Bedside and continuous assessment of arterial load in critically-ill patients by combined analysis of esophageal doppler blood flow and arterial pressure waveform: preliminary results

Objectives To test the usefulness of continuous assessment of arterial load based on a 3-element Windkessel model by a combined analysis of the esophageal Doppler derived-blood flow and arterial pressure waveform, against standard frequency-domain analysis of arterial impedance. Methods Time-domain variables of arterial impedance were obtained by the simultaneous analysis of the Doppler derived-blood flow and arterial pressure. A 3-element Windkessel model was used, consisting on arterial resistance (R = mean arterial pressure/cardiac output), arterial compliance [C = stroke volume (SV)/arterial pulse pressure), characteristic impedance [Zct = maximum derivative of pressure (dPmax/dt)/maximum derivative of aortic blood flow (dQmax/dt)] (Figure 1). Effective arterial elastance (Ea = 90% of systolic arterial pressure/SV) was used as a lumped parameter of the whole arterial load. Frequencydomain analysis of the pressure-blood flow waveforms was obtained to estimate the input impedance (Zin), characteristic impedance (Zc, average of the modulus of 4