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Bedside Clinical Interpretation of Arterial PO2 and PCO2 Measurements

Adequacy of O2 exchange can be assessed at the bedside by examining the relationship between PO2 and inspired O2 fraction (FIO2 ). Although the A-a gradient (see Equation 6) varies with FIO2 , the a/A ratio does not. A low a/A ratio (<0.8) implies abnormal gas exchange. A simpler alternative is to calculate PaO2 /FIO2 (i.e., P/F ratio). Although this varies with FIO2 , it depends less on FIO2 than the A-a gradient ( Table 36-2 ). PaO2 /FIO2 normally exceeds 400 mm Hg. As gas exchange worsens, this ratio declines.


TABLE 36-2 -- A-a gradient and PaO2 /FIO2 in a person with normal pulmonary gas exchange *
FIO2 PAO2 a/A Ratio PaO2 PaO2 /FIO2 A-a Gradient
0.21 102 0.85  87 412  15
0.50 312 0.85 265 530  47
1.00 673 0.85 572 572 101
0.21 102 0.50  51 242  51
0.50 312 0.50 156 312 156
1.00 673 0.50 337 337 337
A-a gradient, alveolar-arterial gradient; PaO2 /FIO2 (P/F ratio), arterial oxygen tension/fractional inspired oxygen concentration.
*Assuming PaCO2 = 40 mm Hg, barometric pressure = 760 mm Hg, body temperature = 37°C, and respiratory exchange ratio (R) = 0.8.



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TABLE 36-3 -- Calculation of the VD/VT ratio *
V̇E V̇E × PaCO2 VD/VT
 5  200 0.4
 6  240 0.5
 8  300 0.6
10  400 0.7
15  600 0.8
30 1200 0.9
Interpretation of the adequacy of carbon dioxide exchange can be inferred from the VD/VT ratio. Although this measurement is usually not readily available, it can be estimated indirectly by examining PaCO2 in the context of the minute ventilation using the simple equation of V̇E × PaCO2 .
PaCO2 , partial pressure of arterial carbon dioxide; V̇E, minute ventilation; VD/VT, dead space expressed as a fraction of the tidal volume.
*Calculated from measured V̇E at PaCO2 = 40 mm Hg.



CO2 exchange can be assessed from the VD/VT ratio (Equation 12). Although the VD/VT ratio is usually not readily available, it can be estimated indirectly by examining PaCO2 in the context of the minute ventilation (V̇E) by the simple calculation of V̇E × PaCO2 (normally about 200 L/min/mm Hg during spontaneous breathing and 400 to 500 L/min/mm Hg during mechanical ventilation). As VD/VT increases (worsening CO2 exchange), the minute ventilation required to maintain a normal PCO2 rises, as does the V̇E × PaCO2 product ( Table 36-3 ).

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