RVOT obstruction is the sine qua non of TOF. It is traditional to believe the obstruction in TOF is dynamic and is located in the infundibulum. It may be true in a broad sense .But in reality the blood flow faces too many hurdles within the RV before it reaches the pulmonary artery and lungs there after.
The commonest and most important one being the mal-aligned conal septum encroaching the infundibulum .(This anterior migration of conal septum is responsible for the aortic over ride and VSD as well) .It is erroneous to think the RVH in TOF is simply an after effect of RVOT obstruction .There can be intrinsic defects in the RV trabecuale that hypertrophies and traverses the RV cavity in randon fashion.
Soto described 6 types of obstruction in TOF in elegant anatomic and pathologic study in 1981. Every cardiology fellow must read this original article before going to the Board examination. http://circ.ahajournals.org/content/64/3/558.full.pdf+html
For some reason God has not arranged the RV inflow , body and out flow in a linear fashion . ( ? Meant for haemic acceleration in the low pressure venous circuit ) .In TOF this becomes important. It is curious to note even minor muscle bundles that criss cross the RV body act as a speed breaker and alter the stream and direction of blood flow .This is why , TOF can generate systolic murmurs in various shapes and time over the left para-sternal area .(In TOF one can get a murmur right from left 2nd space to well down the lower sternal area )
What are the fixed obstruction in TOF ?
The resistance to blood flow within the RV is often multiple , extend from RV body to pulmonary arterial branch points. It is important to realise few of the obstructions are fixed in nature. Differentiation of dynamic vs static obstruction is important in therapeutic aspect also. The efficacy of beta blockers is directly related to the ratio of dynamic vs fixed resistance .
Hypertrophied trabecuale sept0 margianlis (TSM) usually offers fixed resistance. The infundiubulm is the only place where one can expect a dynamic component . If the annulus and valvualr PS caused more of a fixed obstruction
So, fellows beware if some one asks this question “Where is the site of obstruction in TOF ” .Be ready with an elaborate answer . It is better to classify according to sites of obstruction with specific reference to dynamic or static nature .