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J. VarshneyElectrocardiography in Veterinary

10. Cardiac Arrhythmias

J. P. Varshney1 

Veterinary Medicine, Shri Surat Panjarapole Prerit Nandini Veterinary Hospital, Surat, Gujarat, India


Cardiac arrhythmias, disturbances in rhythm, and rate of the heart are frequently encountered in canine practice. There is hardly any dog that has not experienced arrhythmia in his or her lifetime. Any impulse that generates outside the sinoatrial node (usual pace maker) causes arrhythmias. The abnormal impulses generating outside the usual pace maker (in the SA node) are termed as ectopic impulses. The ectopic impulses may originate from the atrium, junction, or ventricle and are referred to as atrial, junctional, supraventricular, or ventricular ectopic beats on the basis of their seat of origin. When the ectopic beat is occurring earlier than the next expected normal sinus impulse, it is termed as premature beat or premature complex. The ectopic beat occurring late or after the normal sinus impulse is called escape rhythm. The ectopic beat or abnormal impulse may occur singly or in multiples. Occurrence of premature impulses in three or more is generally referred to as episode of tachycardia. Brief bouts of tachycardia are called paroxysmal tachycardia. When bouts of tachycardia are prolonged, it is called sustained tachycardia. Asymptomatic arrhythmias are benign having no clinical significance and require no particular therapeutic attention, but arrhythmias associated with clinical manifestations (symptomatic arrhythmias) are serious enough to threaten the life and need proper differential diagnosis and immediate therapeutic intervention. As has been emphasized earlier, electrocardiogram is an effective tool in differentiating the type of arrhythmias. Surveys done in other countries revealed that the incidence of arrhythmias in dogs varied from 3.17% to 42% in different circumstances. In India large-scale studies on the prevalence of cardiac arrhythmias are lacking. A study at this hospital has revealed the prevalence of cardiac arrhythmias as 3.04% in a population of 20,000 canine cases (Varshney et al. 2013). The arrhythmias were grouped into three categories as sinus arrhythmia, abnormalities of impulse formation, and abnormalities of impulse conduction with a prevalence rate as 1.295%, 1.415%, and 0.33%, respectively (Table 10.1).

Table 10.1

Prevalence of arrhythmias in dogs (Varshney et al. 2013)

Types of arrhythmias

No. of cases of arrhythmias

Prevalence among arrhythmias % (n = 608)

Prevalence of arrhythmias among population of 20,000 new cases

1. Sinus arrhythmias




 Sinus bradycardia



 Sinus tachycardia



 Sinus arrhythmias



 Wandering pace maker



2. Abnormalities of impulse formation




 (a) Atrial arrhythmias



   Atrial flutter



   Atrial fibrillation



   Atrial premature complex



   Atrial tachycardia



   AV junctional premature complex



   AV junctional tachycardia



 (b) Ventricular arrhythmias



   Ventricular flutter



   Ventricular fibrillation



   Ventricular premature complex (VPC)



   Ventricular tachycardia



   Ventricular asystole



3. Abnormalities of impulse conduction




 (a) A-V Block



   First degree



   Second degree



   Third degree



 (b) SA block



 (c) Atrial standstill



 (d) Sick sinus syndrome



Researches in canine cardiology have indicated that an arrhythmia may arise from either cardiac or non-cardiac origin. Causes of arrhythmias of cardiac origin are congenital or hereditary diseases of the heart, cardiomyopathy, congestive heart failure, ischemia, trauma, neoplasm, pericardial effusions, or valvular heart diseases, while non-cardiac causes of arrhythmias include respiratory problems (respiratory arrest, pulmonary edema, pneumonia, brachycephalic airway disease, pulmonary shunting, etc.), hypoxic state (anemia, hypervolemia, shock), neurologic defects (cerebral lesions, spinal cord lesions, increased intracranial pressure, vagosympathetic disorders), gastrointestinal tract diseases (acute gastric dilatation, pancreatitis, peritonitis, volvulus), endocrine abnormalities (hypothyroidism, hyperthyroidism, diabetes mellitus, hypoadrenocorticism, hyperadrenocorticism, hypocalcemia, hypercalcemia), urogenital diseases (renal failure, pyometra, acute prostatitis), and blood and lymphatic abnormalities (anemia, lymphosarcoma, hemangiosarcoma). Arrhythmias are also observed during sedation, anesthesia; administration of sympathomimetic, sympatholytic, parasympathomimetic, or parasympatholytic drugs; electrolyte therapy; cancer therapy; pain; hypothermia; hyperthermia; exercise; or excitement/fear.

