Pulmonic Stenosis
History. A 6-month-old female schnauzer is referred to your clinic because of a heart murmur that was detected during a routine health care visit. The puppy is fairly active but is slightly smaller than her female Iittermates.
She also tires more quickly than her Iittermates when they play together.Clinical Examination. All physical parameters are normal except for a systolic heart murmur that can be heard best over the left third to fourth intercostal space. Femoral pulses are normal, and the jugular veins are not distended. Electrocardiography reveals that the dog is in normal sinus rhythm with a heart rate of 118 beats/min. The PR interval is normal. However, the major QRS deflection is negative in leads I and aVF. Also, deep S waves are noted in leads II and III, and the QRS complexes are slightly prolonged as a result of the wide S waves. Thoracic radiographs show right ventricular enlargement; the right border of the cardiac silhouette is more rounded, and closer to the right thoracic wall, than normal.
A catheter is inserted into the jugular vein, and the following pressures are measured as the catheter is advanced through the right side of the heart and into the pulmonary artery: central venous pressure (mean right atrial pressure), 8 mm Hg (normal, 3 mm Hg); right ventricular systolic pressure, 122 mm Hg (normal, 20 mm Hg); and pulmonary artery systolic pressure, 16mm Hg (normal, 20 mm Hg).
The jugular catheter is withdrawn until the catheter tip is in the right ventricle. Additional radiographs are then taken while a radiopaque dye is injected through the catheter. These radiographs reveal that the right ventricular outflow tract is narrowed just below the pulmonic valve and that the pulmonic valve does not open widely during ventricular systole.
Comment. The young age of this dog and the absence of other signs of illness suggest that the murmur results from a congenital cardiac abnormality.
Murmurs are graded on a scale of I through VI, with VI being the most severe. This dogs murmur is graded IV. A systolic murmur can result from aortic or pulmonic stenosis, mitral or tricuspid regurgitation, or a ventricular septal defect (see Figure 21-8). On the basis of the location from which this murmur can be heard best, aortic or pulmonic stenosis is the most likely cause. All the additional clinical evidence supports a diagnosis of pulmonic stenosis.The electrocardiogram indicates that the sinoatrial node is acting as the pacemaker and that the AV node is conducting each atrial action potential into the ventricles. However, the abnormalities observed in the polarity and shape of the QRS complex are indicative of right ventricular hypertrophy, and the radiographs corroborate this finding. Pulmonic stenosis leads to right ventricular hypertrophy, because the right ventricle must generate much higher pressures than normal during systole in order to eject blood through the narrow outflow tract.
Normally, the pulmonic valve opens widely during systole, and the ventricular systolic pressure closely matches the pulmonary artery systolic pressure. In this dog, there is a difference ot 106 mm Hg between right ventricular systolic pressure and the systolic pressure in the pulmonary artery just beyond the pulmonic valve. This difference indicates a severe pulmonic obstruction. The degree of obstruction can be visualized on the radiographs taken during dye injection.
Right ventricular hypertrophy is one of two adaptive responses that help this dog maintain a near-normal right ventricular stroke volume, despite the pulmonic stenosis. The other adaptive response is that the mean right atrial pressure is higher than normal (8 vs. 3 mm Hg). The right atrial pressure is elevated because blood backs up or accumulates in areas upstream from the stenosis (i.e., in the right ventricle, right atrium, and systemic veins). The elevated atrial pressure is adaptive because it increases the right ventricular preload, which increases the end-diastolic volume, which (according to Starling’s law of the heart) helps keep the right ventricular stroke volume at a normal level, despite the stenosis.
The right atrial pressure is not quite high enough in this dog to cause systemic edema or abdominal ascites (see Chapter 23). However, both these signs are sometimes seen in dogs with severe pulmonic stenosis because excessively elevated right atrial pressure leads to marked increases in capillary hydrostatic pressure (upstream from the right atrium).The combined effects of right ventricular hypertrophy and elevated right ventricular preload allow this dog’s heart to pump a near-normal stroke volume during rest. However, the pulmonic obstruction limits the increase in the stroke volume that can occur during exercise. The resulting limitation in cardiac output accounts for this dog’s lack of stamina during exercise. Over a prolonged period, such a limitation in cardiac output can also stunt growth.
Treatment. Theoretically, the best treatment for pulmonic stenosis is to remove the obstruction surgically. A valve dilator can be used, or an artificial conduit can be installed across the stenotic valve. Although seriously affected dogs require such interventional treatment, dogs with mild to moderate pulmonic stenosis can lead sedentary lives without any treatment.
Some evidence indicates that the adverse effects of pulmonic stenosis can be minimized by the administration of β-adrenergic antagonists (e.g., propranolol) or calcium channel blockers (e.g., verapamil). Although the mechanism and efficacy of these drugs remain unclear, there is speculation that these drugs are beneficial because they limit ventricular contractility, which limits the work of the heart. Because an increase in cardiac work is the stimulus for hypertrophy, a drug that limits the increase in work also limits the hypertrophy. Although moderate hypertrophy can be adaptive (as explained earlier), excessive hypertrophy is detrimental for two reasons. First, the enlarged ventricular muscle can crowd the pulmonic outflow tract, worsening the stenosis. Second, the coronary circulation may be unable to deliver the increased amounts of blood flow required by the massive ventricular muscle.