Figure 1

Typical tracings of CO and CH4 during exhalation of the single breath-hold CO test. An example of the standard method used to derive the alveolar volume during exhalation after the dead spaces have been entirely washed is reported for a healthy subject (left). This sample, which corresponds to the end of the knee on the CH4 and CO tracings after the hold time, usually measures between 500 and 1000 ml. An example of the method used to calculate the alveolar volume during exhalation subdividing the whole expirate into five sized quintiles in a COPD patient is reported (right). The increased slope of CO and CH4 tracings during the emptying phase when compared with the normal subject on the left is caused by the increased time-constant of emptying of lung units.