Local anaesthetics formulation

After local anaesthetic injection, onset of nerve block and duration depends mainly on lipid solubility and on the region in where the diug is injected. In some formulations adrenaline is added to prolong the blocking action by inducing regional vasoconstriction and hereby reduce absorption and metabolisation. 

New modified polymeric electrodes selective to procaine and other local anaesthetic compounds were reported [72]. The electrode was constructed by incorporating the ion-pair complex of procaine with tungsto-phosphoric acid into ethylene-vinyl acetate copolymer. Best results were obtained with 1 1 nitrobenzene-dioctyl phthalate as a plasticizer. The calibration graph was linear fi om 18 p to 10 mM of procaine. When these electrodes were applied to the determination of the drug in pharmaceutical formulations, the recoveries were found to be quantitative. 

Epidural and intrathecal opioids are widely used for postoperative and obstetric analgesia. In contrast to local anaesthetics, spinal opioids cause minimal sympathetic efferent and motor blockade. Pethidine, which has local anaesthetic activity, can produce sensory and motor blockade. Because remifentanil is formulated with glycine as a vehicle, it should not be used epidurally or intrathecally, since glycine is neurotoxic. 

Some products are formulated as aqueous or aqueous-alcoholic liquids or gels. They tend to be diluted fairly rapidly and washed away from the site of application by saliva, requiring frequent reapplication. Pastilles or sore-throat lozenges containing a local anaesthetic, placed close up against lesions and allowed to dissolve slowly, may produce a more prolonged effect. 

Products containing combinations of constituents with local anaesthetic and analgesic effects, such as lidocaine, choline salicylate and phenol, counterirritants such as ammonia solution and menthol, and astringents such as zinc sulphate and tannic acid, are marketed to reduce discomfort and promote faster healing of sores while the infection takes its course. Some are formulated with alcoholic bases, which may have a drying effect on sores and speed up healing. The bland cream bases of some products may have a soothing effect. 

Gingival solutions with antiseptics or local anaesthetics may also be formulated in mixtures of water and glycerine. In this case glycerine serves the purpose of raising the viscosity of the vehicle and improving adherence to the mucosa. This is relevant because gingival solutions are used on local ailments in the oral cavity. 

Non-physiological osmolality and pH or increased buffer concentrations in formulations can be responsible for pain at the injection site. Therefore some formulations for intramuscular use contain a local anaesthetic. Needle-free injection... 

---