between. (A) The upper trace shows the end tidal PCO2 and (B) the lower, the inspired PCO2. a filtered mixture of oxygen, carbon dioxide, and nitrogen in concentrations determined by a blender and breathing out through a large tube acting as an alveolar gas reservoir followed by a one-way expiratory valve in parallel with a loaded inspiratory valve. Ventilation was monitored using a pneumotachograph in the expiratory limb. The inspired and expired PCO 2 was measured at the mouth. Once the subject was familiar with the breathing system they were introduced to an electronic breathlessness score. A light was moved between seven verbal scores from 0 = no breathlessness to 7 = extreme breathlessness. 2 Scores were measured every 30 s throughout the study. Measures were recorded for two PCO2 levels after an initial trial run. The breathlessness scores were similar at the same PCO2 but significantly different between the two PCO2 levels (P<0.01 [Mann–Whitney U-test]). In this small group of subjects (Fig. 3) the stability of the breathing system and the repeatability of the scores at the different PCO2 levels indicate that the method may be applicable to testing in patients. Acknowledgement: We thank the Wellcome Trust for a Vacation Scholarship for MN
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