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 (ZL) and transverse (ZT) impedances during early rapid-filling (Doppler E-wave). ZL and ZT were calculated from 578 cardiac cycles of simultaneous transmitral flow and high fidelity LV pressure data in 17 subjects with normal LV function. Average ZL was 0.7 ± 0.4 mmHg s/cm4 and average ZT was 238 ± 316 mmHg s/cm2. Direct comparison, in the same units is achieved by computing ZT over the ≈10 cm2 cross-sectional area of LV (denoted ŽT) revealing that ZL is ≈34 times smaller than ŽT. This quantifies the physiologic preference for longitudinal LV volume accommodation. Lowest ZL and ZT values occurred in the first harmonic with monotonically increasing values with higher harmonics. We conclude that ZL and ZT characterize longitudinal and transverse chamber properties of DF and therefore, diastolic dysfunction can be viewed as a state of impedance mismatch.