Syncope is a classical feature of LVOT obstruction especially with valvular aortic stenosis.The mechanism of exertional syncope in Aortic stenosis is traditionally attributable to the fixed obstruction .This fixed obstruction is not able to cope up with increased cardiac output as demanded by the exercising muscles . But exercise induced reflex as well as local vaso-dilatation mechanism is intact . The consequence is predictable. A critical fall in SVR amidst a obstructed LVOT precipitating a syncope.
However , If the above mechanism is the sole reason for syncope in Aortic stenosis , we have a problem to explain why syncope is rare even in critical mitral stenosis which is also fixed LV inflow obstruction ?
Is there some thing unique in LVOT obstruction that causes syncope ?
No, it is nothing to do with LVOT .To generate a true pathological syncope, reduction in cardiac output per-se may not be enough . It appears there should be an inappropriate systemic vasodilatation as well to precipitate a syncope.This can happen only if the parasympathetic system gets activated by some means . The trigger is located in the mechano- receptors of left ventricle . Hypertophied left ventricle with high Intra cavitory pressure (Often above 200mmhg) generated due to LVOT obstruction activates the syncope circuit.The same rule may apply for RVOT as well .One could get syncope with critical valvular PS or severe pulmonary hypertension when RV mechanical receptors get a triggered.
What happens in mitral stenosis ?
In mitral stenosis , LV is under- filled , wall thickness is normal .There is little likely-hood of LV mechno-receptors to get stimulated as the LV wall stress is normal. This is the reason syncope is less common in mitral stenosis. However , this is not absolute rule , syncope can still occur in severely narrowed orifice of mitral valve due to low flow state alone or a ball valve thrombus and paroxysms of arrhythmia .