The indications and technique for continuous ambulatory electrocardiographic recording in dogs

Continuous ambulatory electrocardiographic recording is a noninvasive test used to evaluate heart rhythm disturbances and to help diagnose the cause of unexplained syncope or presyncope. The utility of Holter recording lies in the ability to continuously examine the heart rate and rhythm over 24 to 48 hours. In contrast to a one-minute electrocardiogram (ECG), which contains only 0.07% of the 24-hour heart rhythm data, Holter recording permits extended ambulatory activity and diurnal ECG evaluation in a variety of situations and environments over at least one sleep-wake cycle.

The physicist Norman J. Holter developed an ambulatory ECG monitor and reported its usefulness in 1961 after the sudden death of a friend.1 Early recorders were bulky and heavy, but today's state-of-the-art digital recorders are about the size of a cell phone (e.g. Lifecard CF digital Holter recorder—Delmar-Reynolds Medical, Delmar, Calif.).

Several decades of clinical experience in dogs and people have demonstrated that Holter monitoring is a cost-effective tool for diagnosing and assessing heart rhythm disturbances, aiding in sudden death risk-stratification in cardiomyopathic Doberman pinschers and boxers, and evaluating antiarrhythmic treatment.2-10 Holter recordings are most valuable when made in a patient's home environment with normal or user-determined periods of activity and timed diary entries describing the patient's activity and clinical signs.

INDICATIONS FOR HOLTER RECORDING

Holter recording is indicated in numerous clinical situations, typically involving the detection and assessment of the severity of supraventricular or ventricular tachyarrhythmias and bradyarrhythmias. It can be used to

1. Evaluate the complexity and frequency of cardiac arrhythmias detected during static ECG recording or auscultation.

2. Assess the efficacy of antiarrhythmic treatment of heart rhythm disturbances.

3. Help determine the differential diagnoses in dogs with syncope and presyncope when heart rhythm disturbances are not evident during auscultation or static ECG recording and when event recording is unavailable.

4. Screen Doberman pinschers and boxers for markers of cardiomyopathy, such as ventricular premature contractions (VPCs).

5. Determine the atrial fibrillation ventricular response rate of dogs during treatment.

6. Assess the functional integrity of implanted artificial pacemakers.

A number of articles in the veterinary medical literature have described the use of Holter recordings for some of these indications.2,3,7-9,11-16

Evaluating arrhythmia severity

When a veterinarian identifies a heart rhythm disturbance on auscultation in an asymptomatic dog and a static ECG records occasional VPCs, it is inappropriate to assume that the arrhythmia is benign. A static ECG records only a brief sample in time, and the arrhythmia is often either more or less severe than estimated on a short electrocardiographic recording.2,10

Not all ventricular arrhythmias are dangerous. In fact, most are hemodynamically insignificant (Table 1). Slow ventricular tachycardia (< 180 beats/min) is usually benign and is best referred to as accelerated ventricular rhythm.17 Most lethal ventricular arrhythmias in ambulatory dogs are associated with cardiomyopathy, severe congenital subaortic stenosis, and, specifically in young German shepherds, inherited ventricular tachycardia.6-8,11,13,14,16

Table 1: Disorders in Dogs Associated with Certain Ventricular Tachyarrhythmias

Although the cause of an arrhythmia is often of more prognostic value than the arrhythmia rate or complexity, ventricular tachycardia with a rate exceeding 250 beats/min is always of concern. Lethal ventricular tachycardia occurs at a rate of > 300 beats/min. At this rate, ventricular tachycardia can degenerate into ventricular fibrillation.7-9,11-13 Dogs with subaortic stenosis are an exception because ventricular tachycardia can degenerate into ventricular fibrillation at rates as slow as 170 beats/min. So all Doberman pinschers, boxers, and other dogs with subaortic stenosis should be evaluated by Holter recordings to determine the severity of the arrhythmia.2,3,7-11,13,14,16

Assessing antiarrhythmic treatment efficacy

The increased sensitivity of the ambulatory ECG in detecting spontaneous arrhythmias has been proven.2-10 These arrhythmias usually have great inherent variability, which often necessitates a pretreatment and post-treatment Holter recording to determine whether medical therapy is proarrhythmic or antiarrhythmic or whether it lacks efficacy.3,12,18 If a post-treatment static ECG reveals numerous VPCs or ventricular tachycardia, then Holter recording is probably not indicated at that time and treatment can be adjusted. However, if a post-treatment static ECG reveals no or only occasional VPCs, it is inappropriate to assume that the arrhythmia is well-controlled.

