A SEMI-AUTOMATED NON-INVASIVE SYSTEM FOR MEASUREMENT OF HUMAN AORTIC INPUT IMPEDANCE

M. Karamanoglu, D. Hopkins*, M.P. Feneley.

Cardiology Department and Victor Chang Cardiac Research Institute, St Vincent's Hospital, Sydney, New South Wales.

The aortic input impedance spectrum Z_AA(w), uniquely describes the left ventricular afterload, but its use is limited by the need for invasive measurements of aortic flow and pressure.  We have developed a semi-automated system which calculates the volumetric aortic flow from the aortic flow velocity and aortic valve annulus area using pulsed Doppler spectra and 2D-echocardiography.  The system automatically derives a series of Doppler flow velocity envelopes from captured video images of the Doppler spectra, which are subsequently calibrated and averaged.  The aortic pressure waveform is synthesised non-invasively from calibrated finger (FINAPRES) pressure measurements and an individualised pressure transfer function of the upper limb.  Simultaneously measured, calibrated finger and non-calibrated carotid pressure waveforms are used for this purpose.  The entire operation takes between 5 and 10 minutes to perform.  We tested this system in 10 healthy subjects (3 males, aged 53.6±11.3 years, mean±SD) before and after 0.3mg sublingual nitroglycerin.

Results: Z_AA(w), values were similar to those published in the literature for a similar group:  peripheral resistance (1880.1±526.3 dyn.sec.cm^-5), characteristic impedance (76.0±100.1 dyn.sec.cm^-5) and the first zero crossing frequency of the phase (4.7±0.4 Hz).  Nitroglycerin did not alter these parameters but reduced the modulus at first harmonic (230.8±69.0 vs 196.3±46.6 dyn.sec.cm^-5, p<0.05, paired t-test).

Conclusion:  This system permits completely non-invasive determination of arterial impedance for assessing ventricular/vascular interaction in human subjects.

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