Comparison of End-Tidal PCO2 and Average Alveolar Expired PCO2 During Positive End-Expiratory Pressure

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“…As depicted in Fig. 2, the gradient between PETCO 2 and PaCO 2 depends on VD alv (ie the amount of lung regions with high or infinite V̇A/ ratios) [2,25]. Lung regions with high V̇A/ ratios can result from high alveolar pressures (eg large VT, positive end-expiratory pressure [PEEP]), low pulmonary perfusion pressures (eg low T, upright position), and obstruction of pulmonary blood flow (eg thrombus, gas, or fat embolism).…”

“…If one assumes no diffusion defect for carbon dioxide, then the partial carbon dioxide tension (PCO 2 ) of arterial blood (PaCO 2 ) leaving the lung is the perfusion-weighted average alveolar PCO 2 (PACO 2 ). Note that pulmonary shunt will add mixed venous blood with high PCO 2 (PVCO 2 ) to arterial blood and slightly increase PaCO 2 [2]. V̇A is the product of respiratory frequency and expired tidal volume (VT).…”

“…where PĒCO 2 is the mixed expired PCO 2 [2]. In turn, VD phy is partitioned into anatomic dead space (VD ana ; conducting airways that do not participate in gas exchange) and alveolar dead space (VD alv ; ventilated alveolar units that are devoid of perfusion; Fig.…”

“…where PACO 2 is the alveolar PCO 2 , estimated either from PETCO 2 or PA̅E̅CO 2 [2] (see below). The PaCO 2 -PETCO 2 gradient results from the presence of VD alv or high alveolar ventilation-to-blood flow (VA/Q) lung regions (see also Capnography during weaning from mechanical ventilation, below).…”

“…Accordingly, the PA̅E̅CO 2 [2,3], which is the volume-averaged alveolar PCO 2 , is a better index of PACO 2 than is PETCO 2 , which is just a single measurement of PCO 2 at the end of exhalation [2]. A more informative determination of pulmonary carbon dioxide elimination is VCO 2,br , which is starting to garner clinical acceptance.…”