Case Studies In Small Animal

Cardiovascular Medicine

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Case 6

Case Discussion

This cat was clinically cyanotic at presentation. Clinical evidence of cyanosis in a patient with heart disease usually means the patient has a right-to-left shunt between cardiac chambers or great vessels. It also means the hypoxemia is severe with the partial pressure of oxygen in the systemic arteries usually being below 40 mm Hg (normal is around 100 mm Hg).

The radiographs from this cat showed an abnormally conformed heart. The right sided enlargement along with the pulmonary artery and ascending aortic enlargement combined to cause this abnormal configuration.

The identification of severe right ventricular concentric hypertrophy (i.e., thickened walls or pressure overload hypertrophy) on the echocardiogram was consistent with a lesion producing a right-to-left shunt since in these situations there commonly is either pulmonary hypertension (i.e., increased pulmonary artery) or pulmonic stenosis causing an increase in right ventricular systolic pressure (i.e., a pressure overload).

The contrast echocardiogram was preformed to identify or rule-out a right-to-left shunt. In a normal animal, microbubbles should be visualized in the right heart only after injection into a peripheral vein. Once they leave the right ventricle, they are filtered out by the pulmonary capillaries (the bubbles are larger than the capillaries) and so never reach the left heart. With a right-to-left shunting ventricular septal defect (Eisenmenger's complex), microbubbles can be seen to cross over into the left ventricle. With a right-to-left shunting atrial septal defect they can be seen in the left atrium and left ventricle. No microbubbles crossed over at the cardiac level which ruled out an intracardiac right-to-left shunt. With a right-to-left shunting PDA, there should be no microbubbles present in the left heart but bubbles should be seen distal to the region of the shunt - in the caudal descending aorta. In Nastasia's case, we were not sure if they were present or not. It appeared that they were not present.

With further interrogation of cardiac structures, we were able to identify a structure between the proximal descending aorta and the pulmonary artery. This structure was consistent with a patent ductus arteriosus. Flow in this structure was low velocity (and so was not turbulent and would not produce a heart murmur) and appeared to flow from the pulmonary artery to the aorta (right-to-left).

A right-to-left shunting patent ductus arteriosus was confirmed at cardiac catheterization via angiography. The pressure waveform findings were consistent with a right-to-left shunting PDA since systolic pressures in both chambers and both great arteries were equal. In a right-to-left shunting PDA, the PDA is wide open providing a wide-open conduit between the aorta and pulmonary artery. Consequently, blood can flow between the two circulations as it pleases. Consequently, systolic pressures on both sides are always equal. Flow travels through the two circulations in direct proportion to the relative impedances (mostly resistances) of the two circulations. If pulmonary vascular resistance is 1/3 of systemic vascular resistance, flow through the pulmonary vasculature will increase to three times normal and pressures will equalize (remember that pressure is equal to flow times resistance). In this case flow through the PDA will be left-to-right. If pulmonary vascular resistance and systemic vascular resistance are equal, no net flow will occur through the PDA. If pulmonary vascular resistance is twice systemic vascular resistance, pulmonary blood flow will be one-half of systemic blood flow and blood flow through the PDA will be right-to-left as in Nastasia.

Follow-Up Information

Nastasia recovered well from anesthesia. Blood gas values taken from the dorsal pedal arterial catheter were as follows: On 100% O2 @ 10:29am O2 saturation = 89%, @ 10:39am 92%. On room air @ 1600 O2 saturation = 82%, at 18:39 78%. Her arterial oxygen tension at 18:39 was 45 mm Hg (normal is greater than 90 mmHg). This marked desaturation was from the right-to-left shunt through her PDA. However, the hypoxemia was not severe enough to result in polycythemia. This usually requires an oxygen tension less than 40 mm Hg. From the angiogram it was clear why Nastasia has generalized instead of differential cyanosis - the desaturated blood coming through the PDA flowed forward all the way to the root of the aorta and so into the cranial vessels (brachiocephalic trunk and left subclavian).

Upon recovery, a prolapsed right third eyelid and a meiotic pupil were noted, which was thought to be Horner's syndrome, a transient event due to manipulation of the carotid sheath containing the vagosympathetic trunk. Bruising was noted around her catheter sites and from previous catheterizations. The following day she was BARH and eating. The signs of Horner's syndrome were subsiding. One week later she was completely back to her baseline condition. Since Nastasia does not have polycythemia, she does not need medical therapy. Dogs with a right-to-left shunting PDA generally die soon after ductal ligation, presumably from the severe pulmonary hypertension. Consequently, it was decided not to ligate Nastasia's PDA.

After living in Dr. Thomas' lab for a week, he took her home where she now resides with five other cats. Nastasia has taken control of all the other cats (she's the boss!).

18 months later

Dr. Thomas brought Nastasia in to the clinic because she was having a lot of difficulty breathing. Thoracic radiographs showed that she had pleural effusion. On her echocardiogram her right heart had enlarged further and she had jugular vein distension. This led to the diagnosis of right heart failure. As much of the pleural effusion as possible was withdrawn but it was difficult to withdraw. She died soon after.:(

At post mortem, Nastasia's heart weighed 50 grams. Normal heart weight for a cat is less than 20 grams. Our previous record for cat heart weight was a cat with hypertrophic cardiomyopathy that weighed 38 grams. This was one amazing heart.

View of the heart from above, behind, and to the right

This view of the heart shows the markedly enlarged right atrium and marked fibrosis of the ventricles.

View of the heart from in front and to the right

   

This view shows the markedly enlarged main pulmonary artery (MPA). The ductus (arrow) can be seen coming off the MPA and going into the descending aorta (DAo).

We have not previously seen a dog or a cat with a right-to-left shunting ductus develop right heart failure. Presumably the extreme hypertrophy of the right ventricle resulted in inadequate myocardial perfusion, cell death, and fibrosis. The fibrosis then lead to increased stiffness of the right ventricular chamber which increased the diastolic pressure in the right ventricle and so the pressure in the right atrium (resulting in the enlargement) and systemic veins (i.e., right heart failure).

 

 

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