The severity of ventricular tachyarrhythmias over a 24- to 48-hour period is variable.2,10 Arrhythmias may appear to be well-controlled when the patient is at rest, only to become severe during exertion or excitement or for no apparent reason.3,8,13,15,16 In both treated and untreated cardiomyopathic boxers, rapid ventricular tachycardia may appear after 10 to 12 hours of relatively benign arrhythmia. Holter recording is the most accurate noninvasive method for verifying antiarrhythmic treatment efficacy in veterinary patients.

Diagnosing the cause of syncope when heart rhythm disturbances are not evident

Syncope due to cardiac arrhythmia is common in dogs. Syncope can be secondary to bradycardia, rapid ventricular tachycardia, or, less often, rapid supraventricular tachycardia. Holter recording may be a useful diagnostic test in patients with syncope and episodic weakness when the cause of syncope is not readily apparent on a static ECG.3,11

Neurally mediated bradycardia. Neurally mediated bradycardia and syncope are common in elderly small-breed dogs.19 Situational syncopes result from vagal triggers caused by coughing, gagging, retching, vomiting, urinating, or defecating.19 Neurocardiogenic syncope is an adrenergic-triggered, vagal-reflex bradycardic syndrome associated with excitement or exertion in elderly small-breed dogs with advanced mitral valve disease, severe left heart enlargement, and high preload.19 Other than the bradycardia during the syncopal episodes, the heart rates and rhythms of dogs with neurally mediated bradycardia are usually normal.19 Holter recording documents the heart rhythm, as would event recording during syncope.

Neurally mediated bradycardia is also a cause of syncope in some large-breed dogs, notably boxers and golden retrievers.20 Some normal dogs experiencing severe fight, fright, or flight stimuli demonstrate an adrenergic-triggered, vagal-reflex bradycardia.20 When cardiomyopathy is absent, these dogs have normal echocardiogram results and no or few VPCs.20 Cardiomyopathic boxers also experience an excitement-exertion-startle-triggered neurocardiogenic bradycardia but also have ventricular tachyarrhythmias of variable severity, with or without echocardiographic abnormalities.20 Cardiomyopathic Doberman pinschers can faint because of transient neurocardiogenic bradycardia in response to exertion-excitement or just because of a sudden change from less activity to more activity.11 In Doberman pinschers, neurocardiogenic bradycardia along with syncope is a marker of cardiomyopathy.11 Holter recording helps differentiate neurocardiogenic bradycardia from ventricular tachycardia when no or few VPCs occur in a dog with recent syncope.20

Bradycardia due to sick sinus syndrome. Not all bradyarrhythmias are neurally mediated. Sick sinus syndrome is usually associated with American cocker spaniels, miniature schnauzers, and West Highland white terriers. When the bradycardia of this syndrome causes syncope, it is usually apparent on a static ECG. Advanced atrioventricular (AV) conduction blocks are usually due to degeneration of conduction pathways. AV conduction blocks severe enough to cause syncope are usually evident on a static ECG.

Ventricular arrhythmia due to myocardial disease. Patients with myocardial disease (most notably Doberman pinschers and boxers) may present with syncope secondary to ventricular tachycardia. When experiencing syncope, boxers may or may not have evidence of dilated cardiomyopathy (DCM), whereas Doberman pinschers almost always have obvious DCM. In patients with syncope secondary to ventricular tachycardia, if a static ECG fails to document couplets, triplets, or ventricular tachycardia, Holter recording is indicated. Holter monitoring is especially indicated in boxers, who may have syncope secondary to neurocardiogenic bradycardia or ventricular tachycardia.