Diagnosis of arrhythmia is easy by systematic evaluation of the ailing dog and its electrocardiogram. Use of multiple leads and a long strip is of great assistance in evaluating the wave forms of an arrhythmic dog. A precise diagnosis of arrhythmia is essential to determine its cause and severity, adopting rational therapeutic approach.

10.1 Classification of Cardiac Arrhythmias

As per definition arrhythmias are due to disturbances in cardiac rate/rhythm or both or conduction disturbances. They may be of different origin (atrial, AV junctional, ventricular) or may be due to conduction defect. Keeping these variation in view, arrhythmias can be grouped under various categories. Though arrhythmias can be classified in various ways, no additional information is derived except similarity of a character.

10.1.1 Arrhythmias due to Variation in Heart Rate

Arrhythmias can be categorized on the basis of heart rate (increase or decrease in heart rate without any disturbance of heart rhythm) as sinus bradycardia (heart rate is lower than the normal), or sinus tachycardia (heart rate is higher than the normal). In this category the similar character is the heart rate. Under the category of sinus bradycardia, there are various types of arrhythmias such as sinoatrial standstill (no “P” wave), ventricular escape rhythm, complete heart block, sinus arrest, advanced second-degree heart block, and junctional escape rhythm. Sinus tachycardia is characterized by heart rate higher than the normal limit. Various types of arrhythmias such as atrial tachycardia, atrial fibrillation/flutters, junctional tachycardia, and ventricular tachycardia are in the category of sinus tachycardia.

10.1.2 Arrhythmias due to Rhythm Irregularities

Another way of looking at arrhythmias may be whether the rhythm is normal or not. Disturbance in the heart rhythm without change in heart rate is termed as sinus arrhythmia. Wandering pace maker, atrial premature beat, sinus arrest, junctional premature beat, junctional tachycardia, ventricular premature beat, heart blocks (first and second degree), bundle branch blocks, and Wolff-Parkinson-White syndrome are the arrhythmias under this category.

10.1.3 Arrhythmias due to Variation in Heart Rate as well as Rhythm Irregularities

Arrhythmias such as bradyarrhythmia, sinus arrest, sick sinus syndrome, and second-degree heart block (high grade) are characterized by slow heart rate and irregular rhythm and therefore can be grouped under arrhythmias with slow heart rate with irregular rhythm. On the other hand, atrial or supraventricular premature contractions, paroxysmal atrial or supraventricular tachycardia, atrial flutter/fibrillation, ventricular premature contraction, and paroxysmal ventricular tachycardia are characterized by fast heart rate with irregular rhythm and therefore can be grouped under arrhythmias with fast heart rate with irregular rhythm. These types of arrhythmias have disturbances of both heart rate (increased or decreased heart rate) and rhythm (irregular rhythm).

10.1.4 Arrhythmias due to Abnormal Impulse Generation in the Seat of Origin

Arrhythmias can also be grouped on the basis of abnormalities in impulse generation in the sinoatrial node (SA node), atrioventricular junction (AV junction), or ventricle and are termed as supraventricular, AV junctional, or ventricular arrhythmias, respectively. Sinus arrest, atrial premature complexes, atrial tachycardia, atrial fibrillation, and atrial flutter are under the category of supraventricular arrhythmia. The category of AV junctional arrhythmias includes AV junctional premature complexes, AV junctional tachycardia, and AV junctional escape rhythm. Ventricular arrhythmias arising due to abnormality of impulse formation in ventricle consist of ventricular premature complexes, ventricular tachycardia, ventricular fibrillation, ventricular flutter, ventricular asystole, and ventricular escape rhythm.

