Difference between the He bolus and N2 technique for measuring closing volume.

We measured closing volume in sixteen healthy subjects simultaneously and separately with a bolus of He (using a rapid catharaometer) and with the N2 technique. In another group of 35 active workers (some with airway obstruction), closing volume was measured separately with those two methods. In both groups the He closing volume was significantly higher than the N2 closing volume. We attribute this difference to a less marked vertical N2 concentration gradient, leading to a less clearly defined separation between phase III and IV and resulting in an underestimation of the N2 closing volume. Indeed, increasing the N2 gradient in the lung, by inspiring O2 from a higher than residual volume level, increased the N2 closing volume which became comparable to the He closing volume. We also found, for both He and N2 tracings, a significant between-observers difference in reading of the closing volume. However, the difference in reading of ts difference in reading of the closing volume. However, the difference was less important for He closing volumes. We conclude that the bolus method improves the resolution of closing volume and decreases the interobserver variability.