Case Studies In Small Animal

Cardiovascular Medicine

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Case 3

Case Discussion

This dog had a small ventricular septal defect (VSD) that produced little hemodynamic change. Consequently, no intervention was recommended and he has continued to do well. Note in this dog that the VSD was small despite the dog having a very loud heart murmur (which is common). The murmur is loud because it is high velocity and so develops a lot of force (power) and it is directed right at the free wall of the right ventricle which lies right under the chest wall where a stethoscope is placed.

The thoracic radiographs were difficult to read because of the dog's chest configuration (i.e. bulldog configuration). Consequently, the radiologists were hesitant to read too much into them. They did, however, think the dog might have some left heart enlargement and did not think the pulmonary vasculature looked enlarged. They noted the spinal abnormality at the eighth thoracic vertebra.

The ECG was interesting. The orientation to the initial part of the QRS complexes was normal although the complexes themselves were small. The terminal portion of the QRS complexes, however, were abnormally oriented. If you look at leads I and II, which are recorded simultaneously, you will see that there is an S wave in lead II. During the time that this S wave is inscribed, lead I has no wave at all (it is at baseline). Leads III and aVF also have S waves. This means the terminal portion of the ECG is directed at the head (-90 degrees). In this case, this could be a terminal right axis deviation or left axis deviation. The terminal portion is important since it represents the last parts of the ventricles to be depolarized. In the case of hypertrophy, the largest ventricle is usually depolarized last. In the case of a intraventricular conduction abnormality (e.g., left or right bundle branch block) the portion of the ventricle depolarized last is the part that has lost its conduction system. Since this dog had minimal cardiac enlargement, the abnormality in this dog is most likely due to a conduction system abnormality. Its exact nature, however, cannot be specifically determined.

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The size of the defect appeared to be small anatomically on the two-dimensional echocardiogram. This, however, can be misleading. However, the left ventricular chamber was only mildly enlarged and the velocity of the jet was close to 5 meters/second. These confirmed that the defect was small. A small defect results in a small amount of left-to-right shunting and a small increase in venous return to the left ventricle. Consequently, the left ventricle is only mildly increased in size. With a large defect there is a large amount of left-to-right shunting and so a large increase in venous return to the left ventricle and a marked increase in left ventricular size (assuming that there is no pulmonary vascular disease). Also with a ventricular septal defect, the velocity of the jet decreases as the orifice becomes larger, just the opposite to what occurs with a stenotic lesion. Also as the orifice increases in size, more flow goes through it increasing pulmonary blood flow. As pulmonary blood flow increases, systolic pulmonary artery and right ventricular systolic pressures increase. Left ventricular systolic pressure stays the same so the pressure gradient (i.e., difference) across the interventricular septum decreases (i.e., left ventricular systolic pressure minus right ventricular systolic pressure decreases). Since flow velocity is related to pressure gradient by the modified Bernoulli equation (4 times the square of the velocity), flow velocity decreases as the orifice becomes larger.

Schematic Drawing of a Large VSD

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This schematic shows the blood flow (arrows) in a large VSD. Oxygen saturation is depicted as the numbers in circles. Blood pressures are depicted as systolic over diastolic (e.g., 65/4 for the right ventricle). The pulmonary vasculature is distended ("overcirculation") and there is pulmonary hypertension. The pulmonary hypertension at this stage is primarily due to the increase in blood flow through the pulmonary vasculature. Pulmonary blood flow is approximately 3 times systemic blood flow.

 

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