mitral stenosis- hemodynamics and pathophysiology
TRANSCRIPT
Anatomy of the normal mitral valveThe whole valve is like an apparatus, made up of
two leaflets suspended by about 120 chordae tendinae to two papillary muscles.
The valve is located between the LA and LV and projects into the LV.
It is essential to note that the whole apparatus should function properly in order that the valve functions properly.
It is a bicuspid valve.
There are two leaflets- anterior mitral leaflet(AML) and posterior mitral leaflet(PML).
The PML has 3 scallops and are named P1,P2, and P3.
The corresponding areas of the AML are known as A1,A2 and A3, though they do not have any scallops.
Mitral valve as seen from above.
The normal valve has an orifice of 4-6 square cms.
The valve functions as a door, allowing the entry of blood into the left ventricle during diastole and closing appropriately during systole to prevent the back flow of blood.
In mitral stenosis, the valve orifice becomes less than the normal size.
Minimal mitral stenosis is valve orifice>2.5 sq cmMild- 1.4-2.5sq cmModerate- 1.0-1.4 sq cmSevere- <1.0 sq cm
So when the valve orifice is stenosed,there is obstruction to flow of blood from the LA to the LV.
Whenever there is stenosis in the valves, there is development of a hemodynamic entity called pressure gradient.
What is pressure gradient?
In stenosis the pressure in the upstream chamber(LA) is really higher than the pressure in the downstream chamber(LV) during flow of blood from LA to LV because the LA tries very hard to empty its blood into the LV.
This is called a pressure gradient.
The mitral valve orifice area and the pressure gradient are inversely related to each other.
So if valve area decreases, the pressure gradient increases and if valve area increases, the pressure gradient decreases.
The pressure gradient also depends on the flow rate of blood through the chamber.
For a normal valve, as the flow rate increases(as in tachycardia due to exercise) the pressure gradient also increases, but only to a little extent.
However in a stenosed valve, the pressure gradient increases to a greater degree with the increase in the flow rate, and in severe stenosis, a pressure gradient may be present even at rest.
Diastolic pressure gradient(mm hg)
Flow rate ml/sec
The numbers 0.5,1,2,3,4 and 6 indicate the area of the mitral valve orifice in sq cm.Hashed area shows normal flow rate.
In the previous slide a graphical representation has been shown to describe the relation between the pressure gradient and flow rate.
It can be easily seen that for the valve having area 6 sq cm, changes in flow rate have a very mild effect on the pressure gradient.
However for the valve having area 0.5,even a normal flow rate produces an appreciable pressure gradient.
For valves having areas about 2-4sq cm, only increase in the flow rate(exercise) produces a significant increase in the pressure gradient.
Now due to the development of a pressure gradient, the LA has to work really hard to keep on pumping the blood and hence the LA pressure gradually increases.
In the initial stages it might be only on exertion, but later it might be elevated even at rest.
The elevated left atrial pressure is reflected on the pulmonary veins resulting in pulmonary venous hypertension and later pulmonary artery hypertension.
Pulmonary artery hypertension can be of two types, passive pulmonary hypertension(passive backward transmission of the LA pressure) or it can be of the reactive type, in which there is reflex constriction of the pulmonary arterioles in response to the elevated pulmonary vein pressure and the LA pressure.
What are the consequences of this pulmonary hypertension?
The main result is that it poses a great load to the RV because it has to now, pump blood against tremendous pressure continously, and hence after some time the RV undergoes hypertrophy and finally dilation.
What will happen if dilation occurs?
Normally the heart is elliptical in shape with a well defined apex. This shape contributes a lot to the contractile ability of the heart.
When dilation occurs, the shape of the heart becomes more spherical and the apex is lost.
The apical shape of the heart allows maximum shortening and lengthening of the muscle fibres and allows maximum contraction. So if this shape is lost contractile efficacy will be lost.
It is usually the spiral arrangement of the muscle fibres that gives the blood a real push towards the opposite side.
However when the original shape is lost, the muscle fibres become more transverse and hence the spiral shape is lost and so the ability of the heart to push blood in a specific direction is further decreased.
Can mitral stenosis cause tricuspid regurgitation? Yes , sometimes due to RV dilation, the tricuspid ring also gets dilated, due to the movement of the papillary muscle towards the lateral wall of the heart.
This type of regurgitation is called functional regurgitation.
What about the LA?
LA undergoes dilation, and that is why it is more prone to, atrial fibrillation and due to blood stasis, it is prone to develop thrombi.
Will any problem arise with the LV?
The ejection fraction will reduce because there is less amount of blood coming into the LV( there is a decrease in the preload).
The previous slide shows a flow chart of the pressures of the various chambers of the heart.
In the first panel, there is no abnormality in the valve and all the pressures are normal.
In panel2 the person has mitral stenosis and passive pulmonary hypertension but no reactive hypertension.
In panel3 the person has the same amount of mitral stenosis as in panel2 but here reactive pulmonary hypertension has developed.
Thank You!