10.1.5 Arrhythmias due to Abnormal Impulse Conduction

Abnormal impulse conduction also leads to arrhythmias. In this category impulse is formed in a normal way, but its conduction is affected. Sinoatrial block (SA block), atrial standstill, and atrioventricular block (AV block) fall under this category.

10.1.6 Arrhythmias due to Abnormal Impulse Generation and Conduction

Arrhythmias may also occur when there is disturbance of impulse formation and its conduction. Parasystole and Wolff-Parkinson-White syndrome are the arrhythmias which are associated with the disturbance of impulse formation and its conduction.

10.1.7 Arrhythmias due to Differing Pace Maker Site

Arrhythmias can also be grouped on the basis of their origin, whether they have originated from pace maker in the SA node or other than the SA node. Sinus arrhythmia, sinus arrest, sinoatrial block, wandering pace maker, sinus tachycardia, and sinus bradycardia are due to pace maker disturbance in the SA node and therefore can be grouped as sinoatrial arrhythmias. When the pace maker lies at a site/focus other than SA node, arrhythmias originated from such focus are called ectopic arrhythmias. The focus of ectopic beat may be supraventricular or ventricular and are accordingly termed as supraventricular or ventricular arrhythmias. In supraventricular arrhythmias the foci of ectopic beat lie above the ventricle; therefore arrhythmias such as atrial premature beat, junctional premature beat, atrial fibrillation/flutter, and junctional tachycardia are called supraventricular arrhythmias. When foci of ectopic beat lie in ventricular mass (pace maker is found in the bundle of His, bundle branches, and Purkinje fibers), these arrhythmias are termed as ventricular arrhythmias. Ventricular premature beats, ventricular tachycardia, and ventricular fibrillations/flutters are thus called ventricular arrhythmias.

10.1.8 Conduction Disturbances

Atrioventricular (AV) conduction time is the interval between onset of atrial activation and the onset of ventricular activation. In ECG, it is represented by P-R interval. P-R interval denotes the total travel time taken by the depolarization wave from the SA node to the AV node. P-R intervals in clinically healthy dogs vary from 0.08 to 0.12 s. P-R interval is of small duration in small breed dogs as compared to that of giant breed dogs. There are four types of conduction disturbances, namely, heart blocks or AV blocks, sinoatrial standstill (SA standstill), bundle branch blocks (BBB), and Wolff-Parkinson-White syndrome (WPW syndrome). The heart blocks are further grouped as first-degree, second-degree, and third-degree heart blocks depending on the disturbance or delay in the conduction of supraventricular impulse through the AV node and bundle of His. The bundle branch blocks (BBB) are also further divided into two categories, viz., right bundle branch block (RBBB) and left bundle branch block (LBBB) depending on the site of the bundle branch affected.

10.2 Factors Precipitating Arrhythmias

Arrhythmias are precipitated by various cardiac or non-cardiac factors as detailed below.

10.2.1 Cardiac Factors

Concurrent cardiac abnormalities may precipitate both atrial and ventricular arrhythmias.

  1. (a)

    Atrial Arrhythmias—Atrial arrhythmias are precipitated by many cardiac diseases such as valvular insufficiencies (mitral valve insufficiency, tricuspid valve insufficiency), cardiomyopathies (dilated cardiomyopathy, myocardial fibrosis), congenital malformation, ischemia, intra-atrial catheterization, and cardiac tumors; drugs (anesthetics, digitalis, bronchodilators); and increased sympathetic tone.


  2. (b)

    Ventricular Arrhythmias—Ventricular arrhythmias are precipitated by many cardiac diseases such as cardiomyopathies (Boxer cardiomyopathy, Doberman Pinscher cardiomyopathy), myocarditis, pericardial disease (pericarditis), cardiac tumors, ischema, cardiac trauma/chest trauma, congenital heart diseases, dirofilariasis, degenerative valvular diseases with myocardial fibrosis, ventricular dilation, pace maker wire, and cardiac catheterization and drugs (anesthetics, sympathomimetic, tranquilizers, anticholinergic, and digitalis).


