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