Multiple breath nitrogen dead space

Summary. Ventilatory efficiency for eliminating CO₂ is expressed by the physiological dead space, V_Dphys = (1-PE CO₂/Pa CO₂) ×V_T, where PE is the mixed exhaled and Pa the arterial CO₂-tension and V_T the tidal volume. We used data from the multiple breath N₂-wash-out with oxygen for calculating a functional dead space for nitrogen. V_DFN₂= (1 -FEN₂/FEN₂/FidN₂) ×V_T. F_EN₂ is the mixed exhaled N₂-fraction and FidN₂ the calculated mean alveolar N₂-fraction during a wash-out with the same number of breaths to reach 2% N₂ in end tidal air, but having completely even distribution. FidN₂ is shown to be 0·20±0·01 for wash-outs using 20–150 breaths. The method was applied to wash-outs from 21 healthy volunteers, 18 patients with chronic obstructive lung disease and two subjects with acute bronchospasm. V_DF was well related to V_D phys CO₂ (r= 0·78) but higher than the latter. In subjects with lung disease V_DF was inversely related to the degree of obstruction expressed by forced expiratory volume in one second per cent of vital capacity (r= 0·85). The subjects with bronchospasm had very high V_D/V_TMF, in relation to their FEV%. If airway dead space predicted from height and sex is subtracted from V_DF, the resulting alveolar dead space will be a good expression for uneven gas distribution in the lungs. We also deduced a direct mathematical relation between lung clearance index and V_D/V_TF. The documented good reproducibility of LCI is thus also valid for V_D/V_TF, while the latter better expresses ventilatory efficiency.