10.2.2 Non-cardiac Factors

There are many factors other than cardiac factors that may precipitate arrhythmias. These factors are listed below for both atrial and ventricular arrhythmias.

  1. (a)

    Atrial Arrhythmias—Atrial arrhythmias are precipitated by many non-cardiac factors such as catecholamine, electrolyte imbalance, acidosis, alkalosis, electric shock, thoracic surgery, severe anemia, thyrotoxicosis, and hypoxia.


  2. (b)

    Ventricular Arrhythmias—Ventricular arrhythmias are also precipitated by many non-cardiac factors such as abnormal potassium level, hypoxia, fever, heat stroke/sun stroke, hypothermia, acidosis, alkalosis, electric shock, pancreatitis, uremia, septicemia, thyrotoxicosis, toxemia, abdominal or chest tumors, lower respiratory tract diseases (lung diseases), gastric dilation and volvulus, anxiety, pain, vagal stimulation, sympathetic stimulation, and viper envenomation.


10.3 Diagnostic Criteria for Arrhythmias

Diagnosis of arrhythmias is based on changes in electrocardiographic features with respect to presence or absence of “P” wave, its morphology, uniformity, and regularity; morphology and pattern of “QRS” complex; and variations in the measurement of P-R and R-R intervals.

Site of the arrhythmia can be differentiated by the configuration of QRS complex and P wave. Normal QRS complex indicates that ventricular depolarization is normal; hence rhythm disturbances are supraventricular (originating above the bundle branches in the the atria, AV node, or bundle of His). Wide QRS complexes preceded by related P waves suggest abnormality of the ventricular conduction pathway (bundle branch block or ventricular enlargement). Wide QRS complex unassociated with P wave indicates that the beat is ventricular in origin. Further differentiation of arrhythmias is based on the presence of P wave and its relation to QRS complex. Sometimes identification of P wave in tachyarrhythmia and low-voltage complexes may become difficult. Differentiation of supraventricular tachycardia becomes impossible as it is difficult to evaluate “P” wave because of increased heart rate. It becomes possible only when heart rate slows down sufficiently to evaluate the presence of P waves. P wave unrelated to QRS complexes suggests junctional or ventricular arrhythmia.

10.4 Electrocardiographic Features of Arrhythmias

Arrhythmias are associated with variations in the electrocardiogram. The electrocardiographic alterations with respect to different types of arrhythmias are illustrated below.

Type of arrhythmias

Electrocardiographic features

Normal sinus rhythm (Fig. 10.1)

Heart rate remains within range.

R-R intervals remain almost the same.

There is a P wave for every QRS complex.

P-R interval is almost constant.

The variations in P-P interval are <0.12 s.

Sinus arrhythmia (Fig. 10.2)

Heart rate remains within range.

Pauses are shorter than twice the normal R-R interval.

There is a P wave for every QRS complex.

P-R interval is almost constant.

There are variations in R-R interval.

Sinus rhythm is more irregular with more P-P variations.

Sinus arrest (Fig. 10.3)

Pauses are equal to or greater than twice the normal R-R interval.

Sinoatrial block (Fig. 10.4)

Pauses are exactly twice or <twice of normal R-R interval.

Wandering pace maker (Fig. 10.5)

There are wide variations in the amplitude of “P” waves.

P-R interval may vary from normal to short in duration.

Sinus tachycardia (Fig. 10.6)

There is marked increase in heart rate as compared to normal heart rate.

Heart rhythm is regular.

R-R intervals are normal or may vary slightly.

There is “P” wave for every QRS complex.

P-R interval is almost constant.

Severe tachycardia may be associated with P wave hidden in preceding T wave.

Configuration of both “P” and “QRS” is normal.

Sinus bradycardia (Fig. 10.7)

There is marked decrease in heart rate as compared to normal heart rate.

