Fixed 50:50 mixture of nitrous oxide and oxygen to reduce lumbar-puncture-induced pain: a randomized controlled trial

Background and purpose: Lumbar puncture (LP) has been frequently performed for more than a century. This procedure is still stressful and often painful. The aim of the study was to evaluate the efficacy of a fixed 50% nitrous oxide-oxygen mixture compared to placebo to reduce immediate procedural pain and anxiety during LP. Methods: A randomized controlled trial was conducted involving adults who needed a cerebrospinal fluid analysis. Patients were randomly assigned to inhale either a fixed 50% nitrous oxide - oxygen mixture (50% N2O-O-2) or medical air (22% O-2-78% N-2). Cutaneous application of a eutectic mixture of local anaesthetics was systematically done and all LPs were performed with pencil point 25G needles (20G introducer needle). The primary end-point was the maximal pain level felt by the patient during the procedure, the maximal anxiety level being a secondary outcome, both measured using a numerical rating scale (0-10). Results: A total of 66 consecutive patients were randomized. The analysis was intention to treat. The maximal pain was 4.9 +/- 2.7 for the 33 patients receiving air and 2.7 +/- 2.7 for the 33 receiving 50% N2O-O-2 (P = 0.002). Similarly, the maximal LP-induced anxiety was 4.5 +/- 3.1 vs. 2.6 +/- 2.6 (P = 0.009), respectively. The number needed to treat to avoid one patient undergoing significant pain (pain score >= 4/10) was 2.75. Body mass index > 25 kg/m(2) was significantly associated with higher pain intensity (P = 0.03). No serious adverse events were attributable to 50% N2O-O-2 inhalation. Conclusions: Inhalation of a fixed 50% N2O-O-2 mixture is efficient to reduce LP-induced pain and anxiety.