Staircase assessment of the magnitude and time-course of 50% nitrous-oxide analgesia

Prospective, double-blind, randomized trial of equimolar mixture of nitrous oxide/oxygen to prevent pain induced by insertion of venous access ports in cancer patients

BACKGROUND

To assess the efficacy of equimolar mixture of nitrous oxide/oxygen (EMNO) to prevent pain induced by venous access ports (VAPs) implantation in cancer patients.

PATIENTS AND METHODS

In a randomized, double-blind study on an adult population not knowing the effects of EMNO, cancer patients were randomly assigned to breath via a facial mask, EMNO or a placebo mixture comprising 50% oxygen and 50% nitrogen. The primary end-point was the patients' assessment of the severity of pain evaluated using a visual analog scale (VAS, 0 to 100) and the proportion of patients suffering pain in each group. The secondary criteria were side effects, tolerability of EMNO, and the level of satisfaction of both the patients and the medical team.

RESULTS

Eighty-three adults (42 in the EMNO group and 41 in the placebo group) were included. VAPs were implanted in the jugular vein in 95% of patients. In the placebo group, 78% of the patients declared that they found VAP implantation painful vs. 34% in the EMNO group (p=0.001). The severity of the pain was reduced by 50% in the EMNO group in comparison with placebo (p=0.0125). Although the median time to perform implantation was strictly identical in both groups (20 min), the estimated duration of surgery seemed longer to patients in the control group. Patient and investigator satisfaction indexes were >90% in both groups.

CONCLUSION

EMNO provides an effective solution for the prevention of pain during placement of VAPs.

Nitrous oxide analgesia in humans: acute and chronic tolerance

Electrical tooth stimulation was used to investigate whether humans develop tolerance to nitrous oxide (N(2)O) analgesia within a single administration as well as over repeated administrations. In a double-blind cross-over experiment, 77 subjects received a 40-min administration of 38% N(2)O at one session and placebo gas at the other. The sessions were separated by 1 week and the order of gas administration was counterbalanced. Acute analgesic tolerance developed for pain threshold but not for detection threshold. There was no evidence of a hyperalgesic rebound effect following cessation of the N(2)O administration. In a second double-blind experiment, 64 subjects received both 30-min of placebo gas and 30-min of 35% N(2)O, separated by a 35-min gas wash-out period, during each of five sessions. Sensory thresholds were assessed prior to drug or placebo administration (baseline) and between 7-12 and 25-30 min of gas administration. A control group of 16 subjects received only placebo gas at these five sessions. During a sixth session, the experimental procedures were similar to the previous sessions except that the control group received N(2)O for the first time and the experimental group was sub-divided to test for conditioned drug effects. For both detection and pain threshold measures, acute tolerance developed during the initial N(2)O exposure and chronic tolerance developed over repeated administrations. Although chronic tolerance developed, a test for Pavlovian drug conditioning found no evidence of conditioned effects on sensory thresholds. In conclusion, acute and chronic tolerance develop to N(2)O's analgesic effects in humans.

Nitrous oxide-induced hypothermia in the rat: acute and chronic tolerance

Although inhalation of nitrous oxide (N2O) causes hypothermia in rats, there is a paucity of information as to whether tolerance develops to this effect. The purpose of this study was to determine whether tolerance to N2O hypothermia develops within a single administration as well as over repeated administrations. Temperature was measured telemetrically by implanting intraperitoneal thermal sensors/transmitters in male Long-Evans rats. Experimental rats received an initial 2-h exposure to 60% N2O and became hypothermic relative to controls breathing placebo gas. Only a few rats demonstrated evidence of acute tolerance over the 120 min. Over the next 10 days, the experimental rats received five additional 30-min exposures to 60% N2O and five 30-min exposures to placebo while the control rats received only placebo gas exposures. Chronic tolerance developed to N2O hypothermia over these repeated administrations. A test for Pavlovian drug conditioning found no evidence that conditioned temperature effects contributed to chronic tolerance development. In a second experiment, naive rats were given a 380-min exposure to 60% N2O and a 380-min exposure to placebo gas in a counterbalanced order. Acute tolerance did develop to N2O hypothermia, with the recovery of temperature beginning after a mean of 141 min of gas administration. Hence, both acute and chronic tolerance develop to N2O's hypothermic effects in rats.

Response to nitrous-oxide and oxygen among dental phobic patients

The present study evaluated the records of 27 patients (20 men, 7 women) who were treated under nitrous-oxide (N2O) and oxygen sedation due to extremely high dental anxiety. Corah's DAS and Kleinknecht's dental fear survey were completed by the patients. Patients who responded positively to the gas were categorised as N2O+ patients, whereas those who responded negatively were categorised as N2O- patients. A mean DAS score of 19.3 +/- 1.6 was observed for the entire population. No significant difference was found between N2O+ patients and N2O- patients in any DAS item, nor in the total scores (19.4 +/- 1.35 and 18.67 +/- 2.24 respectively). Only in answer to the question, 'Has fear of dental work ever caused you to cancel or not appear for an appointment?' were the scores of N2O- patients significantly higher than of N2O+ patients. No differences were found between patients who reported pain as the source for their dental anxiety and those who did not; between patients with gag reflex and those without, between patients who had never had any dental treatment and those who had, and between patients who required two or three appointments before administration of the gas with respect to successful treatment under N2O and oxygen sedation. Among the N2O- group, significantly more patients did not attend for two appointments or more. The results suggest that lack of keeping previous dental appointments may indicate patients' possible negative response to nitrous-oxide and oxygen sedation.