Mitral inertance in humans: critical factor in Doppler estimation of transvalvular pressure gradients

The pressure-velocity relationship across the normal mitral valve is approx imated by the Bernoulli equation DeltaP = 1/2 rho Deltav(2) + M . dv/dt, wh ere DeltaP is the atrioventricular pressure difference, rho is blood densit y, v is transmitral flow velocity, and M is mitral inertance. Although M is indispensable in assessing transvalvular pressure differences from transmi tral flow, this term is poorly understood. We measured intraoperative high- fidelity left atrial and ventricular pressures and simultaneous transmitral flow velocities by using transesophageal echocardiography in 100 beats (8 patients). We computed mean mitral inertance ((M) over bar) by (M) over bar = integral(DeltaP - 1/2 . rhov(2))dt/integral (dv/dt)dt and we assessed th e effect of the inertial term on the transmitral pressure-flow relation. (M ) over bar ranged from 1.03 to 5.96 g/cm(2) (mean = 3.82 +/- 1.22 g/cm(2)). DeltaP calculated from the simplified Bernoulli equation (DeltaP = 1/2 . r hov(2)) lagged behind (44 +/- 11 ms) and underestimated the actual peak pre ssures (2.3 +/- 1.1 mmHg). (M) over bar correlated with left ventricular sy stolic pressure (r = -0.68, P< 0.0001) and transmitral pressure gradients ( r = 0.65, P< 0.0001). Because mitral inertance causes the velocity to lag s ignificantly behind the actual pressure gradient, it needs to be considered when assessing diastolic filling and the pressure difference across normal mitral valves.