Effects of pH and buffer capacity modulation on the analgesic efficacy of lignocaine and pethidine solutions

Background. Objectives were to study the effects of variations in the pH and the buffer capacity of solutions of lignocaine or pethidine on their analgesic efficacy in peripheral nerve block. Methods. Infraorbital nerve blocks (IONB) were induced in rats during light pentobarbitone anaesthesia employing 0.2 mi of test solutions containing 10 mg . ml(-1) of lignocaine or pethidine dissolved in normal saline, 50 mmol and 150 mmol tris-hydro-xymethylaminomethane (THAM) hydrochloride (THAM . HC1) of pH 4.8 and 4.5, respectively, or 100 mmol THAM+ THAM . HC1 of pH 7.4. The pH of solutions in saline was varied from 3.0 to 9.3 by adding HCl or NaHCO3. The analgesic efficacy is expressed as the interval from injection to elicitation of a minimal response to electric stimulation at various intensities (threshold intensities) within the blocked area (IONB 3 to 10), and in terms of the area under the curve (AUC, threshold intensities vs, time). Results. When dissolved in saline, pH 7.4, pethidine exerted more pronounced effects than lignocaine [AUC by 3.5 times (P<0.001), IONB 3 to 10 by 2.7 (P<0.05) to 3.6 (P<0.001) times]. Variations of the pH of solutions did not affect their analgesic efficacy Lignocaine exerted more pronounced local analgesic effects when mixed with 150 mmol THAM . HC1 than when mixed with saline (pH 7.4), but manifested even more pronounced effects when dissolved in THAM+THAM . HC1 [AUC by 3.7 times (P<0.001), IONB 3 to 10 by 2.7 (P<0.01) to 3.8 (P<0.001) times]. When dissolved in THAM+THAM . HC1, the analgesic efficacy of pethidine increased by 2.0 to 2.8 times (P<0.001), as compared to a solution in saline at the same pH. Conclusions. It is concluded that variations in the pH of solutions of lignocaine or pethidine in saline does not affect the analgesic efficacy of the drugs, presumably due to the low buffer capacity of the medium, whereas when dissolved in THAM+THAM . HC1, their effect is enhanced.