The role of gas exchange in phase IV of the single-breath nitrogen test.

Because at the end of a prolonged expiration the respiratory quotient is less than 1, ongoing gas exchange could add to the rising N2 concentration in phase IV of the single-breath nitrogen (SB-N2) test. To verify this, 7 normal subjects performed SB-N2 and reversed gradients (SB-R) tests with and without a 20-s apnea at the onset of closing volume. In the SB-R test, the residual gas is 100% O2 and the final VC inspiration is room air. This gives a downward slope of phase IV (delta N2IV). Apnea, by prolonging total expiratory time, should increase the contribution of gas exchange and therefore increase the delta N2IV in the SB-N2 test and decrease it in the SB-R test. In the SB-N2 test, the delta N2IV increased, with the apnea, from 5.58 +/- 1.35 to 8.12 +/- 1.52 (mean +/- SD, p less than 0.01). In the SB-R test, the delta N2IV decreased from 4.31 +/- 1.65 to 1.43 +/- 1.45 (p less than 0.01). These results suggest that 10% of the N2 changes in the delta N2IV in the normal SB-N2 test may be related to gas exchange.