Heart rhythm is almost regular.

There is a “P” wave for every QRS.

P-R interval is prolonged.

“P” wave amplitude may vary (wandering pace maker).

All “QRS” complexes are alike.

Supraventricular premature complex

Heart rate may be normal.

Impulses originate above the AV node (either in the atrium or the AV junction).

Rhythm is irregular and is broken by a premature beat followed by pause.

P-R interval is either slightly shorter or longer.

There is a P wave for every QRS complex.

P wave of premature beat may be hidden in preceding “T” wave.

Premature “P” may be altered.

In case of junctional (nodal) premature beat, “P” is inverted, i.e., negative.

In case of atrial premature beat, “P” wave is positive but different from normal P.

Atrial premature contractions (Figs. 10.8 and 10.9)

Configuration of premature (P′ wave) P′ is different.

QRS-T complex is normal for normal P wave.

QRS-T complex is absent for premature (P′) wave.

P-R interval of APC may be prolonged with altered morphology of QRS.

Atrial tachycardia or paroxysmal atrial tachycardia (Figs. 10.10 and 10.11)

Tachycardia may be intermittent or continuous.

Tachycardia may be regular (R-R interval same).

Premature (P′) wave is positive in lead II.

There is a P wave for every QRS in a normal complex.

Configuration of P′ wave is different.

P′ wave may be hidden in preceding T wave.

P′ wave during tachycardia is positive but different from P wave when heartbeat is normal.

Atrial fibrillation (Fig. 10.12)

Heartbeat is exceeding the normal limit.

Heart rhythm is irregular but may not be apparently visible when heart rate is rapid.

There are no P waves or P-R intervals.

P wave is replaced by fine baseline undulations (“f” wave).

QRS is normal without P wave.

Atrial flutter (Fig. 10.13)

Heart rate is normal or rapid.

Rhythm is regular or irregular.

There are no P-R intervals.

Ventricles respond in a periodic manner (2:1, 3:1, or 4:1).

Normal P wave is replaced by coarse sawtooth oscillations.

Sino atrial standstill (Fig. 10.14)

Sometimes heart rate is low.

Rhythm is regular or irregular.

P wave is absent.

QRS is without P wave.

There are no P-R intervals.

QRS is normal if it arises near the bundle of His or bizarre if it arises in ventricular muscle mass.

Junctional (nodal) escape rhythm (Fig. 10.15)

Hear rate is normal or slow.

Rhythm is irregular with long pauses in isolated escape beat.

Rhythm is regular when junctional rhythm persists.

There is a P wave for every QRS complex.

P wave occurs before, during, or just after QRS complex.

P wave is negative.

P-R interval is slightly shorter when present.

QRS complexes are normal.

AV junctional tachycardia (Figs. 10.16 and 10.17)

Hear rate is normal or rapid.

Rhythm is usually normal.

Tachycardia may be in bursts.

There is a P wave for every QRS complex.

P wave may be hidden in preceding T wave.

P-R interval may be constant.

P waves are negative, while QRS complexes are normal.

AV junctional beats occurring in high numbers (more than 60 bpm) are termed as AV junctional tachycardia.

Ventricular premature beat (Fig. 10.18)

Heart rate is generally normal.

Normal rhythm is broken by premature beats followed by pause.

There is no P wave for premature beats.

QRS is related to P wave in normal beats.

Abnormal beats do not have consistent P-R interval.

QRS complex of premature beat is bizarre.

If there is one VPC, it is called single.

Two VPCs in succession are called pair.

Three VPCs in succession are called run.

If all VPCs are similar, they are from a unifocal origin.

When VPCs vary in conformation, they are of multifocal origin.

If VPC is alternating (i.e., one beat sinus and other VPC), it is called ventricular bigeminy.

When two beats are sinus and third beat is VPC or one beat is sinus and two beats are VPC, it is called trigeminy.

When major deflection of VPC is negative in lead II, the focus of ectopic beat is considered in the left ventricle.

When major deflection of VPC is positive in lead II, the focus of ectopic beat is considered in the right ventricle.

