The P wave morphology may appear different from the normal sinus P wave.

Atrial Fibrillation Definition I. Atrial fibrillation is caused by multiple simultaneously occurring disorganized atrial impulses that bombard the AV node. During atrial fibrillation there is rapid atrial activation and the atrial rate can exceed 500 depolarizations/minute. II. ECG characteristics A. QRS complexes are usually narrow and upright in leads II, III, and aVF. The QRS complex can be wide in appearance if a bundle branch block is present. B. P waves are absent. C. Atrial activity is represented by fibrillatory (f) waves of varying amplitudes. D. Ventricular rhythm is irregular. E. At very rapid rates the rhythm can appear regular.

Atrial Flutter Definition I. Atrial flutter is a form or reentry tachycardia that utilizes the anatomy of the right atrium to sustain a loop of continuous depolarization. II. ECG characteristics A. Rapid regular rhythm; may be irregular B. Saw tooth undulation of the baseline (flutter waves) C. Atrial rate usually > 300 BPM D. Supraventricular appearance of QRS complexes Ectopic Atrial Tachycardia Definition I. Atrial tachycardia (AT) occurs when localized regions in the atria (other than the sinus or AV node) develop the ability to fire rapidly on their own (abnormal automaticity). The arrhythmia becomes clinically significant when the rate is maintained above 180 BPM in the dog and 240 BPM in the cat. Atrial tachycardia is defined as a run of atrial premature beats (³3 APCs). There is paroxysmal (short runs) and sustained atrial tachycardia (incessant atrial tachycardia). II. ECG characteristics A. Ectopic P waves are different in morphology from sinus P waves. B. Differentiation of automatic versus reentrant mechanisms may be determined by the presence of a warm-up or cool-down period at the onset and termination of the arrhythmia. Arrhythmias caused by abnormal automaticity usually show gradual acceleration and deceleration (warm-up and cool-down phenomena). C. Atrial rate is usually > 180 BPM in dogs. D. Usually there are narrow, upright QRS complexes 1. Tachycardia may cause a functional bundle branch block, where the QRS complexes are wide but associated with a P wave. a. In rapid tachycardia, a P wave may be buried in the preceding T wave and not be visable. E. The ventricular rhythm is irregular when there is physiologic second degree atrioventricular block where the atrial rate is so rapid that not every atrial beat is conducted to the ventricles.

Sinus Bradycardia Definition I. Sinus bradycardia is a sinus rhythm in which the sinus node discharge rate is low (< 60 BPM in an awake dog, < 120 BPM cats). II. Sinus bradycardia of 45 - 60 BPM during sleep is normal. Pathologic bradycardia often persists during excitement or exercise. III. ECG characteristics A. Sinus rhythm with upright P waves in the inferior leads (II, III, aVF) , normal upright, narrow QRS complexes unless concurrent conduction disease is present B. Slow sinus discharge rate (dog < 60 BPM, cat < 120 BPM) C. Escape beats may be present from either the atrioventricular node (narrow QRS complex) or the ventricular Purkinje fibers (wide QRS).

Sick Sinus Syndrome Definition I. Sick sinus syndrome (SSS) is a disease process in which the spontaneous sinus node discharge is either slower than normal (primary sinus bradycardia) or intermittently absent (sinus arrest). II. The subsidiary pacemaker tissue (AV node and Purkinje fibers) is usually also abnormal, resulting in inadequate escape beats, such that asystole (pauses) can be > 6- 8 seconds. III. It occurs primarily in small breed dogs such as the miniature schnauzer, American cocker spaniel, West Highland white terrier and dachshund. IV. Doberman pinschers and boxers may have syncope associated with long sinus pauses, suggestive of SSS. V. The underlying cause is unknown, but it has been associated with fibrous replacement of the sinoatrial node.

