Effects of the sitting position on the distribution of blood volume in patients undergoing neurosurgical procedures

The use of the sitting position in neurosurgery is often associated withdecreased arterial pressure (MAP) and stroke volume index (SVI). A shift inblood from the intra- to the extrathoracic compartment may be responsiblefor this cardiovascular response. However, little is known of the amount ofshift in blood volume after transfer from the supine to the sittingposition. Therefore, we measured simultaneously changes in intrathoracicblood volume (ITBV) caused by a change in body position in anaesthetizedpatients. Measurements of cardiac index (CI), ITBV, pulmonary (PBV) andtotal circulating (TBVcirc) blood volumes were performed in the supine andsitting position. CI, ITBV, PBV and TBVcirc were measured using a thermodyedilution technique. Fluid input was restricted to 14 ml kg-1 beforeinduction of anaesthesia. Change in body position caused a significantdecrease in ITBV and was accompanied by a significant decrease in CI, SVIand MAP. Changes in ITBV correlated (r = 0.78) with changes in SVI. Thus achange in blood volume distribution between the intra- and extrathoraciccompartment occurred after a change from the supine to the sittingposition. Indicator dilution enables quantification of this shift and maybe helpful in guiding fluid therapy in selected patients.