Effect of expiratory flow rate on the nitrogen concentration vs. volume relationship.

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Abstract
The N concentration versus volume relationship of a single expiration following an inspiration of O2 was studied in 3 normal subjects. Three commonly performed clinical tests with different pre-inspiratory lung volumes and inspired volumes of O2 were studied. For each test all inspiratory variables were held constant and only the expiratory flow rate was varied. Calculated dead space was consistently lower, the alveolar plateau less steep and the mean N concentration of the expirate was greater with rapid expiratory flow rates. The results indicate a different sequence of lung emptying and imply a different end-expiratory regional volume distribution when expiration is rapid. Expiratory flow rate should be comparable for repeat studies in a given subject when either dead space measurement or distribution information is derived from the expired N concentration versus volume relationship.