Venous pressure and arm volume changes during simulated Bier's block

In investigating the possible causes of an observed complication of intravenous regional anaesthesia, cubital fossa venous pressures were measured distal to an inflated tourniquet as standard 40 ml volumes of normal saline were injected. The maximal pressures obtainable were limited by tourniquet pressure since the veins compressed under the tourniquet acted as ‘Starling’ spillover resistors. Once the compressing pressures were reached, with continued injection, fluid passed easily into the systemic circulation with little further increase in venous pressure. The rates of rise and maximum values of venous pressures tended to be increased by increased injection rates, and by failure to exsanguinate the arm, but the choice of injection site was paramount. Compared with more distal injections, cubital fossa venous injections are more likely to lead to leakage under the tourniquet and should never be used for intravenous regional anaesthesia.