A low concentration of nitrous oxide reduces dyspnoea produced by a combination of hypercapnia and severe elastic load

We have measured how a low concentration of nitrous oxide affected respirat ory sensation and ventilation. Severe dyspnoea was induced in nine normal s ubjects by a combination of hypercapnia and inspiratory elastic load (50 cm H2O litre(-1)). Subjects were asked to rate their sensation of respiratory discomfort using a visual analogue scale (VAS) while breathing either 20% nitrous oxide or 20% nitrogen gas mixture. We compared the effects of each gas mixture on respiratory sensation and ventilation using steady-state val ues of ventilatory variables and VAS scores obtained before, during and aft er inhalation of each gas mixture. Inhalation of 20% nitrous oxide reduced the sensation of respiratory discomfort from a median VAS score of 6.5 (ran ge 5.0-8.1) before inhalation to 3.6 (2.4-5.9) during inhalation (P < 0.05) . There was no significant change in minute ventilation but tidal volume in creased during inhalation of 20% nitrous oxide (P < 0.05) while respiratory frequency decreased (P < 0.05). Inhalation of 20% nitrogen did not alter V AS scores or ventilatory variables. We found that a low concentration of ni trous oxide greatly alleviated the intensity of dyspnoea without changing r espiratory load compensation.