Subcutaneous infiltration with ammonium sulphate 10% does not prolong the local anaesthetic duration of lidocaine in humans

Background: The aim of the present study was to investigate whether a solut ion of ammonium sulphate and lidocaine had a prolonged duration of action c ompared with lidocaine alone after subcutaneous infiltration in human volun teers. Methods: In a double-blind, randomised, controlled trial 12 male volunteers had either 4 mi lidocaine 1% (40 mg) or a solution of 4 mi ammonium sulpha te 10% and Lidocaine 1% (40 mg) subcutaneously infiltrated in corresponding skin areas on the right and left calf. The duration of the local anaesthet ic action of the test meditations was assessed 1-6 h after the infiltration s using thresholds to mechanical (von Frey hairs) and thermal stimulation ( heat detection thresholds: HDT, and heat pain dectection thresholds: HPDT), and by calculation of areas under the curve (AUC) for electronic visual an alogue scale (VAS) pain ratings (sampled at 2 Hz) during a heat stimulation on the test areas for 30 s with a 45 degrees C warm thermode. Results: At no assessment were thresholds to mechanical or thermal stimulat ion increased in favour of ammonium sulphate treated legs compared with con trol legs. On the contrary, HPDT were significantly lower in ammonium sulph ate treated legs than control legs at the 3 h and 4 h assessment after infi ltration (P<0.02). Similarly, the AUC for the VAS ratings was significantly higher in treatment legs compared with control legs 2 h and 3 h after infi ltration (P<0.05). Conclusion: No improvement in local anaesthetic duration of ammonium sulpha te 10% plus lidocaine 1% compared with lidocaine 1% alone was demonstrated early after subcutaneous infiltration in human volunteers.