Case Studies In Small Animal
The ECG tracings above were recorded from a 6-year-old German short haired pointer that weighed 40 kg. She was presented to the referring veterinarian because she had a swollen mammary gland and was coughing. An arrhythmia was ausculted. An ECG was interpreted by Cardiopet as having atrial bigeminy. The dog was started on 30 mg diltiazem q8 hours. Thoracic radiographs were taken and right heart enlargement and pulmonary vascular changes were noted to be typical of heartworm disease. A heartworm antigen test was positive. The dog was treated with two doses of an adulticide (thiacetarsemide). The dog's ALT increased to 707 the following morning. Although the dog was still eating, adulticide therapy was discontinued (Note: This was inappropriate - the thiacetarsemide should have been continued). The diltiazem was also discontinued.
The owner brought Tilly to the VMTH two months later for an ovariohysterectomy. She was still coughing occasionally although the frequency had decreased. Her mammary glands were normal. An arrhythmia was ausculted. The above ECG was recorded. The ECG is a lead II recorded at 25 mm/second. Calibration is 1 cm = 1 mV. In the top trace, the first and second complexes originate from the sinus node. The second complex is followed by a burst of tachycardia. The QRS complexes appear almost identical to the sinus complexes so they are supraventricular. The rhythm is slightly irregular but the rate is around 200 beats/minute (7.5 mm or 0.3 seconds between each QRS complex). There may be a P wave in front of some of the QRS complexes, especially after the first one. After the seventh complex the tachycardia breaks and is followed by a sinus beat. This is immediately followed by a supraventricular premature beat, which again appears to have a P wave in front of it. This rhythm alteration of supraventricular premature complexes and tachycardia continues to occur over both traces.
Before taking her to surgery, it was desirable to control the arrhythmia. Since it was thought that the arrhythmia would most likely be present for a prolonged period, it was decided to titrate her up to an effective dose of diltiazem orally. Doses of 30 mg of diltiazem were administered at 10:00 AM, 12:00 PM, and 2:00 PM with no effect. The following morning at 8:00 AM, 90 mg diltiazem was administered PO. The top trace was recorded at 9:00 AM and the middle trace at 10:00 AM. Another 30 mg was administered and the bottom trace recorded at 2:00 PM. This trace shows only one premature complex (third complex). She was continued on 120 mg diltiazem (3 mg/kg) q8 hours while she was in the hospital and she remained in normal sinus rhythm. She successfully underwent surgery. Surprisingly, her heartworm antigen test was negative and her thoracic radiographs were improved. Consequently, it was decided not to treat her again with an adulticide. She was sent home on a heartworm preventative (ivermectin).
When the dog was examined again two weeks later at the time of suture removal, it was found out that the owner had decided not to continue to treat Tilly with the diltiazem or at least not until she showed clinical signs due to the arrhythmia. The ECG at that time was similar to the baseline ECG at the top of this page. The arrhythmia was still present when Tilly was examined one and three years later at the VMTH. She still had no structural heart disease on her echocardiogram at the third year examination. Consequently, it appears that the owner probably made the right decision.
©Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.