Atrial fibrillation due to advanced heart disease. Ventricular tachycardia is not the only tachyarrhythmia that can be associated with syncope and episodic weakness. These signs can also occur after the onset of atrial fibrillation in dogs with advanced heart disease. In these patients, atrial fibrillation is evident on a static ECG. The assumption is that poor cardiac function, loss of atrial contraction, and the rapid, irregular rhythm that result from this arrhythmia produce a marked decrease in cardiac output, resulting in syncope. However, in dogs with cardiomyopathy and atrial fibrillation, syncope may also result from paroxysmal ventricular tachycardia. Holter recording can be a useful tool in diagnosing syncope in which the cause is not readily apparent and is the result of atrial fibrillation or ventricular tachycardia.3,11-16

Screening Doberman pinschers and boxers for cardiomyopathy

Cardiomyopathy is common in Doberman pinschers and boxers.7-9,12,16 Owners and breeders of Doberman pinschers and boxers have become aware of the high incidence of cardiomyopathy in these breeds and are increasingly seeking diagnostic screening from veterinarians. The earliest marker for cardiomyopathy in these breeds is VPCs.8,9,15,16

Holter recording is a proven diagnostic procedure for detecting occult (presymptomatic) ventricular arrhythmias in Doberman pinschers and boxers.8,9,12,13 Cardiomyopathy is likely if more than 50 to 100 VPCs occur in 24 hours and if any complexity of arrhythmia, such as couplets, triplets, or ventricular tachycardia, is present, unless another cause can be discovered. We recommend all apparently healthy adult Doberman pinschers and boxers be examined by cardiac ultrasonography and Holter recording once a year (Figure 1).

Figure 1. Selected Holter monitor ECG rhythm strips demonstrating ventricular tachycardia in a Doberman pinscher.

Determining the atrial fibrillation ventricular response rate

Atrial fibrillation is a common disorder in dogs and is usually associated with advanced heart disease. Often by the time atrial fibrillation is detected by an ECG, it is a persistent, rapid, and irregular heart rhythm. Rate control, rather than rhythm control, is a common management strategy for dogs with atrial fibrillation.

When dogs are examined in the hospital setting, particularly during restraint for diagnostic testing, their sympathetic nervous system activity may be increased. Sympathetic dominance decreases the refractory period of the AV nodal tissue and increases its conductivity. For this reason, when a static ECG is performed in a dog being treated for atrial fibrillation, the ventricular response rate to the fibrillating atria is increased compared with when the patient is calm.21

Holter recording in the home environment allows accurate measurement of the post-treatment heart rate characteristics over a 24-hour period in a natural setting. In most patients, the heart rates in the home setting are significantly slower than in the hospital (Figure 2).21

Figure 2. Selected Holter monitor ECG rhythm strips demonstrating atrial fibrillation. The top rhythm strip (three leads) was obtained in the hospital shortly after applying the Holter monitor. Note the rapid ventricular rate (210 beats/min). The rhythm strip at the bottom was recorded in the patients home environment. Note the slower ventricular rate (73 beats/min).

When a dog being treated for atrial fibrillation is re-examined in the hospital setting and the static ECG reveals an atrial fibrillation ventricular response rate > 180 beats/min, it may be necessary to add an additional drug to the treatment regimen to reduce the ventricular rate. In patients with cardiac disease, our goal for atrial fibrillation is a ventricular response rate in the home environment < 140 beats/min for 90% of the 24-hour Holter monitoring period.

Assessing the functional integrity of implanted artificial cardiac pacemakers

Pacemaker implantation is the recommended treatment for dogs with symptomatic bradyarrhythmias unresponsive to anticholinergics. In dogs with permanent artificial cardiac pacemakers, periodically confirm normal pacemaker function. Evaluation can be performed by using a computerized program system analyzer. Holter recording can also provide information about pacemaker function, such as quantification of normal rhythm vs. paced rhythm time, sensitivity of the rate-responsive feature to activity levels, and appropriateness of inhibition to spontaneous ectopic activity. Because of the limited examination time during routine hospital visits and the pacemaker's limited ability to store abnormal ECG recordings, a Holter monitor can facilitate the diagnosis of pacemaker malfunction by recording the patient's rhythm over 24 hours or more during daily activities.

