This sellekchem sequential emptying contributes to the rise of thealveolar plateau; the greater the V/Q heterogeneity, the steeper the expiredCO2 slope. Accordingly, the slope of the alveolar plateau has beenshown to correlate with the severity of airflow obstruction [8].Figure 1The three phases of capnography tracings. Phase I contains gas fromthe apparatus and anatomic dead space (airway), phase II representsincreasing carbon dioxide concentration resulting from progressive emptyingof alveoli, and phase III represents alveolar …Physiologic dead space (Vdphys) can be easily calculated from the Enghomodification of the Bohr equation by using arterial partial pressure of carbondioxide (PaCO2) with the assumption that PaCO2 is similar toalveolar PCO2:Vdphys/VT=(PaCO2-PECO2)/PaCO2,where VT is the tidal volume and PECO2 is thepartial pressure of CO2 in mixed expired gas and is equal to the meanexpired CO2 fraction multiplied by the difference between theatmospheric pressure and the water vapor pressure.

Vdphys is increasedin acute respiratory distress syndrome (ARDS), and a high dead space fractionrepresents an impaired ability to excrete CO2 because of any kind ofV/Q mismatch. Several authors [9,10] have demonstrated that increased Vdphys values areindependently associated with an increased risk of death in these patients.Since Vdphys/VT measures the fraction of each tidal breath that iswasted on alveolar dead space (Vdalv) and airway dead space(Vdaw), the Vdaw must be subtracted from Vdphys/VTto obtain the Vdalv/VT [11]. By using the PetCO2 instead of PECO2 in theequation, the Vdalv can be calculated.

Equating the alveolar PCO2to the arterial PaCO2 is, however, valid only in healthy subjects. Inpatients with high right-to-left shunt, PaCO2 is higher than alveolarPCO2 because of the shunted blood with high PvCO2 (partialpressure of carbon dioxide in mixed venous blood). Without correction [12], it must be remembered that a high dead space also includes the shunteffect.PaCO2 can be grossly estimated by PetCO2. Monitoring PetCO2can also help to track PaCO2 when changes in PaCO2 are to beavoided (especially in critically ill, neurological patients with normal lungs). Thegradient between PaCO2 and PetCO2 widens in ARDS and correlatesacross the different levels of Drug_discovery Vdphys [13]. The difference between PaCO2 and PetCO2 is reduced byusing the maximal values of PetCO2 over time [14].In patients with sudden pulmonary vascular occlusion due to pulmonary embolism, theresultant V/Q mismatch produces an increase in Vdalv. When volumetriccapnography is used as a bedside technique, the association of a normal D-dimer assayresult plus a normal Vdalv is a highly sensitive screening test to rule outthe diagnosis of pulmonary embolism [15].