Anaesthesia and circulating blood volume

Background and objective: The exact change in circulating blood volume (BV) during general anaesthesia is still unknown because there is no standard method of evaluating BV. We evaluated the changes in BV by general anaesthesia using simple and easy estimation methods. Methods: Fourteen patients scheduled for minor surgery under general anaesthesia were enrolled. Propofol and vecuronium bromide were used for the induction of anaesthesia, and anaesthesia was maintained with sevoflurane and nitrous oxide. Haematocrit (Hct), total protein concentration (TP), as well as colloid osmotic pressure (COP) measured using a colloid osmometer, were determined before anaesthesia, and 30, 60 and 90 min after the induction of general anaesthesia. BV was calculated using Allen's formula and the changes in Hct, TP and COP. The estimated BV was compared with directly measured BV using indocyanine green dilution method (BV I C G ). Results: Hct, TP and COP significantly decreased after the induction of anaesthesia (Hct: 42.1-39.4%; TP: 7.3-6.9 g dL - 1 ; COP: 23-19mmHg). The calculated BV as well as BV I C G significantly increased after induction of anaesthesia (calculated by COP: 4.13-5.03 L; BV I C G : 4.54-5.56L). The change rate in BV calculated by the change of COP was larger than other calculated BVs, and was approximated to the change rate in BV I C G . After emergence from anaesthesia, all values tended to return to baseline. Conclusions: General anaesthesia increases BV. The value of BV calculated from the change in COP was most changeable.