| At expiration, the first part of the exhaled gas originates from the anatomical dead space where no gas exchange occurs. Therefore, this gas has the same PCO2 as inspired gas, usually close to zero (fig. 11). After a short transitional phase consisting of a mixture of anatomical dead space and alveolar gas, the rest of the expiration consists of mixed alveolar gas. Measurement of PCO2 of exhaled gas at the end of the expiration, PETCO2, has been used as a measure of mixed alveolar gas and as a noninvasive measure of PaCO2. There are, however, several considerations with this approach. In diseased lung the proportion of airflow from different lung regions might change during expiration, with different regions emptying sequentially rather than in parallel. In this situation, late emptying of low V′A/Q′ regions with a high PACO2 causes the PCO2 to increase throughout expiration and potentially be greater than the mean PACO2 at the end of exhalation [24]. Even with healthy lungs, a similar phenomenon occurs when breathing with large tidal volumes during exercise [25, 26]. In this case, one important cause is that PACO2 increases during the course of expiration due to more CO2 being added to alveolar gas [27]. In these situations PETCO2 might in fact even be greater than PaCO2, the upper limit being the Pv¯CO2 [24, 25]. Obviously, a PETCO2 greater than the mixed PACO2 will cause the PaCO2–PETCO2 to underestimate wasted ventilation. |