Ventricular fibrillations/flutters (Figs. 10.19 and 10.20)

There is no coordinated heartbeat.

Specific P waves and QRS complexes are absent.

There is a series of baseline undulations.

Coarse undulations are called flutters.

Fine undulations are called fibrillations.

Ventricular escape beat (Fig. 10.21)

Heart rate is within range or slow.

Heart rhythm is irregular when there is isolated ventricular escape beat.

Idioventricular rhythm is regular.

P wave may or may not occur.

There is no P wave for escape beats.

P wave may be normal but QRS complex is bizarre.

R-on-T phenomenon (Fig. 10.22)

Reported first in humans by Smirk (1949).

A VPC is occurring on T.

It has been reported in dogs also (Engel et al. 1978).

There is a superimposition of an ectopic beat (R) on

T wave of preceding beat.

Ventricular tachycardia (Fig. 10.23)

Four or more VPCs in a row are called ventricular tachycardia.

Enhanced ventricular rhythm (60–100 bpm).

HR is usually slow than sinus tachycardia.

Ventricular asystole (Figs. 10.24 and 10.25)

There is no ventricular rhythm (QRS).

Configuration of P is normal with severe third-degree AV block.

No atrial and ventricular rhythm.

Atrioventricular heart block (AV blocks)

 First-degree AV blocks (Fig. 10.26)

Heart rate is usually normal.

Heart rhythm is sinus.

There is a P wave for every QRS complex.

P-R interval is consistent but increased (more than 0.13 s).

P wave and QRS complex is normal.

 Second-degree AV blocks

Heart rate is normal or slow.

Rhythm is broken by absence of one or several QRS complexes.

There is a P wave for every QRS complexes, but some P waves are without QRS complex.

Normal complexes have consistent P-R interval.

Normal P wave and QRS complexes are of normal configuration.

Second-degree AV blocks are of two types (Mobitz types I and II).

 Second-degree AV blocks (Mobitz type I) (Fig. 10.27)

There is a progressive prolongation of the P-R interval until a non-conducted P′ wave.

 Second-degree AV blocks (Mobitz type II) (Fig. 10.28)

Atrial rate is higher than ventricular rate.

Rhythm is irregular owing to absence of one or more QRS complexes.

P wave is usually normal.

P-R interval is constant and may be normal or prolonged.

 Third-degree AV blocks (Fig. 10.29)

Ventricular heart rate is slow.

Heart rhythm is regular.

There are more P waves than QRS complexes.

P-R interval is varying.

There is no consistent relationship between atrial and ventricular beats.

Configuration of P waves is normal.

QRS complexes near normal or bizarre.

Sick sinus syndrome (Fig. 10.30)

It is due to abnormalities of SA node.

It is also called bradycardia-tachycardia syndrome.

Tachycardia and bradycardia are alternating.

T-on-P phenomenon

There is sinus tachycardia.

QT interval is prolonged (mainly due to ST segment).

T is closely followed by next P or P is buried in the downstroke of preceding T.

Ventricular preexcitation

Heart rate is usually normal.

Rhythm may be sinus or arrhythmic.

There is a P wave for every QRS complex.

P-R interval is constant but short.

P wave is normal.

QRS complexes may be wide and aberrant.

Upstroke of the R wave (delta wave) is slurred (Fig. 10.31).

Wolff-Parkinson-White syndrome

Heart rate is extremely high.

P wave is unrecognizable.

QRS may be normal, wide with delta wave.

Lown-Ganong-Levine preexcitation

P-Q interval is shortened.

QRS complex is normal.

Atrioventricular accessory pathway arrhythmia

It is an uncommon but fatal arrhythmia.

Labrador and Golden Retrievers are predisposed.

Electrical impulse travels in reverse direction.

Congestive heart failure is due to tachycardia.

Heart rate is very high exceeding 240.

When rhythm is sinus, ECG is normal.

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Jul 17, 2021 | Posted by in INTERNAL MEDICINE | Comments Off on Arrhythmias
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