The relationship between aortic flow and pressure is described by a three-element model of the arterial input impedance, including continuous correction for variations in the diameter and the compliance of the aorta (Modelflow). We computed the aortic flow from arterial pressure by this model, and evaluated whether, under orthostatic stress, flow may be derived from both an invasive and a non-invasive determination of arterial pressure. In 10 young adults, Modelflow stroke volume (MFSV) was computed from both intra-brachial arterial pressure (IAP) and non-invasive finger pressure (FINAP) measurements. For comparison, a computer-controlled series of four thermodilution estimates (thermodilution-determined stroke volume; TDSV) were averaged for the following positions: supine, standing, head-down tilt at 20 ° (HDT20) and head-up tilt at 30 ° and 70 ° (HUT30 and HUT70 respectively). Data from one subject were discarded due to malfunctioning thermodilution injections. A total of 155 recordings from 160 series were available for comparison. The supine TDSV of 113±13 ml (mean±S.D.) dropped by 40% to 68±14 ml during standing, by 24% to 86±12 ml during HUT30, and by 51% to 55±15 ml during HUT70. During HDT20, TDSV was 114±13 ml. MFSV for IAP underestimated TDSV during HDT20 (-6±6 ml; P < 0.05), but that for FINAP did not (-4±7 ml; not significant). For HUT70 and standing, MFSV for IAP overestimated TDSV by 11±10 ml (HUT70; P < 0.01) and 12±9 ml (standing; P < 0.01). However, the offset of MFSV for FINAP was not significant for either HUT70 (3±8 ml) or standing (3±9 ml). In conclusion, due to orthostasis, changes in the aortic transmural pressure may lead to an offset in MFSV from IAP. However, Modelflow correctly calculated aortic flow from non-invasively determined finger pressure during orthostasis.

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