Pain perception with pH buVered peribulbar anaesthesia: a pilot study

Aims—To determine the relation between pH of anaesthetic solutions and patient perception of pain with peribulbar injection of local anaesthesia. Methods—This prospective randomised controlled double blind pilot study involved 60 consecutive patients who received a peribulbar block with either a standard acidic local anaesthetic of 5 ml 2% lignocaine and 5 ml of 0.5% bupivacaine (solution A), or an alkalinised solution composed of the same anaesthetic agents but with a pH of 7.44 (solution B). Before surgery patients were asked to grade the pain of both the preoperative dilating drops and the peribulbar injection using a visual analogue scale. Results—The mean pain scores were similar in the two treatment groups— slightly higher (4.97) in group B who received the buVered solution, compared with group A (4.84) who received the plain solution. The small diVerence (−0.13, 95% confidence limits −1.6 and +1.3) was not significant. There was, however, a highly significant association between pain threshold (“drop pain”) and injection pain levels (p<0.0001). Conclusion—This study showed no diVerence in the reduction in the pain experienced by patients undergoing peribulbar anaesthesia with pH buVered local anaesthetic. The study suggests the importance of “pain threshold” as a confounder and also showed the considerable pain felt by some patients on instillation of the preoperative dilating drops. (Br J Ophthalmol 2000;84:1041–1044) Local anaesthetic is the commonest form of anaesthesia for patients undergoing cataract surgery in the UK. DiVerent techniques have been described for administering the anaesthesia and diVerent solutions have been suggested. Modification of local anaesthetics by the addition of agents to reduce the onset time or to prolong duration of blockade has been practised and investigated extensively. The pain often experienced on injection of local anaesthetic solutions is in part related to pH and this is particularly true for solutions with added adrenaline (epinephrine). There is some evidence to suggest that adding preservative free sodium bicarbonate to local anaesthetic solutions results in less discomfort on injection. We have therefore investigated the use of a buVered solution of lignocaine (lidocaine) and bupivacaine in peribulbar anaesthesia to study the eVect on pain perception in patients undergoing cataract surgery. Patients and methods Sixty patients (35 female, 25 male) admitted for elective cataract surgery were randomised to receive either a standard solution of local anaesthetic (solution A) or an alkalinised solution (solution B). Patients who did not have a good understanding of spoken English were excluded. Solution A was prepared using 5 ml of 2% lignocaine and 5 ml of 0.5% bupivacaine (total 10 ml, both manufactured by Antigen Pharmaceuticals Ltd) with a resultant pH of 4.87. Solution B was prepared as above with the addition of 1ml of 8.4% preservative free sodium bicarbonate with a resultant pH of 7.44 (1 ml of solution B was discarded to achieve an equal volume to solution A). To ensure stability of pH, the solutions were prepared in polypropylene syringes at the beginning of each operating list. The pH of each solution was tested using a Corning 240 pH meter (subjected to a two point calibration using commercially available buVered pH solutions). Preoperative pupil dilatation using phenylephrine 10% eyedrops (2.5% in hypertensives) and cyclopentolate 1.0% eyedrops were given topically at 60, 45, 30, and 15 minutes before surgery and proxymetacaine 0.5% eyedrops were given topically before peribulbar anaesthesia. All peribulbar injections were given by two ophthalmologists who were unaware of the choice of solution (AI or MM). The peribulbar anaesthetic was administered at the junction of the middle and lateral thirds of the lower lid using a 1] (15/16 ̋) 25 gauge needle (0.25×24 mm). Before surgery the patients were asked to grade the pain of both the preoperative dilating drops and the peribulbar injection on a standard visual analogue scale 14 which was enlarged to A4 size to facilitate reading in these visually impaired patients—0 represents no pain and 10 the most severe pain imaginable. The Table 1 Distribution of the possible prognostic factors in the two treatment groups