The influence of pH on the intravenous delivery of lidocaine solutions

In order to evaluate the surface tension lowering effect of lidocaine 0.4% on a 5% glucose solution, drop size has been measured in intravenous sets of 60 drops per ml. The relationship between the number of drops per ml and the pH of the solution was sigmoidal, from 63 drops/ml at pH5 to 95 drops/ml at pH 10. Without lidocaine, the effect of pH on the drop count of the 5% glucose solution was negligible. The surface tension lowering effect of lidocaine may account for significant decreases in apparent bioavailability and delivered lidocaine when high pH solutions are used with drop counting infusion pumps.