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Physiology II
Cardiovascular Case A

atrial septal defect 

the patient: a 13-year-old boy 

reason for admission: a heart murmur

history: The patient was well and physically active with no cardiac symptoms. A murmur had been heard during a routine school sports physical examination performed by a second-year UNE/COM student on a preceptorship assignment.

clinical examination: The first heart sound of this boy was essentially normal with perhaps some enhancement of the component related to closure of the tricuspid valve. A midsystolic pulmonary ejection murmur was heard, and the second heart sound was widely split and did not change over the respiratory cycle. A mid-diastolic rumbling murmur was heard at the lower left sternal border. The electrocardiogram showed a right axis deviation (+ 120 degrees) in the frontal plane and prominent R waves in the right precordial leads. X-ray examination of the chest revealed enlargement of the right atrium and ventricle, dilation of the main pulmonary artery and its major branches, and evidence of abnormally large pulmonary blood flow (prominent "vascular markings"). Echocardiographic examination revealed normal cardiac valves and confirmed the increased right ventricular volume. During cardiac catheterization, measurement of relative oxygen content in the superior vena cava, inferior vena cava, right atrium, right ventricle, and pulmonary artery revealed increased content in the right atrium compared to the vena cava. The ratio of pulmonary blood flow to systemic blood flow was calculated to be 3:1.

STUDY QUESTIONS:

1 . What is the pattern of blood flow through the chambers of this patient's heart? How does it compare to the normal flow pattern?  Blood is flowing from the Left Atrium to the Right Atrium.  

2. What is meant by the "split" of the second heart sound and what is the significance of this finding?  The second heart sound is the closure of the pulmonic and aortic valves.  When they are split, that means that they are closing at different times.  Do to the extra volume entering the right side of the heart, the right ventricle will be stretched larger and would have a greater contractility.  The extra volume and strength would take longer to discharge the blood so the pulmonic valve will close a little later than the aortic valve.

3. Are the right ventricular and left ventricular preloads normal? Why or why not?  The right ventricular preload is higher than normal related to the increased amount of blood coming from the LA into the RA into the RV.  This extra blood would create a larger preload.  Since blood is flowing from the LA into the RA, there is not as much blood as expected flowing into the LV so its preload is less than normal.

4. How was the 3:1 ratio of pulmonary to systemic blood flow calculated or measured? What is this ratio normally?  Normally the ratio is 1:1 because the same amount of blood going into the LV just came from the RV.  It is measured by checking and calculating off from the oxygen "tagged" RBC's.

5. How could this patient be physically active and yet asymptomatic?  His heart has grown larger to compensate for the lack in oxygenated blood.

 


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