Review Question - QID 100636

This patient's echocardiogram reveals the classic findings of tetralogy of Fallot (TOF): pulmonic stenosis, VSD, RVH, and overriding aorta. The degree of pulmonic stenosis is the best predictor of the degree of cyanosis in patients with TOF.

In TOF, the VSD is usually so large that right ventricular pressure is equal to left ventricular pressure. Consequently, blood flow direction is determined by the path of least resistance (i.e., through the PA or the aorta), and hence by the degree of pulmonic stenosis. If there is not a great degree of pulmonic stenosis, RV outflow tract resistance will be lower than LV outflow tract resistance through the aorta. Consequently, blood will flow from the LV through the VSD into the RV (left-to-right shunt). This results in an acyanotic condition. If there is a high degree of pulmonic stenosis, RV outflow tract resistance will be higher than LV outflow tract resistance, and blood will from from the RV through the VSD into the LV (right-to-left shunt). This results in a cyanotic lesion.

Saenz et al. present an overview of the care of infants with congenital heart disease like TOF. They note that suspicion of congenital heart disease should be heightened in patients with tachypnea and feeding difficulties, diaphoresis and subcostal recession, and severe growth impairment.

Castleberry et al. studied the stenting of the RV outflow tract (RVOT) in high-risk infants with cyanotic TOF prior to definitive surgical repair. They report placement of an RVOT stent in 5 patients, who showed a significant improvement in oxygen saturation (71% to 94%) 24 hours after stent placement.

Illustration A shows EKG findings in TOF.
Illustration B shows pressure considerations in TOF.
Video V shows a video explanation of the structure of the heart in TOF, complete with animation.

Incorrect Answer:
Answer 2: As described above, the size of the VSD is usually so great in TOF that it does not directly affect the degree of cyanosis.
Answer 3: Degree of RVH is not the best predictor of cyanosis in TOF patients.
Answer 4: Degree to which aorta overrides the RV is not the best predictor of cyanosis in TOF patients.
Answer 5: An S3 or S4 is uncommon in TOF patients.