Analysis of mitral inertance: a factor critical for early transmitral filling

The upslope of the transmitral E wave depends on the combined influence of the rate of change of the atrioventricular gradient and the inertial mass of blood within the mitral apparatus (inertance). To use observed transmitral velocity to predict the atrioventricular pressure (delta p) difference requires knowledge of the magnitude of mitral inertance (M, inertial mass divided by effective orifice area), closely related to the length over which blood accelerates and decelerates on passing through the valve. To define the magnitude and determining factors for mitral inertance in typical valvular geometries, we used an in vitro model in which a known atrioventricular gradient (delta p, range 3.8 to 39 mm Hg) was applied instantaneously to orifices (areas of 0.5, 1, 1.5, 2, and 2.5 cm2) and conduits (volume 2.5 to 24 ml). Continuous wave Doppler spectra were recorded and the slope (dv/dt) of the tangent to the upslope was measured manually. From slope and pressure difference, inertance was calculated as delta p/(dv/dt). In 103 combinations of pressure gradients and orifices or conduits, inertance ranged between 1.9 and 12.7 gm/cm2. Linear regression showed that inertance M was highly correlated with orifice diameter D (M = 3.17 D; r = 0.84; p < 0.0001) and, in the conduits, with diameter and length L (M = 4.1 D + 0.7 L-1.8; r = 0.87; p < 0.0001). Inertance was not significantly related to the pressure gradient. In conclusion, inertance depends mainly on the mitral apparatus geometry and most strongly on orifice diameter. Knowledge of mitral inertance should help to extract quantitative data on atrioventricular pressure difference from the upslope of the transmitral E wave.