Single-breath test for CO
at ordinary and long postinspiratory pause in subject 4, depicting fraction of CO
at airway opening, FCO
, against expired volume. The blue loop shows the SBT-CO2 from an ordinary breath, and the magenta loop, a breath with a prolonged TP. The blue area corresponds to VTCO2 of an ordinary breath. The additional volume of CO2 eliminated at the longer TP, ΔVTCO2, indicated by hatched area, is caused partly by a lower-airway dead space (indicated by interrupted lines) and partly by a higher level of the alveolar plateau.