Mitral Regurgitation

Mitral Regurgitation: Primary vs. Secondary Mechanisms

Mitral regurgitation (MR) is classified by mechanism, which determines both severity thresholds and treatment strategy:

• Primary (organic/degenerative) MR: intrinsic leaflet or chordal pathology — myxomatous degeneration with prolapse or flail leaflet, ruptured chordae, rheumatic disease, infective endocarditis, or a cleft leaflet. The valve itself is diseased.
• Secondary (functional) MR: structurally normal leaflets that fail to coapt because of left ventricular remodeling (ischemic or non-ischemic cardiomyopathy) or annular dilation, which displace the papillary muscles and tether the leaflets below the annular plane. The problem is ventricular/annular geometry, not the valve.

The Carpentier functional classification links leaflet motion to mechanism:

• Type I — normal leaflet motion (annular dilation or leaflet perforation).
• Type II — excessive leaflet motion (prolapse/flail), the classic primary-MR pattern.
• Type IIIa — restricted motion in both systole and diastole, typically rheumatic.
• Type IIIb — restricted motion in systole only, the classic secondary/ischemic MR pattern from leaflet tethering.

Severity Grading

Because secondary MR carries a worse prognosis at lower regurgitant volumes than primary MR (a smaller leak in an already-failing ventricle is proportionally more consequential), the ASE 2017 guideline sets different severe thresholds for primary and secondary MR:

An EROA of 0.35 cm², for example, is only moderate-to-severe for primary MR but is graded severe for secondary MR — a clinically important distinction because it changes eligibility for transcatheter edge-to-edge repair.

Quantifying MR: The PISA Method

The proximal isovelocity surface area (PISA) method is the standard quantitative technique:

1. On color Doppler, the Nyquist limit is shifted toward the direction of flow (typically to 20–40 cm/s) to create a visible, single hemispheric flow-convergence zone proximal to the regurgitant orifice.
2. The radius (r) of that hemisphere is measured at mid-systole.
3. EROA is calculated as: EROA (cm²) = (2π × r² × V_alias) ÷ V_peak,MR, where V_alias is the Nyquist aliasing velocity and V_peak,MR is the peak MR jet velocity from continuous-wave Doppler.
4. Regurgitant volume = EROA × velocity-time integral (VTI) of the CW MR jet.
5. Regurgitant fraction = regurgitant volume ÷ total transmitral (or total LV) stroke volume.

PISA assumes a flat, circular orifice and a hemispheric convergence zone — assumptions that break down with multiple jets, eccentric/non-circular orifices (common in secondary MR and bileaflet prolapse), or when the convergence zone is truncated by an adjacent structure. 3D vena contracta area (3D VCA), measured directly by 3D color Doppler viewed en face on the regurgitant orifice, avoids the circular-orifice and single-jet assumptions and is increasingly used to cross-check or replace PISA-based EROA, particularly in secondary MR.

Vena Contracta Pitfalls

2D vena contracta width, like its counterpart in aortic regurgitation, assumes a circular orifice; in eccentric or multiple jets (common with bileaflet prolapse or annular dilation) it can misrepresent true regurgitant severity. Averaging measurements from orthogonal planes (parasternal long-axis and apical views) or using 3D VCA improves accuracy when the jet is non-circular.

Jet Direction as a Mechanism Clue

On color Doppler, an eccentric jet direction is a useful sonographic clue to the underlying leaflet pathology, though it should always be confirmed with 2D/3D structural imaging rather than used alone. A jet that hugs and is directed posteriorly (toward the posterior LA wall) suggests anterior leaflet prolapse or flail, while a jet directed anteriorly suggests posterior leaflet prolapse or flail — the jet is deflected away from the abnormally mobile leaflet by the Coanda (wall-hugging) effect. Eccentric wall-hugging jets characteristically underestimate true severity on color-flow jet-area assessment, which is why vena contracta, PISA-derived EROA, or 3D VCA are preferred over jet area whenever a jet is eccentric.

LV Size and Function as Intervention Triggers

As with chronic AR, chronic severe primary MR can remain asymptomatic while progressively remodeling and eventually decompensating the LV, so serial LV size and EF trending guides timing of intervention. Current ACC/AHA guidance targets repair before the LV reaches an ejection fraction <60% or an LV end-systolic dimension ≥40 mm, since a preoperative LVEF below 60% is associated with a higher risk that LV function will not normalize even after successful repair. This is a key reason serial, reproducible LV volumetric and linear-dimension measurement (Chapter 6) is emphasized for every patient carrying a diagnosis of severe primary MR, symptomatic or not.

Key Testing Points

• Primary MR = intrinsic valve/chordal disease (Carpentier Type II most common — prolapse/flail); secondary MR = normal leaflets tethered by LV/annular remodeling (Carpentier Type IIIb).
• Severe primary MR: EROA ≥0.40 cm², regurgitant volume ≥60 mL/beat, vena contracta ≥0.7 cm, regurgitant fraction ≥50%.
• Severe secondary MR uses lower thresholds: EROA ≥0.30 cm², regurgitant volume ≥45 mL/beat.
• PISA-derived EROA = (2πr² × aliasing velocity) ÷ peak MR velocity; regurgitant volume = EROA × CW VTI.
• 3D vena contracta area avoids PISA's circular-orifice/single-jet assumptions, useful in secondary MR and multiple jets.