Case Studies In Small Animal

Cardiovascular Medicine

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Case 29

Clinical Diagnosis

bulletTetralogy of Fallot without cyanosis at rest (so-called "pink tet")


bulletECG: HR 220 beats/minute; normal sinus rhythm; wide, tall P waves in lead II with a duration of 0.03-0.04s and an amplitude of 0.3mV; PR interval = 0.06s; short, splintered QRS complexes in lead II with a normal duration of 0.03s and amplitude of 0.3mV; deep S wave present in left chest lead (V4); Mean electrical axis in the frontal plane = 170degrees.
bulletECHO: Butorphanol was given IM for sedation to facilitate cooperation for echocardiography. Marked right ventricular concentric hypertrophy with a right ventricular free wall thickness of 6mm and a left ventricular free wall thickness of 4mm. Large ventricular septal defect (VSD) with laminar flow from the right ventricle and left ventricle into the aorta; narrowed right ventricular outflow tract with malpositioning of pulmonic valve and pulmonic stenosis; highest velocity recorded across pulmonic valve region = 5.4 m/s (pressure gradient of 117; therefore approximate RV and LV pressure of 140 mmHg assuming the systolic pulmonary artery pressure is 20 to 25 mmHg). Overriding aorta visualized.
bulletThoracic radiographs: two view thorax: enlarged heart (vertebral heart score=10.25); right heart enlargement; hypoperfusion of the pulmonary vasculature with diminutive caudal lobar vessels and pulmonary arteries.
bulletPCV:38%; total protein=7.4
bulletArterial blood gas: pH 7.4, PCO2 - 30 mmHg, PO2 - 36 mmHg, K - 4.5 mEq/l, Na - 150 mEq/l, Ca - 1.36 mmol/l, oxygen saturation 64%, acid base: HCO3 - 17.1 mmol/L, tCO2 - 18 mmol/L, base deficit - 6.8 mmol/L

The ECG findings are consistent with right ventricular enlargement although the limb lead axis could also be consistent with a so-called posterior fascicular block. The deep S wave in V4 is consistent with right ventricular enlargement.

Echocardiographic findings documented the presence of tetralogy of Fallot with right ventricular outflow tract narrowing and/or pulmonic stenosis, an overriding aorta, a large ventricular septal defect, and right ventricular concentric hypertrophy of 6mm. Right-to-left shunting of blood flow was present with laminar flow evident from both the right and left ventricles through the large VSD into the aorta.

An arterial blood gas sample obtained from the femoral artery revealed a PO2 of 36 mmHg. This documents severe hypoxemia due to shunting of venous blood into the arterial circulation. Clinically Nimbus was not cyanotic at rest. When severely stressed by restraint (as she was during femoral artery puncture for blood gas analysis), she did progress to open-mouth breathing with increased abdominal effort and cyanosis. This resolved within minutes with rest.

The PCV of 38% is in the normal range for a cat of her age (average normal is 41%); normal PCV values are usually attained by four months of age. The normal PCV verifies that Nimbus does not have hypoxemia severe enough to cause polycythemia at most times in her life. Clinically significant polycythemia does not occur until the arterial oxygen tension decreases into the 30 to 50 mmHg range. With stress the arterial oxygen tension did decrease into this range and Nimbus did turn blue. However, when not stressed she was pink (see the picture of her oral mucous membranes on the first page).

bulletThe diagnosis of tetralogy of Fallot is rare in cats and the prognosis for Nimbus will depend on whether or not her disease progresses over time. At this time the owner has been advised to monitor for development of signs of increased respiratory rate or effort with play or stress, to have a PCV taken every 3 months by her referring veterinarian, and to return for a progress check in 6-7 months. Client was also instructed to call Dr. Wess if the PCV is greater than 50%, indicating worsening polycythemia.

Discharge Instructions

bulletThank you for bringing Nimbus to see us today. She is a darling kitty. As we discussed with you, we found that she does have a congenital heart defect called tetralogy of Fallot which consists of four lesions of the heart: the narrowing of the outflow tract of the right ventricle (pulmonic stenosis), the ventricular septal defect (the hole between the right and left ventricles), the malpositioned aorta, and the thickening of the muscle of the right ventricle.
bulletWhat this means is that some of the venous blood from the right heart that is supposed to go to the lungs is going out into the circulation to her body instead. That means that her blood is not carrying as much oxygen as normal (she is at about 35-40 mmHg when she is stressed instead of 100 mmHg oxygen in her blood).
bulletWe would like you to observe her for any development of clinical signs, which could include open-mouth breathing, panting, increased abdominal effort of breathing, or purplish or bluish color to her gums or tongue. Exercise, stress, or exuberant play may make these signs worse. If she appears to be having difficulty breathing that does not get better with rest, please call your veterinarian or us.
bulletIn order to monitor her condition we recommend that you have your veterinarian check her PCV (packed cell volume) every 3 months. This is a measure of how many red blood cells are in her blood. If the PCV is greater than 50% please call us. Today her PCV is normal at 38%.
bulletWe would like to see her back for a recheck cardiac ultrasound in 6 or 7 months.
bulletThank you for bringing her here; she is a cutie and we enjoyed meeting her.

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