A cardiac arrhythmia in the horse: Is the ECG normal?

WHAT IS YOUR DIAGNOSIS?

The editors welcome contributions to this section. Submissions should be in a case study format, including: brief summary of the case, ECG of publishable quality (glack and white glossy print(s), labeled as to lead(s), paper speed and calibration voltage), ECG interpretation and mechanism, and therapy and discussion. - - Michael S. Miller, MS, VMD, DipABVP and John D. Bonagura, DVM, MS, DipACVIM, editors

A CARDIAC ARRHYTHMIA IN THE HORSE: IS THE ECG NORMAL? a

Michael S. Miller, MS, VMD, DipABVP

The electrocardiogram (ECG) is a record of the average electrical potential generated in the heart muscle, graphed in terms of voltage and time, and recorded during the different phases of the cardiac cycle? Variations in voltage (y axis) are produced by depolarization, repolarization and conduction of current through individual muscle cells of the atria and ventricles. Normally each segment of the ECG arises from a specific area of the heart and occurs in a sequential fashion (x axis). The P-wave, QRS complex and T-wave are the recognizable deflections of the ECG trace (Figure 1) and indicate atrial depolarization (P), ventricular depolarization (QRS) and ventricular repolarization (T). The ECG is obtained by attaching eleclrodes at various Author'saddress:Cardiopet,Inc., 25 LumberRoad,Roslyn, New York 1157 aA lecture presented at the 1987 World Veterinary Congress, Montreal, Canada, with permission. Volume 7, Number 5, 1987

points on the body and interfacing these recording eleclrodes to a galvanometer (electrocardiograph) to display the electrical field generated by the heart. This machine combines the electrodes into specific combinations (leads) which allow evaluation of the electrical activity of the heart from different angles or aspects of the body. Since the ECG complexes of the traditional limb leads are sometimes small in the horse, monitoring leads have been developed to magnify the ECG complex size. An example of such a monitor lead is the base apex lead (Figure 1). The ECG is well established in veterinary medicine as a diagnostic test that accuratelydefines cardiac arrhythmias and conduction disturbances and provides limited information about cardiac chamber size. TM The ECG is best interpreted in conjunction with other available parts of the data base and the significance, prognosis, and need for therapy can only be determined with a complete evaluation. Although there are numerous indications for ordering an electrocardiogramin the 285

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Figure 4. Electrocardiogram from a horse with ventrlcular premature complexes (VPC's). The normal complexes are labeled with arrows and the VPC's are much larger and wider than the normal sinus complexes. This arrhythmla is often seen with myocarditls. From Bonagura JB, Miller MS,J Equine Vet Sci, Vol $(6), 1985.

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FlgureS.Ventricular tachycardiain a horse. The left portion ofthe strip shows a wide and bizarre QRS-T configuration with dissociated P-waves buried In the QRS-T complexes. Spontaneous conversion to normal sinus rhythm occurs (arrow); (25 mndsec) paper speed. This horse had a systemic infection. From Bonagura JD, Miller MS,./Equine Vet Sci, Vol 5(6), 1985.

horse, 13 an auscultable irregular cardiac rhythm, especially when it occurs in a horse with a history of poor performance, quitting, or syncope is the chief indication. Other indications are thoroughly discussed in the referenced articles. The normal cardiac rhythm, sinus rhythm, occurs when the cardiac impulse originates in the sinoatrial node, is conducted through normal pathways to the ventricular myocardial cells and results in a resting heart rate of 25 -50 beats minute. Variations in normal sinus rhythm are often encountered in the horse~s yet these irregular rhythms are not associated with either pathologic or functional cardiac disorders. These findings have contributed to confusion concerning the practical use of the ECG in equine practice, and the mistaken assumption that this diagnostic test is not of major benefit when evaluating horses with suspected heart disease. Accordingly, the clinician must recognize that normal rhythm variations (especially involving the sinus mechanisms) frequently occur in the resting horse while appreciating that other frequent or repetitive atrial and ventricular arrhythmias must be identified as they often indicate myocardial disease. Although there are numerous factors that effect discharge (automaticity) and conduction from the sinus node and AV node, variations in vagal and sympathetic efferent activity to the heart accounts for the majority of these rhythm changes. The resting horse may have a high vagal tone resulting in sinus bradycardia, sinus arrhythmia, sinus block, or AV block. When sympathetic activity (as occurs with stress or exercise) is high, these vagally induced irregularities disappear in the normal horse. Excitement, stress, exercise and administration ofvagolytic drugs like atropine accentuate sympathetic aclivVolume 7, Number6, 1987

