Feb 29, 2012

Hemodynamics of Mitral Stenosis(MS)

Normal Pulmonary artery (PA) pressure is more than  pulmonary veins But this difference is less than 10mmhg
In early stages of Mitral stenosis with increase in pulmonary venous pressure there is corresponding increasing in pulmonary artery pressure also, but this increase is proportional to increase in pulmonary venous pressure.  By this, difference between pulmonary venous and pulmonary artery pressure never exceed more than 10mmhg.
With advanced mitral stenosis  (Mitral valve area less than 1.0 cm2) increase in pulmonary artery pressure is disproportional i.e. more than 10mmhg . This is because vasoconstriction of pulmonary artery and obliterative changes in pulmonary vascular bed. This called “Second Stenosis”
Note:-Left atrial pressure (LAP) is equal to pulmonary venous pressure and this is equal to pulmonary capillary wedge pressure (PCWP)
“If pressure difference between PCWP or LA pressure to  PA pressure is more than 10mmhg than it is considered as reactive pulmonary hypertension
In initial stages of MS major symptoms of MS are secondary to back pressure changes i.e. pulmonary venous hypertension presenting with orthopnea and PND , with progression of severity of obstruction i.e. Mitral valve are less than 1.0 cm2, symptoms of congestion of lungs reduce, due development “Second Stenosis”.  Symptoms of second stenosis produced i.e. easy fatigability due decreased cardiac output and symptoms of right heart failure develop like ascites and pedal oedma
C diagram shows development of reactive or oblitrative pulmonary artery pressure,  because difference between pulmonary artery mean and that of pulmonary veins mean pressure is more than 10 mmhg

Stages of MS with MVA and clinical features
Mitral valve area (MVA) in cm2
More than 2.5
1.4- 2.5
Dyspnea with exertion
Dyspnea , paroxysmal nocturnal Dyspnea, +/- pulmonary edema
Less than 1.0
Resting Dyspnea, NYHA class IV
Reactive  pulmonary hypertension
Less than 1.0
Easy fatigue , right heart failure signs

Heart rate and Mitral stenosis
Normal cardiac output is 5L/min, with normal mitral valve area of 4-5cm2 , this is achieved with minimal mitral valve gradient. With reduction of valve area to less than 2cm2  mitral valve gradient starts to increase,  at 1 cm2 mitral valve area  resting gradient of 8-10mmhg is needed to maintain 5L/min cardiac output.
As heart rate increase from normal of 72beats /min, diastole time interval reduces and to maintain normal cardiac output of 5L/min,  flow rate across mitral valve has to increase, this leads to increase pressure gradient across mitral valve (if exercise is demanding increased cardiac output i.e. more than 5L/min, than the mitral valve gradient increase is even more)
Normal oncotic pressure is 25mmhg. If pulmonary venous pressure goes above this, pulmonary edema develops
Patients who are otherwise asymmetric with mitral valve area of 2cm2 when develop atrial fibrillation, present with symptoms of pulmonary edema, is because of tachycardia (as explained above).