Friday, December 2, 2016

Dynamic LVOT obstruction

Dynamic LVOT obstructin can occur due to several causes:
1. The most well-known cause is hypertrophic cardiomyopathy (HCM). The hypertrophied basal septal (the most common cause of HCM) causes narrowing of LVOT that is dependant on the preload and afterload. Sometimes the obstruction is not obvious and requires provocation with Valsalva's maneuver or vasodilators.
2. Narrowing of LVOT due to other causes (e.g.: subaortic membrane) can cause dynamic obstruction. So, with subaortic membrane part of the obstruction is fixed due to the presence of membrane itself. but sometimes there is a dynamic component due to the associated systolic anterior motion (SAM) of the anterior mitral leaflet. This is due to the Venturi effect, as increased velocity in the LVOT bulls the anterior mitral leaflet causing further narrowing of the LVOT.
3. With increased contractility of the basal portions of LV. This can occur in different situations. For example compensatory hypercontractility of the basal segments in cases of acute myocardial infarction involving the apical and mid segments. Another example is in case of stress cardiomypathy that typically involves the apical segments with the famous apical ballooning picture. The basal parts usually tries to compensate with increased contractility to keep the cardiac output unaffected. The third condition is increased contractility due to dminstration of dobutamine as during dobutamine stress echocardiographym, dynamic LVOT obstruction can occur and is one of the possible causes of hypotension that is worsened by the vasodilator effect of dobuamine. The presence of concentric LVH with small cavity is predisposing condition in all the three situation given here.
4. Cardiac amyloidosis should be suspected in dynamic LVOT obstruction. It can cause septal hypertrophy in association with reduced LV filling. Both togeather may lead to LVOT obstruction.
5. Mitral valve repair with ring: Here the anterior mitral leaflet is pushed rather than pulled into the LVOT in contrary to the other types of LVOT obstruction. Small LV cavity and long posterior leaflet predisposes to this situation.
6. Increased velocity in the LVOT with increase flow as in exercise, sepsis and severe anemias has been shown to cause LVOT obstruction in some cases. Here there is reduction of the afterload associated with tachycardia that causes reduced filling time and smalled LV cavity predisposing to dynamic LVOT obstruction.
7. Aortic valve replacement for treatment of aortic stenosis. Long standing aortic stenosis causes significant LVH. But as there is increased afterload due to the fixed obstruction at the valve level, this keeps the LV walls apart and no dynamic component is present. As the fixed obstruction is relieved with aortic valve replacement, the dynamic obstruction is unmasked. Dynamic obstruction here is more commen to happen at the midcavity level rather than LVOT.
8. Sigmoid septum in elderly patients can cause dynamic LVOT obstruction due to abnormal aortic angle but yet the exact mechanism is unclear.
9. In cases of L-TGA, the LVOT is open into the low pressure pulmonary circulation while the right ventricle is exposed to the systemic higher pressure circulation. This causes bulging of the interventricular septum to the left ventricle and subsequently may cause dynamic LVOT obstruction of a unique mechanism

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