ity and usually lead to regular and rapid sinus nodal discharge (sinus tachycardia) with abolishment of sinus bradycardia and AV block. Failure of vigorous exercise to abolish these arrhythmias is often taken as a sign of heart disease. Sinus arrhythmia is distinguished from normal sinus rhythm by the irregular generation of P-waves with a P-wave to P-wave interval that differs greater than 10%. The P-wave and QRS complexes are generally normal with sinus arrhythmia; however, variation in P-wave configuration (wandering pacemaker) may also be observed. Sinus arrhythmia as well as sinus bradycardia, sinus block and sinus arrest are all normal rhythm variations in the horse at rest, and all are associated withhighlevels ofvagal tone to the sinus andAV nodes. Sinus bradycardia is a sinus rhythm, occurring at a rate less than 25 beats per minute. With sinus block, the sinus node impulse is formed but is blocked in the sinoatrialjunction, and no P-wave is seen. Sinus block is suggested by a pause that is exactly equal to 2 P-wave intervals or a pause that is proceeded by progressively shortening P-wave intervals. Sinus arrest is an actual failure of an impulse formation caused by depressed automaticity within the SA node, and although the pauses tend to be quite long (greater than 2 average P to P intervals), it may be difficult to differentiate from sinus block. Second degree AV block (Figure 2) is the most common normal rhythm variation encountered in the resting horse? This rhythm may occur independently or in conjunction with the previously discussed rhythm variations. Mobitz type 1 (Wenckebach) AV block is typified by a variable P-R interval which either prolongs or shortens prior to blocking of Pwaves. Most horses block a single P-wave; however 2 con287

secutive blocked P-waves are not uncommon and may be a normal variation in some horses. Again, this arrhythmia should resolve with exercise. Submaximal exercise, including lunging, may be inadequate to totally resolve second-degree AV block, and particularly fit horses may immediately revert to blocking of P-waves during the post-exercise rest period. Sinus tachycardia is a sinus rhythm characterized by P-QRST complexes that occur at a more rapid rate (greater than 50 beats per minute). This rhythm is expected in foals, immature or excited horses, and following exercise. A number of pathologic conditions lead to sinus tachycardia including colic, shock, sepsis, fever, pain, and heart failure, but the rhythm is compensatory in these conditions and serves to maintain cardiac output in times of stress. These normal rhythm variations in addition to descriptions of different lead systems and cardiac enlargement patterns accounted for the confusion and complexity previously ascribed to equine electrocardiography. It should be emphasized that these normal slow rhythm variations can easily be abolished with exercise or vagolytic drugs, thereby focusing on detectionof serious arrhythmias such as frequent atrialpremature complexes, frequent ventricular premature complexes (Figure 4), atrial tachycardia, atrial fibrillation (Figure 3) and ventricular tachycardia (Figure 5) that may support the existence of serious heart diseaseg-13and usually result in poor performance, exercise, intolerance or sudden death. Systemic problems with resultant electrolyteor acid base disorders may also result in cardiac arrhythmias that affect morbidity or mortality, therefore, the ECG rhythm strip is an important part of the "sick horse" data base. If used in the above framework, electrocardiographycan be an extremely useful clinical aid to evaluate the equine heart and complementauscultation and/or echocardiography (ultrasound examination of the heart), and is a vital part of the complete cardiac data base. Proper recognition of serious arrhythmias is mandatory as they are often treatable with drugs4-16 such as quinidine sulfate, or digoxin. Appropriate diagnosis and therapy will allow stabilization of serious cardiac arrhythmias that may result in clinical signs such as syncope, congestive heart failure and sudden death. The veterinarian should anticipate normal rhythm variations in the horse. Palpation of the arterial pulse and auscultation of the heart are the first indicators of these arrhythmias.

