Sinus Respiratory Arrhythmia 

Decreased vagal tone during inspiration causes the heart rate to accelerate.  Conversely, vagal tone increases and the heart rate decelearates during expiration.

 This is the most common arrhythmia seen, and it is the normal resting rhythm for dogs.    

Sinus Tachycardia

Sinus Tachycardia can be recognized by the gradual acceleration or deceleration of heart rate.  It is a normal response to stress, pain, excitement, etc...

Atrial Premature Complexes (APCs)

The P-wave of the upcoming beat is superimposed on the T-wave of the preceding beat.  The morphology of the complex appears normal, since the beat is originating in the atria.  However, the timing of the beat indicates that it is not originating from the SA node.  It is, therefore, by definition, an ectopic beat.  Since there is less time for filling between these two beats, a pulse-deficit may be associated with the premature beat.

Paroxysmal Atrial Tachycardia (or Supraventricular tachycardia)

 This rhythm can be differentiated from a sinus tachycardia by its sudden onset and ending (the sinus node will accelerate and decelerate gradually). 

A sinus tachycardia would be expected with exercise, stress, or pain and is a normal rhythm.  Conversely, paroxysmal atrial tachycardia is an abnormal rhythm originating from an ectopic focus in the atria.  The QRS complexes appear normal because they are supraventricular in origin.

Atrial Flutter

Atrial Flutter is rare in small animals.  It is due to the firing of an ectopic focus in the atria, which sets up reentry pathways that rapidly stimulate the AV node.  Only some of these impulses conduct to the ventricles.

Unlike atrial fibrillation, there is an underlying pattern to this rhythm.

Atrial Fibrillation

Atrial Fibrillation is described as an "irregularly irregular" rhythm because it is unpredictable and chaotic with no underlying rhythm.

There are no organized P-waves produced in A-fib.  Rather, F-waves (fibrillation waves) are seen and are caused by multiple coexisting reentrant wavelets that circulate within the atria and produce rapid stimulation of the AV node.  Those depolarizations, which conduct to the ventricles, produce normal QRS complexes since they originated in the atria.  However, the irregular rhythm results in variable filling time and, therefore, variable pulse pressure and cardiac output.

Ventricular Premature Complexes (VPC or PVC)

R-on-T is a phrase used to describe VPCs that are closely coupled to the preceding beat.

VPCs that occur too early to allow sufficient filling of the ventricles will be associated with a pulse deficit.

Paroxysmal Ventricular Tachycardia 

 The impulses are generated so rapidly that the ventricles may not adequately fill between beats.  The ventricular ectopic complexes are so closely coupled in this strip that the R wave occurs at the end of the preceding T-wave.  This close coupling is called R-on-T.  Poor ventricular filling, low blood pressure, and weak to no pulses can be expected; fainting and even sudden death may result.  Note the abrupt onset of wide complex tachycardia.  The QRS complexes are wide because they are originating in the ventricles.   

Accelerated Idioventricular Rhythm

Accelerated Idioventricular rhythms are ventricular ectopic rhythms (faster than a ventricular escape rhythm of 20-40bmp) that depolarize at a rate similar to the sinus rate (100-180bpm).  These rhythms are often seen in hospitilized patients with severe metabolic derangements, but they often do not have underlying primary cardiac disease.  It is commonly observed in dogs following gastric dilatation-volvulus surgery, in dogs with severe trauma or metabolic disease, etc...

Ventricular Flutter & Fibrillation

Ventricular flutter rarely results in a measurable cardiac output due to the rapid rate (and inadequate filling time).  ECG oscillations are often large and coarse and may have a sine wave appearance.  This arrhythmia may degrade to ventricular fibrillation.

Venricular Flutter:

Ventricular fibrillation causes cardiac arrest and is often a terminal event.  Cardiac output is essentially zero due to weak, uncoordinated ventricular contractions.  The ECG shows completely irregular chaotic deflections of varying amplitude, width, and shape.

Ventricular Fibrillation:

Atrioventricular Block (AV Block)

 There are 3 types of AV block:

1. First Degree AV Block

     In first degree AV block, transmission through the AV node is slower than normal.  Thus, the P-wave conducts to a QRS but the PR interval is prolonged.

2. Second Degree AV Block

     With second degree (or incomplete) AV block, some impulses are conducted to the ventricles (P wave followed by a QRS), while others are not (P wave not associated with a QRS). There are two types of second degree AV Block: Type I and Type II

  In Type I second degree AV block (Wenckebach periodicity), there is an underlying pattern to the blocked beats.  The PR interval gradually increases until there is a blocked beat.  This is followed by a sinus beat, after which the PR interval again begins to increase and the cycle repeats itself.  The Wenckebach type block is often considered to be a normal arrhythmia and is a function of high vagal tone. 

            Type II

             In Type II second degree AV block, PR intervals remain constant but occasional P-waves fail to conduct.  This type of block is most often caused by organic disease of the AV nodal fibers.

3. Third Degree AV Block

     With third degree (or complete) AV block, none of the P-waves conduct to the ventricles.  Thus, the atria and ventricles beat independently (complete AV dissociation).

The sinus node determines the rate of P-wave formation, while an underlying ventricular escape rhythm is set by the ventricular pacemaker.  This ventricular escape rhythm is slow (40-50bpm) and regular.

Bundle Branch Block (BBB)

 Normally, the two ventricles depolarize practically simultaneously; however, an intraventricular conduction defect may result from a delay or block in one or more of the conduction pathways below the Bundle of His.  The delay in the process of depolarization results in a change in the configuration of the QRS complex, lengthening the duration of the QRS complex beyond its normal limits.

Right Bundle Branch Block (RBBB)

RBBB is a delay or block of conduction in the right bundle branch, so the right ventricle is stimulated following activation of the left ventricle.  The delayed vectors are directed towards the right, resulting in a widened QRS (often notched) which is negative in leads I, II, III and aVF (the negative electrode is on the right side in these leads).

Left Bundle Branch Block (LBBB)

LBBB is a delay or block in conduction in the left bundle branch, so the left ventricle is stimulated following activation of the right ventricle.  The delayed vectors are directed towards the left body wall, and, therefore, the QRS is widened (often notched) and positive in leads I, II, III, and aVF (the positive electrode is on the left in these leads).

Electrical Alternans

 Electrical Alternans is diagnosed when the P, QRS, or T complexes (or any combination) alter their configurations on every other complex, every third complex, etc...  Most commonly, the QRS complex alone is involved and the amplitude variation occurs with every other complex. 

Electrical Alternans occurs with significant pericardial effusions which allow the heart to swing within the pericardial sac with every beat.