Influence of external resistance and minor flow variations on single breath nitrogen test and residual volume.

Inspiratory and expiratory flow are known to influence parameters obtained from the single breath nitrogen test. We assessed the influence of two different flow rates (0.3 and 0.5 1.s-1) with and without the use of an external resistor during inspiration and/or expiration on the indices of the single breath nitrogen test in eight healthy subjects. We found that when an external resistance was used during expiration residual volume was larger and the height of phase IV smaller, because the resistance interferes with complete lung emptying. When applied only during inspiration the resistance did not affect any of the measurements. The height of phase IV was greater at a flow of 0.3 1.s-1 than at 0.5 1.s-1; also residual volume was greater at the lower flow rate. The effect of flow rate can be accounted for by continuous gas exchange during the manoeuvers. The slope of the alveolar plateau and closing volume are not influenced by the resistance or by minor changes in flow rate. We conclude that for commonly used indices (slope of phase III, closing volume) the use of a resistance does not affect the results and is helpful for the subject; residual volumes are affected by both the resistance and the flow rate, but these effects are small.