A new technique for evaluating nitrogen washout efficiency.

Nitrogen washout curves resulting from open circuit N2 washouts of unsedated human and canine subjects were constructed by plotting the breath-by-breath end-expiratory N2 fraction (Fn2) against breath number on a semilog scale. Curves were analyzed graphically, and the slope of the 2nd of the 3 components was expressed by B 1/2, the number of breaths required to reduce Fn2 to half of its original value. The magnitude of B 1/2 was directly proportional to the value of functional residual capacity (FRC) divided by tidal volume (Vt). The quantity B 1/2 (FRC/Vt) was relatively constant within an individual regardless of FRC or Vt and was named the nitrogen dilution constant (NDC). Means and ranges of Vt, FRC, B 1/2, and NDC were determined for normal human subjects and Beagle dogs. The NDC varied less among and within subjects than other simple indices of washout efficiency. It was also more sensitive than other indices in detecting age-related loss of washout efficiency. The use of NDC in evaluating N2 washout of subjects with interstitial lung disease was demonstrated.