TECHNIQUE

Holter recorders are available with three to five electrodes. Holter recorders with five electrodes improve the ability to differentiate movement artifact from pathologic arrhythmia and VPCs from supraventricular premature contractions. However, Holter recorders with three electrodes are easy to apply and usually provide sufficient data (Figure 3).

Figure 3. A three-electrode Holter monitor. The electrode wires are concealed beneath elastic tape, and the recorder is taped tightly to the dorsal midline. When applied in one vertical plane around the thorax, the electrodes and lead wires can be concealed with 2-in elastic tape.

If you carefully apply and secure the electrodes, you will almost always obtain high-quality Holter recordings. It is critical to have excellent contact between the electrodes and the skin. To that end, clip the hair down to the skin at the sites of electrode attachment. Scrub the skin vigorously with a solvent, such as acetone, and allow it to dry. Apply adhesive electrodes, and wrap them tightly with 2-in elastic tape (Figure 4). When the electrode-skin contact is firm and wrapped tightly to prevent electrode movement, you will obtain a high-quality recording during both quiet and active periods. Even vigorous activity is usually not associated with excessive artifact.

Figure 4. Small areas (1 to 2 cm) of skin are clipped free of hair at the sites of electrode placement, and the skin is scrubbed vigorously with a solvent such as alcohol or acetone. The electrodes are held tightly to the skin with elastic tape.

Because the electrodes are applied securely, removal can be challenging. We do not recommend using sharp utensils such as scissors and scalpel blades to remove the elastic tape; you might accidentally cut the leads or the patient. Instead, we recommend carefully applying acetone or a commercial tape adhesive solvent to the elastic tape and electrodes. Avoid contact with the Holter monitor and flash card. If you destroy the adhesive material first, Holter recorders can be removed safely and painlessly.

Holter monitor services

Holter monitors can be obtained from and the data analyzed by numerous Holter scanning services. Although these services use modern systems capable of arrhythmia analysis, remember that they are not designed for canine ECGs. Because of technical problems with recordings and in the analytic processes, fully automated computer scanning systems for Holter analyses are unreliable.5,6 We recommend services that analyze Holter recordings under the supervision of a technician or doctor who is familiar with the scanning computer and who is also competent in analyzing canine ECGs.

Some large commercial laboratories, such as Laboratory Corporation of America (Burlington, N.C.; 800-289-4358) and Biomedical Systems of Atlanta (Atlanta, Ga.; 800-877-6334), provide ambulatory ECG monitoring services. These diagnostic services will express- mail you a Holter recorder and all the ancillary equipment to apply the device to veterinary patients. After about 24 hours of recording, you then remove the recorder and mail it back to the service. The service will fax a summary of the recording to you and mail you a complete report within a few days. The cost to veterinarians is $140 to $150. You can request an evaluation by a veterinary cardiologist for an additional $50 to $100.

CONCLUSION

Continuous ambulatory ECG monitoring with a Holter monitor is a useful and practical diagnostic test available to all veterinarians. Holter recording is essential for the accurate assessment of arrhythmias and their treatment. It is the most sensitive test for detecting cardiomyopathy in Doberman pinschers and boxers, may be useful in determining the differential diagnoses in patients with syncope, and is underused as a means of determining the efficacy of atrial fibrillation ventricular response rate control. The application and removal techniques require attention to detail but are not difficult. Evaluation of the report by a veterinary cardiologist is available through commercial Holter monitoring services.

Justin D. Thomason, DVM, DACVIM (small animal internal medicine)

Department of Clinical Sciences

Center for Veterinary Health Sciences

Oklahoma State University

Stillwater, OK 74078

Tiffany L. Fallaw, BS, RVT

Clay A. Calvert, DVM, DACVIM (small animal internal medicine)

Department of Small Animal Medicine and Surgery

College of Veterinary Medicine

University of Georgia

Athens, GA 30602

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