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.
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.