Early left ventricular diastolic function quantitation using directional impedances.

Impedance has been used in vascular biology to characterize the frequency dependent opposition the circulatory system presents to blood flow in response to a pulsatile pressure gradient. It has also been used to characterize diastolic function (DF) via the early, diastolic left ventricular (LV) pressure-flow relation. In a normal LV, early filling volume is accommodated primarily by wall-thinning and ascent of the mitral annulus relative to the spatially fixed apex (longitudinal chamber expansion). Simultaneously, the endocardial (transverse or short axis) dimension also increases while epicardial (transverse) external dimension remains essentially constant. To quantify these directional filling attributes, we compute longitudinal (Z(L)) and transverse (Z(T)) impedances during early rapid-filling (Doppler E-wave). Z(L) and Z(T) were calculated from 578 cardiac cycles of simultaneous transmitral flow and high fidelity LV pressure data in 17 subjects with normal LV function. Average Z(L) was 0.7 ± 0.4 mmHg s/cm(4) and average Z(T) was 238 ± 316 mmHg s/cm(2). Direct comparison, in the same units is achieved by computing Z(T) over the ≈10 cm(2) cross-sectional area of LV (denoted ŽT) revealing that Z(L) is ≈34 times smaller than Ž(T). This quantifies the physiologic preference for longitudinal LV volume accommodation. Lowest Z(L) and Z(T) values occurred in the first harmonic with monotonically increasing values with higher harmonics. We conclude that Z(L) and Z(T) characterize longitudinal and transverse chamber properties of DF and therefore, diastolic dysfunction can be viewed as a state of impedance mismatch.