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.
|Number of pages
|Anesthesia and Analgesia
|Published - 1996
ASJC Scopus subject areas
- Anesthesiology and Pain Medicine