Experimental Physiology : Translation and Integration Normovolaemia is the normal blood volume of healthy individuals and averages ~75 ml (kg body weight) _1 (Smith & Kampine, 1999). The effective circulating blood volume refers to the part of the volume within the arterial system effectively perfusing the tissues (Schrier, 1990; Abraham & Schrier, 1994) and it is regulated by the interplay between the circulatory system and the kidneys (Guyton et al. 1980). Changes in the venomotor tone can increase, or decrease, the capacity of the venous circulation and thereby partially compensate for variations in the effective circulating blood volume. The effective circulating blood volume is assumed to depend mainly on the central blood volume (CBV), that is, the blood available to the heart. Transition from the supine to the upright posture has little effect on the blood pressure and orthostasis is proposed as the operating set point for human cardiovascular function (Gauer & Thron, 1965). However, in order to maintain blood pressure during standing, an elevated vascular tone is required (Jacobsen et al. 1993; Ray et al. 1993; Ng et al. 1995). Cardiac output (CO) is reduced by the postural fall in venous return and CBV (Blomqvist & Stone, 1984; Matzen et al. 1991) with the gravitational displacement of blood to the dependent regions of the body (Bergenwald et al. 1977; Smith & Ebert, 1990). The need for activation of cardiovascular reflexes for circulatory postural adaptation questions whether, in healthy humans, CBV is optimised to support the circulation in the upright position. A functional definition of ‘normovolaemia’ would be the ability to provide the heart with an appropriate CBV, i.e. cardiac preload (Ejlersen et al. 1995; Jenstrup et al. 1995). However, the usual clinical and haemodynamic parameters are not reliable indices of preload to the heart (Boulain et al. 2002; Pinsky, 2002) and an ‘optimal’ volume is neither defined nor is it an easily measurable entity. Hypovolaemia may be characterised by a reduced preload to the heart, i.e. with stroke volume and CO becoming dependent on CBV. The reported increase in CO with volume loading is taken to imply that a patient is preload- responsive (Pinsky, 2002; Boulain et al. 2002). Conversely, the intravascular volume may be expanded beyond the volume that can provide for a ‘maximal’ CO at rest. By interpolation between hypo- and hypervolaemia, normo- volaemia may be considered as the point in the cardiac Postural effects on cardiac output and mixed venous oxygen saturation in humans Mark P. M. Harms *, Johannes J. van Lieshout *†‡, Morten Jenstrup §, Frank Pott § and Niels H. Secher §¶ * Cardiovascular Research Institute Amsterdam and † Department of Medicine, Academic Medical Centre, University of Amsterdam, 1105 AZ Amsterdam, The Netherlands, and § The Copenhagen Muscle Research Centre and ¶ Department of Anaesthesia, Rigshospitalet, University of Copenhagen, DK-2100 Copenhagen, Denmark (Manuscript received 24 March 2003; accepted 31 June 2003) 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, J) 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, J 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, J was in the range 79–58 %. Neither CO (median 6.0 (range 5.3–8.7) l min _1 ) nor S v, J (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, J 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, J. Maximal values for CO and S v, J during supine rest suggest that the horizontal position provides for an ‘optimal’ central blood volume. Experimental Physiology (2003) 88.5, 611–616. 2580 Publication of The Physiological Society ‡ Corresponding author: j.j.vanlieshout@amc.uva.nl ) by guest on July 14, 2011 ep.physoc.org Downloaded from Exp Physiol (