Case Studies In Small Animal

Cardiovascular Medicine

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Case 12

Case Discussion

This dog has pulmonary thrombosis or thromboembolism based on the fact that there is a large mass compatible with a thrombus on the echocardiogram. Although this mass could be something other than a thrombus, it is extremely unlikely. The mass is round, smooth, and homogeneous, all of which are characteristics compatible with a thrombus.

There are numerous reasons for a dog to have pulmonary thromboembolism. These include reasons for a dog to form a thrombus in a systemic vein or the right heart that becomes dislodged to form an embolus and reasons for a thrombus to form at the site it is found. Reasons for systemic thrombosis include hyperadrenocorticism, diabetes mellitus, autoimmune hemolytic anemia, and protein-losing glomerulonephropathy.

Basic blood work basically ruled out everything except a protein losing glomerulonephropathy. Blood was taken for cortisol concentrations before and after ACTH administration. These were never measured, however, because of the subsequent findings. Urinalysis revealed 3+ protein with a specific gravity of 1.019. A urine protein to creatinine ratio was 5 (normal is less than 1). This confirmed that a protein losing glomerulonephropathy was present but did not identify the cause or if it was causing systemic thrombosis. Protein losing glomerulonephropathies are thought to cause thrombosis by allowing antithrombin III to leak through the glomerular basement membrane resulting in a decrease in the plasma concentration of this substance. Consequently, a plasma antithrombin III concentration was determined. It was 88% of control. Anything greater than 80% of control is considered to be within the normal range so this was ruled out as the cause of the pulmonary thromboembolism.

Because the other common causes of pulmonary thromboembolism had been ruled out and because the radiographic appearance of the right caudal lobar pulmonary artery was very suggestive of heartworm disease, it was decided to go ahead and treat Lucky for adult heartworms. Her owner brought her back 4 days later for treatment with melarsomine. At that time another echocardiogram was obtained. At this time she had more pleural effusion than she had on the previous exam and her right caudal lobar pulmonary artery distal to the thrombus could be visualized (see below).

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An abnormality (arrow) distal to the thrombus (T) can be seen. It has the typical appearance of a heartworm with cuticles (white linear densities) on either side of a digestive tract (black linear echolucency).

 

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The heartworm (arrow) is again demonstrated in the figure above.

In addition to all of her other problems, Lucky also had a supraventricular tachycardia (SVT). This arrhythmia was a bit unusual. Most SVTs are due to reentry and so are extremely regular. This one is irregular. P waves can be identified before each of the QRS complexes (see below). This atrial rate is not regular but tends to speed and slow. The PR interval is also irregular. All of this suggests that this SVT may be due to an automatic (rather than a reentrant) focus although reentry is still possible. Since the P waves are upright in leads I, II, and III, the focus is most likely in the right atrium. With this dog's right atrial enlargement, this would make sense.

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Follow-Up

Because several heartworm antigen tests in this dog were negative, Lucky most likely only had a couple of male worms causing her disease. The heartworm antigen tests available at this time only detect an antigen (i.e., a protein) that originates from the female reproductive tract and eggs of heartworms. Consequently male worms go undetected. Usually only one female worm can be detected with a heartworm antigen test and if two or more are present the tests will almost always detect them. Lucky clearly has the one worm in her right caudal branch of her pulmonary artery and radiographically had evidence that another worm was living in the left caudal lobar artery branch (this was the branch that was enlarged and tortuous on the DV and VD radiographs).

Because she already had severe pulmonary and cardiac disease secondary to her heartworm infestation (heartworm disease class 3), Lucky was treated conservatively with one injection of melarsomine at this visit. One injection primarily kills the male worms so in Lucky's case it may have killed all of her worms. The owner was instructed to bring her back in 4 weeks for a set of two injections (each 24 hours apart). Since dogs can more readily break down thrombi than humans, it was hoped that she would lyse her own clot once the heartworm was gone.

Lucky was sent home on a myriad of drugs. Her dose of Lasix was increased to 25 mg q8 hours and her enalapril dose was increased to 10 mg q12 hours. Ten days later she was taken to an emergency clinic because she was again having tachypnea and a "gurgling" sound. Approximately 1500 ml of fluid was again removed. Consequently, her dose of Lasix was increased to 37.5 mg q8 hours. Her dose of digoxin (0.125 mg q24 hours) was maintained. However, her owner called the Friday following her visit to say she wasn't eating as well. The digoxin was discontinued and within 48 hours her appetite returned. Digitalis intoxication was suspected but not confirmed. Her dose was reduced to 0.0625 mg q24 hours. She was also started on 15 mg diltiazem q8 hours to treat he supraventricular tachycardia. The owner monitored her heart rate at home and reported that it was still very irregular and fast. The dose was increased to 30 mg (approximately 2 mg/kg) q8 hours and her rhythm became regular with a heart rate of around 160 beats/minute.

Lucky was seen again three weeks after the melarsomine injection. The owner reported that Lucky had been more lethargic and had had a poor appetite the last few days. She had also been reluctant to stand up and preferred to lie sternally with her rear legs splayed. She had fallen over several times when going out into the yard; her rear legs went down first and then she would lie down. The owner felt that this was an orthopedic or balance problem. She was taken to her local veterinarian a week and a half ago to have 1.7 liters of pleural fluid drained. Since then her resting respiratory rate had been around 40 bpm and she had been panting less. Her heart rate had stayed around 150 - 160bpm with no noted episodes of tachycardia. She had had bilateral ocular discharge over last couple days. Lucky was currently on digoxin .0625mg SID, diltiazem 30mg TID, enalapril 10mg BID, Lasix 37.5mg BID, and prednisone 15mg BID.

On physical exam she was unwilling to get up. She held her head down and panted. She had a yellowish mucoid discharge from both eyes. There was no nasal discharge. Her ears were slightly dirty and there was a tick in her right ear. Her mucous membranes were pale with a CRT=1 second. Mandibular, prescapular, popliteal nodes were not palpable. Her abdomen was distended cranially (her enlarged liver could be palpated, but a fluid wave was also present so ascites was suspected. She stood with her right rear leg slightly rotated outward. Her femoral pulses were strong. Jugular pulses were present. Her heart sounds were very loud and hammering with occasional irregular beats. The lung sounds were reduced ventrally. P=138, T=100.6.

1.3 liters of fluid was removed from her pleural space. The heartworm could no longer be seen on the echocardiogram but the thrombus had enlarged rather than decreased in size as hoped (see below). Her hematocrit had fallen from 44 to 30, and her BUN had increased from 40 to 75. The creatinine was within normal limits. The ALT had increased from 52 to 163. The WBC count had increased from 25,000 to 39000. No eosinophils were detected this time, but there were a substantial number of basophils. There was a monocytosis of 3,483 and severe lymphopenia with no lymphocytes noted. There were some acanthocytes and schistocytes.

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Options, including thrombolytic therapy and surgical removal of the thrombus, were discussed with the owner. He decided to treat Lucky conservatively at home in the hope that Lucky's fibrinolytic system would break down the clot. The owner phoned one week later to let us know that Lucky died in his arms. He buried Lucky in his yard.

 

Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.