Often there are good auscultatory correlates to these arrhythmias and the experienced clinician learns to correlate atrial sounds with P-waves and fhst and second heart sound with the QRS complexes. Conversely, some of the most serious life threatening arrhythmias (e.g., ventricular tachycardia and atrial tachycardia) have regular rhythms and can easily be missed on auscultation. The ECG offers definitive information, and objective documentation of the overall electrical activity of the heart.

REFERENCES 1. Miller MS, Bonagura JD: Equine electrocardiography: Genesis of the equine electrocardiogram and indications for electrocardiography in clinical practice. JEq Vet Sci 5(1) 23-25, 1985. 2. Detweiler DK, Patterson DF: The cardiovascular system. In: Catcott EJ and Smithcors JF (eds): Equine Medicine and Surgery. Edition 2. Santa Barbara, CA, Amer Vet Pub Inc, pp 277-347, 1972. 3. Fregin GF: The cardiovascular system. In Mannsmann RA and McAllister ES (eds): Eq Med andSurg, Edition 3, Vol 1, Santa Barbara, CA, Amer Vet Pub Ine, Whcaton, II pp 645-704, 1982. 4. Miller MS, Bonagura JD: Equine electrocardiography: Normal ECG complexes. JEq Vet Sci, 5(4) 200-203, 1985. 5. Bonagura JD, Miller MS: Treatment of cardiac arrhythmias and conduction disturbances. In:CurrentVeterinary Therapy IX, Kirk RW, ed., WB Saunders Co, Philadelphia, pp 346-360, 1986. 6. Holmes JR, Alps BJ: The effects of exercise of rhythm irregularities in the horse. Vet Rec, 78:672-683, 1966. 7. Miller MS, Bonagura JD: Equine electrocardiography: normal Cardiac rhythms. JEq Vet Sci, 5(3) 157-159, 1985. 8. Button C, Scmtchfield WL, Clark RG et ah Multiple atrial dysrhythmias in a horse. J Am Vet Med Asso¢ 1177:714-719, 1980. 9. Morris DD, Fregin FG: Atrial fibrillation in horses: Factors associated with response to quinidine sulfate ia 77 clinical cases. Cornell Vet 71:339-349, 1982. 10. B6nagura JD, Miller MS: Equine electrocardiography common atrial arrhythmias. J Eq Vet Sci 5(6):347-350, 1985. 11. Hilwig RW: Cardiac arrhythmias. In: Current Therapy in Equine Medicine, WB Saunders Co, Philadelphia, PA, 131-140, 1983. 12. Bonagura JD, Miller MS: Equine electrocardiography: Ventficular and junctional arrhythmias. JEq Vet Sci 5(6) 347-350, 1985. 13. Hilwig RW: Cardiac arrhythmias in the horse, JAm VetMedAssoc, 170:153-163, 1977. 14. McGuirk, Muir WW: Pharmacology and pharmacokinetics of drugs used to treat cardiac disease in horses. In: Veterinary Clinics ofNorth America: Equine Practice. Edited by JD Bonagura. WB Saunders Co., Philadelphia, PA 353-370, 1985. 15. Muir WW, McGuirk SM: Hemodynamics before and after conversion of atrial fibrillation to normal sinus rhythm in horses. J Am Vet bled Assoc 184:965-979, 1984. 16. McGuirk, Muir WW: Diagnosis and treatment of cardiac arrhythmias. In: Veterinary Clinics of North America: Equine Practice. Edited by JD Bonagura. WB Saunders Co, Philadelphia PA 353-370, 1985.

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