Low gradient severe mitral stenosis has been defined as mean transmitral gradient <10 mm Hg in patients with mitral valve area ≤1.5 cm2 . A low flow subgroup has been defined with left ventricular stroke volume index ≤35 mL/m2. In a study of 101 patients with severe rheumatic mitral stenosis who underwent balloon valvuloplasty, low gradient was present in 55 patients and low flow low gradient in 11 patients .
Low flow low gradient mitral stenosis patients were older, had higher rates of atrial fibrillation, arterial afterload, subvalvular thickening and decreased left ventricular compliance compared to high gradient mitral stenosis . 40% of those with low gradient mitral stenosis did not have symptomatic benefit compared to 18% with high gradient, after valvuloplasty.
European Association of Echocardiography/American Society of Echocardiography recommendation in 2009 had defined severe mitral stenosis as valve area <1 cm2 . 2014 AHA/ACC Guideline defines severe mitral stenosis as valve area ≤1.5 cm2 .
An earlier study of 180 patients who underwent percutaneous balloon mitral valvuloplasty found 36 patients with transmitral gradient ≤10 mm Hg . 24 of these 36 patients (67%) had reduced left ventricular ejection fraction. Those with reduced left ventricular ejection fraction were noted to have left ventricular wall motion abnormalities on ventriculography. Balloon valvuloplasty did not significantly improve symptoms in those who had pre procedure left ventricular ejection fraction ≤35% .
A similar study in patients undergoing mitral valve replacement for very severe mitral stenosis has been done by Cho IJ et al . Among 140 patients who underwent mitral valve replacement for pure valvular mitral stenosis with valve area ≤1 cm2 by planimetry, low gradient of < 10 mm Hg was noted in 82 patients (59%). Low gradient patients were older and more likely to have diabetes mellitus, atrial fibrillation and female gender. Their left atrial volume index was larger and left ventricular strain rate during isovolumic relaxation was lower. Percentage reduction of left atrial volume index after mitral valve replacement was also smaller. Even though pre-operative functional class was similar, persistent symptoms after mitral valve replacement were more common in the low gradient group .