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

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Abstract
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 ml of test solutions containing 10 mg · ml‐1of lignocaine or pethidine dissolved in normal saline, 50 mmol and 150 mmol tris‐hydroxymethylaminomethane (THAM) hydrochloride (THAM · HCI) of pH 4.8 and 4.5, respectively, or 100 mmol THAM‐THAM · HCI of pH 7.4. The pH of solutions in saline was varied from 3.0 to 9.3 by adding HCI 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 intensitiesvs.time).Results.When dissolved in saline, pH 7.4, pethidine exerted more pronounced effects than lignocaine [AUC by 3.5 times (PConclusions.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 · HCI · their effect is enhanced.