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 with decreased arterial pressure (MAP) and stroke volume index (SVI). A shift in blood from the intra- to the extrathoracic compartment may be responsible for this cardiovascular response. However, little is known of the amount of shift in blood volume after transfer from the supine to the sitting position. Therefore, we measured simultaneously changes in intrathoracic blood volume (ITBV) caused by a change in body position in anaesthetized patients. Measurements of cardiac index (CI), ITBV, pulmonary (PBV) and total circulating (TBVcirc) blood volumes were performed in the supine and sitting position. CI, ITBV, PBV and TBVcirc were measured using a thermodye dilution technique. Fluid input was restricted to 14 ml kg-1 before induction of anaesthesia. Change in body position caused a significant decrease in ITBV and was accompanied by a significant decrease in CI, SVI and MAP. Changes in ITBV correlated (r = 0.78) with changes in SVI. Thus a change in blood volume distribution between the intra- and extrathoracic compartment occurred after a change from the supine to the sitting position. Indicator dilution enables quantification of this shift and may be helpful in guiding fluid therapy in selected patients.