Effect of nitrous oxide analgesia on pneumothorax.

Objective: To address whether spontaneous inhalation of 50% nitrous oxide (N2O) for up to 30 minutes adversely affects swine with pneumothorax (PNTX).

Methods: Five fasted, female swine (13-17 kg) were sedated with IV ketamine, intubated, and allowed to spontaneously inhale either 50% N2O or room air (RA) in a sequential crossover design in which each animal served as its own RA control. Small (group 1, 150 mL), medium (group II, 300 mL), and large (group III, 500 mL) PNTXs were created by instilling air into the left pleural space via an 18-Fr three-way Foley catheter. Changes in PNTX volume, heart rate (HR), central venous pressure (CVP), blood pressure (BP), ECG. and arterial blood gas (ABG) parameters were recorded in separate 10- and 30-minute trials. A 15-minute washout period was given between each trial. Data were analyzed using repeated-measures analysis of variance with post-hoc Tukey's tests.

Results: No significant increase in PNTX size was seen during the 10-minute trials. In the 30-minute trials, a statistically significant increase in absolute PNTX size was seen with N2O compared with RA for group I (44 +/- 17 vs 16 +/- 5 mL, p = 0.02), group II (61 +/- 21 vs 26 +/- 4 mL, p = 0.01), and group III (62 +/- 32 vs 40 +/- 5 mL, p = 0.06). No difference in HR, CVP, MAP, ABGs, or ECG were observed between the N2O and RA trials for any size PNTX. No animal developed hemodynamic signs of tension pneumothorax.

Conclusion: In this model, spontaneous inhalation of 50% N2O for up to 30 minutes is associated with little risk of hemodynamic or respiratory compromise. Although PNTX size increases with 50% N2O use, the magnitude observed in this animal model is less than previously reported.