Stephen J. Ettinger, DVM
Arrhythmias discussed will include the bradyarrhythmias, the tachyarrhythmias and the bradycardia-tachycardia complex. Of these dysrhythmias the classification will be further broken down into the supraventricular disturbances and those arising from below the A-V Node in the ventricle.
Clinical signs are anxiety, palpitations, faintness, dizziness, light headedness, syncope, fatigue, exertional intolerance, shortness of breath, seizures and congestive heart failure in human beings. Most of the same signs are present in dogs and cats although some of the subjective signs are more difficult but not always impossible to identify.
The signs, associations, and therapeutic principles will be presented for each of the arrhythmia groups mentioned below. The abnormal conduction patterns are identified as follows in an effort to summarize and have a unanimity of definition while discussing each of the syndromes. Slides of characteristic EKG's will be shown. Space in this manuscript limits this paper to definitions only and a description of the currently available therapeutic modalities for the treatment of arrhythmias in dogs and cats.
Supraventricular Premature Beats are characterized by premature P waves, P waves that differ from normal in both size and or configuration. The ventricular activation is usually normal but aberrancy is possible. The P-R interval is usually longer or shorter than normal. Resetting with or without pause is most likely to be present. Occasionally these beats occur so early that blocking occurs and this is referred to as a blocked supraventricular premature beat.
Ventricular Premature Beats (VPC) have P waves that occur without interruption and the P waves are often obscured since the ventricular complex occurs early and overshadows that cycle. The QRS complexes are usually but not always aberrant but are always premature. Compensatory pause usually characterizes this beat and occasionally true interpolated beats do occur.
Sinus Tachycardia is associated with an increased heart rate, normal complexes and tall P waves. There may be T and P wave superimposition. Vagal maneuvers slow but normally do not break the rhythm.
Atrial Tachycardia is either paroxysmal or continuous in nature. The R-R interval is usually perfectly regular and the P waves are likely to be different from the sinus P waves. The abnormal P wave may be buried in the previous T wave due to an increased rate. Vagal maneuvers break or cause no change in the rhythm and the rate may be 1:1 or there may be variable degrees of block present.
Pre-Excitation Syndromes usually are indicated by short P-R intervals, a delta wave in the initial portion of the QRS complex and QRS complexes that are normal to aberrant and prolonged. During the attack which may be continuous or paroxysmal the heart rate is often significantly increased.
Ventricular Tachycardia is a form of A-V dissociation and is characterized by a rapid, regular or irregular QRS rhythm. When persistent and from one focus the R-R intervals are quite constant. There are regular uninterrupted atrial beats and the presence of capture and fusion are pathognomonic features of this rhythm. The rhythm is described as A-V Dissociation when the ventricular rate is under 100 beats per minute and V. Tachycardia when over 100 BPM.
Atrial Fibrillation is a serious arrhythmia associated with a lack of P waves, irregular, undulating base line and an irregular ventricular rate. The QRS complexes are normal to slightly aberrant. Usually the rate in the untreated atrial fibrillation animal is very rapid and is associated with a marked pulse deficit clinically.
Ventricular Fibrillation is a terminal rhythm recognized by low voltage irregular undulations of the base line with a lack of well defined QRS complexes on the EKG. This is equivalent to cardiac arrest physiologically. There are nonexistent heart sounds, pulses and blood pressure.
Sinus Bradycardia is a slow rhythm often seen in normal athletic dogs. The rhythm may include either sinus rhythm or sinus arrhythmia. The heart rate by definition is under 60 beats per minute. It usually is increased with exercise or atropine injection.
Sinus Arrest and Sinus Pause are normal or abnormal rhythms. The P to P interval equals two or more normal intervals. There may be junctional and or escape beats present.
Atrial Standstill is characterized by a lack of P waves. The heart rate does not increase with either exercise or atropine. There is little or no contractility of the atria on echo or fluoroscopic examination.
First Degree A-V Heart Block (Incomplete Heart Block) is an EKG diagnosis only. It may not be an abnormal finding but usually is a significant problem. The P-R interval is prolonged beyond normal to a level of greater than 0.14 sec. in the dog and 0.08 seconds in the feline.
Second Degree A-V Block is another incomplete heart block. There are two types. Mobitz Type 1 has progressive P-R prolongation until the QRS cycles drops out. This is AV nodal disease electrophysiologically. In Mobitz Type 2 which is an infra HIS bundle disease the P-R interval remains the same and then the QRS complex will periodically drop out.
Advanced Second Degree Heart Block is observed when the heart rate is slow but greater than 40 beats per minute. The ventricle responds to every few P waves but there also may be no relationship between the P and QRS complexes. This is a faster form of block but otherwise looks the same as third degree heart block.
Third Degree (Complete) Heart Block is observed when the ventricular rate drops below 40 beats per minute. In this rhythm disturbance there is no conduction through the A-V node. The atrial rate exceeds the ventricular rate and sounds regular but of low intensity over the thorax. There is an idioventricular independent ventricular rate and is no relationship between the P waves and the QRS complexes. Their occurring together is simply the result of both rhythms occurring simultaneously on the same strip even if they beat independently.
Sick Sinus Syndrome is a combination of many different brady and tachyarrhythmias. The associated arrhythmias include sinus bradycardia, sinoatrial block, sinus arrest, alternating brady-tachyarrhythmia, escape beats and most known atrial and ventricular tachyarrhythmias.
The following tables are from The Textbook Of Veterinary Internal Medicine-6th Edition (2005). It describes the current drugs being used in small animal medicine along with recommended dosages and recommendations regarding use, side effects and efficacy. Each veterinarian needs to develop skill with a limited number of drugs and this table provides a beginning point for the armamentarium of drugs useful in the treatment of identified cardiac arrhythmias.