Stroke volume of the heart and thoracic fluid content during head-up and head-down tilt in humans

Background: The stroke volume (SV) of the heart depends on the diastolic volume but, for the intact organism, central pressures are applied widely to express the filling of the heart. Methods: This study evaluates the interdependence of SV and thoracic electrical admittance of thoracic fluid content (TA) vs. the central venous (CVP), mean pulmonary artery (MPAP) and pulmonary artery wedge (PAWP) pressures during head-up (HUT) and head-down (HDT) tilt in nine healthy humans. Results: From the supine position to 20 degrees HDT, SV [112 +/- 18 ml; mean +/- standard deviation (SD)], TA (30.8 +/- 7.1 mS) and CVP (3.6 +/- 0.9 mmHg) did not change significantly, whereas MPAP (from 13.9 +/- 2.7 to 16.1 +/- 2.5 mmHg) and PAWP (from 8.8 +/- 3.4 to 11.3 +/- 2.5 mmHg; P <0.05) increased. Conversely, during 70 degrees HUT, SV (to 65 +/- 24 ml) decreased, together with CVP (to 0.9 +/- 1.4 mmHg; P <0.001), MPAP (to 9.3 +/- 3.8 mmHg; P <0.01), PAWP (to 0.7 +/- 3.3 mmHg; P <0.001) and TA (to 26.7 +/- 6.8 mS; P <0.01). However, from 20 to 50 min of HUT, SV decreased further (to 48 +/- 21 ml; P <0.001), whereas the central pressures did not change significantly. Conclusions: During both HUT and HDT, SV of the heart changed with the thoracic fluid content rather than with the central vascular pressures. These findings confirm that the function of the heart relates to its volume rather than to its so-called filling pressure

Postural effects on cardiac output and mixed venous oxygen saturation in humans

The activation of cardiovascular reflexes for postural adaptation questions whether, in healthy humans, the central blood volume is optimised to support the upright position. A functional definition of an 'optimal circulating volume' that provides the heart with enough central blood volume to establish a maximal cardiac output (CO) and mixed venous oxygen saturation (S-v,S-O2) at rest was evaluated in nine healthy subjects. Preload to the heart was varied by passively changing the body position from 70 deg head-up to 20 deg head-down tilt. The S-v,S-O2 was compared with simultaneously measured estimates of CO by computer-controlled thermo-dilution. The CO was in the range 8.7-3.8 l min(-1) and S-v,S-O2 was in the range 79-58%. Neither CO (median 6.0 (range 5.3-8.7) l min(-1)) nor S-v,S-O2 (mean +/- S.D. 73.6 +/- 2.6 %) changed from the supine to the 20 deg head-down position. During sustained 70 deg head-up tilt, S-v,S-O2 decreased to 64 +/- 4 % together with a decline in CO to 4.7 (3.9-5.6) l min(-1) (P <0.05). Under conditions of varying tilt angles, a change in CO is paralleled by concordant changes in S-v,S-O2. Maximal values for CO and S-v,S-O2 during supine rest suggest that the horizontal position provides for an 'optimal' central blood volum

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

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