Diagnosis I. Because of the intermittent nature of the sinus pauses in some cases, a Holter monitor or an event monitor may be necessary to definitively determine the cause of collapse. II. Definitive diagnosis requires documentation of sinus node dysfunction with associated clinical signs of SSS. III. Periods of bradycardia or asystole followed by paroxysms of supraventricular tachycardia are common (bradycardia-tachycardia syndrome).

Atrial Standstill Definition I. Atrial standstill is a lack of electrocardiographic evidence of atrial depolarization (no P waves are present/visible on the ECG). II. Two main reasons exist for atrial standstill. A. Hyperkalemia (urethral obstruction, hypoaldosteronism, reperfusion syndrome in arterial thromboembolic disease) B. Primary atrial muscle disease: neoplasia, atrial myopathy, scapulohumeral muscular dystrophy of English springer spaniels. (Kirk, 1992; Tilley, 1997) III. ECG abnormalities A. No P waves are present. B. The escape rhythm is either junctional (60-80 bpm, narrow QRS) or ventricular (20- 40 bpm, wide QRS) in origin. C. If associated with hyperkalemia, there may be tall T waves and wide and bizarre QRS complexes.

Atrioventricular Conduction Abnormalities Definition I. ECG is required for definitive diagnosis. II. First degree AV block is characterized by a PR interval ≥ 0.13 second, and normal P wave and QRS complexes that occur at a 1:1 ratio. III. Second degree AV block has normal P wave and QRS complexes with intermittent P waves not followed by QRS complexes (see Mobitz I , II, and high grade listed above; IV. Complete AV block is characterized by the following A. The atrial rate and ventricular electrical depolarizations are unrelated to each other, with P waves dissociated from the QRS complexes (AV dissociation). B. The atrial rate is faster than the ventricular rate. C. If the ventricular rate is faster than the atrial rate, AV dissociation is present (not AV block). D. The rhythm is usually regular. E. Escape beats may arise from 2 sites of subsidiary pacemakers: the AV node- bundle of His below the site of AV block, or the ventricular Purkinje fibers. 1. AV nodal escape beats have a narrow upright QRS morphology (supraventricular), and their rate is usually 40-60 BPM in dogs, and 80-120 BPM in cats. 2. Ventricular escape beats have wide, bizarre QRS complexes, and are usually slower (20-40 BPM in the dog). Ventricular Premature Contractions/Ventricular Tachycardia Definition I. Ventricular premature contractions (VPC’s) are ectopic impulses that originate in the ventricles (distal to the His Purkinje system). II. ECG characteristics A. Abnormal wide QRS complexes occurring early (can be negative or positive QRS morphology) B. No associated P wave C. Can have relatively narrow upright QRS complex if ectopic beat originates near the bundle of His. D. In Doberman pinschers, the presence of VPC’s is highly suggestive of occult dilated cardiomyopathy, and an echocardiogram should be performed. Evidence suggests that >50 VPC’s/24 hours or 1 couplet or triplet of VPC’s/24 hours, predicts subsequent development of dilated cardiomyopathy (Calvert, 2000). E. Ventricular tachycardia is defined as a run or more of 3 consecutive VPC’s

Idioventricular Rhythm Definition I. Idioventricular rhythm is a form of VA characterized by a ventricular rate that is slow or comparable to the sinus rate with rates between 60-150 BPM in the dog, and > 100 BPM in cats. II. ECG characteristics A. The ventricular rate usually remains within 10-15 BPM of the sinus rate B. The cardiac rhythm “switches” back and forth between the 2 competing pacemaker sites. C. The QRS complexes are wide and bizarre.

Ventricular Fibrillation Definition I. Ventricular fibrillation (VF) is an irregular, chaotic rhythm in which there is no effective ventricular contraction. II. Ventricular fibrillation is always a terminal rhythm if not successfully defibrillated II. ECG Characteristics A. Chaotic and irregular deflections of varying amplitudes B. No P- waves, QRS complexes, or T waves are observed