Effects of hypertonic 75 mg/ml (7.5%) saline on extracellular water volume when used for preloading before spinal anaesthesia

Abstract

Prevention of hypotension during spinal anaesthesia is commonly achieved using fluid preloading. This may result in a substantial amount of excess free water retained in the body after spinal anaesthesia. We aimed to evaluate the effects of 7.5% hypertonic saline on extracellular water volume and haemodynamics when used for fluid preloading before spinal anaesthesia. This randomised double-blind study evaluated the effects of 75 mg/ml (7.5%) hypertonic saline (HS) on extracellular water volume and haematocrit in patients undergoing arthroscopy or other lower limb surgery under spinal anaesthesia. Amounts of 1.6 ml/kg of HS (20 patients) or 13 ml/kg of 9 mg/ml normal saline (20 patients) were administered for preloading before spinal anaesthesia with a 10 mg dose of 0.5% hyperbaric bupivacaine. Etilefrine was administered in order to maintain mean arterial pressure (MAP) at >or=80% of its baseline value. Whole-body impedance cardiography-derived cardiac index (CI) and extracellular water (ECW) were measured. There were no significant differences in demographic data or in the number of blocked segments. ECW remained similar in both groups despite the much smaller amount of infused free water in the HS group. There were no significant differences between the groups in CI values during the study. The amount of etilefrine administered was similar in the treatment groups. Dilution of haematocrit was also similar in both groups. Hypertonic 75 mg/ml (7.5%) saline is an alternative for preloading before spinal anaesthesia in situations where excess free water administration is not desired. It is effective in small doses of 1.6 ml/kg, which increase the extracellular water, plasma volume and cardiac output, and thus maintain haemodynamic stability during spinal anaesthesia.