Continuous stroke volume monitoring by modelling flow from non-invasive measurement of arterial pressure in humans under orthostatic stress

Abstract

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 degrees (HDT20) and head-up tilt at 30 degrees and 70 degrees (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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“…A recent study, comparing SV obtained from ModelXow (using Portapres) versus a rebreathing method, indicates that Modelfow overestimates Q and has large inter-individual variations (Pitt et al 2004). However, Harms et al observe a non-signiWcant oVset of SV by ModelXow for a 70° head-up tilt compared to the direct thermodilution method (Harms et al 1999). Without an invasive calibrating method, absolute individual SV values should be viewed with caution, although relative mean changes in ModelXow-derived values seem to be reliable (Harms et al 1999).…”

“…However, Harms et al observe a non-signiWcant oVset of SV by ModelXow for a 70° head-up tilt compared to the direct thermodilution method (Harms et al 1999). Without an invasive calibrating method, absolute individual SV values should be viewed with caution, although relative mean changes in ModelXow-derived values seem to be reliable (Harms et al 1999). …”

Dietary risk markers that contribute to the aetiology of hypertension in black South African children: the THUSA BANA study

“…Since the pressure waveform is available continuously, computations provide further information on the dynamics of the cardiovascular system on a beat-to-beat basis, similar to intraarterial measurements. 20,[22][23][24] Weight and height Qualified anthropometrists took weight and height measurements. Height was measured to the nearest 0.1 cm and weight was measured to the nearest 0.1 kg by means of a calibrated electronic scale (Precision Health Scale).…”

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“…Subsequently, the subject was connected to a Finapres (finger-arterial pressure) apparatus 43,44 and blood pressure recorded continuously. After at least a 10 min period of rest, resting blood pressure values were obtained.…”

“…The data were stored on magnetic tape by means of a Kyowa RTP-50A four-channel data recorder and digitized for further analysis by means of the Fast Modelflow software program. 44,45 In this way, the cardiac output (CO), stroke volume (SV), heart rate (HR), total peripheral resistance (TPR), the systolic blood pressure (SBP), diastolic blood pressure (DBP), 'Windkessel' compliance of the arterial system (Cw) and mean arterial pressure (MAP) were obtained. Cardiovascular reactivity was calculated as the percentage change from resting to plateau values obtained during application of the laboratory stressor.…”