Quantitative assessment of left ventricular diastolic function via Longitudinal and Transverse flow impedances

Flow impedance has been used to characterize the physical properties of the vascular system by assessing its phasic flow response to pulsatile pressure input in terms of resistance as a function of frequency. Impedance has also been used to characterize global diastolic left ventricular (LV) chamber properties. In early diastole the LV is a mechanical suction pump and accommodates filling by simultaneously expanding in two principal spatial directions: longitudinal (base-to-apex, long-axis) and transverse (radial, short-axis). Total (characteristic) impedance Z_C is the product of longitudinal (Z_L) and transverse (Z_T) impedance as Z_C²=Z_LZ_T where the two impedances reflect the relative spatial propensity for volume accommodation. In this work we compute Z_L and Z_T for the LV in early diastole. We analyze simultaneously recorded dual pressure-transducer and transthoracic echocardiographic flow data obtained during cardiac catheterization in 11 subjects. We found that Z_L was 2 orders of magnitude smaller than Z_T in all subjects, providing the first hemodynamic evidence, in concordance with cine-MRI imaging data that longitudinal volume accommodation is indeed, nature´s preferred spatial filling mechanism. We also investigated the effect of impaired diastolic function on directional impedances and found that Z_L increased (becomes worse) while Z_T decreased (becomes better) indicating that as diastolic function becomes impaired radial filling compensates for decreased longitudinal volume accommodation to preserve stroke volume. These results provide mechanistic insight and show that normal diastolic function defines a properly impedance matched state and that diastolic dysfunction is equivalent to a state of impedance mismatch.