Comparison of end-tidal PCO₂ and average alveolar expired PCO₂ during positive end-expiratory pressure

The measurement of average alveolar expired PCO₂ (PĀĒCO₂) weights each PCO₂ value on the alveolar plateu of the CO₂ expirogram by the simultaneous change in exhaled volume, PĀĒCO₂ can be determined from a modified analysis of the Fowler anatomic dead space (VD(ANAT)). In contrast, end-tidal PCO₂ (PETCO₂) only measures PCO₂ in the last small volume of exhalate. In conditions such as mechanical ventilation with positive end- expiratory pressure (PEEP), where the alveolar plateau can have a significant positive slope, we questioned how much PETCO₂ could overestimate PĀĒCO₂. Accordingly, in six anesthetized ventilated dogs, we digitally measured and processed tidal PCO₂ and flow to determine VD(ANAT). We determined PETCO₂ and PĀĒCO before and after the application of 7.6 cm H₂O PEEP. Alveolar dead space to tidal volume fraction (VD/VT) was determined by [arterial PCO₂-alveolar PCO₂]/arterial PCO₂, where alveolar PCO₂ was determined by either PETCO₂ or PĀĒCO₂. During baseline ventilation, PETCO₂ was 3.4 mm Hg (approximately 11%) greater than PĀĒCO₂. Because PEEP significantly increased the slope of the alveolar plateau from 28 to 74 mm Hg/L, the difference between PETCO₂ and PĀĒCO₂ significantly increased to 6.6 mm Hg (approximately 20% difference). The concurrent increase in VD(ANAT) during PEEP decreased alveolar tidal volume and tended to limit the overestimation of PETCO₂ compared to PĀĒCO₂. When alveolar PCO₂ was estimated by PETCO₂, alveolar VD/VT was 18%, compared to an alveolar VD/VT of 26% when alveolar PCO₂ was estimated by PĀĒCO₂. This difference was significantly magnified during PEEP ventilation. The overestimation of PĀĒCO₂ by PETCO₂ can result in a falsely high assessment of overall alveolar PCO₂. Moreover, the use of PETCO₂ to estimate alveolar PCO₂ in the determination of the alveolar dead space fraction can result in falsely low and even negative values of alveolar dead space.