O2-CO2 diagram as a tool for comprehension of blood gas abnormalities

Abstract

The O 2 -CO 2 diagram is an often neglected but very useful tool to understand the implications of arterial blood gas (ABG) analysis, which is a challenging concept not only to the undergraduate medical student but also to practicing clinicians. Here, we present a simplified description of the O2-CO2 diagram that was developed in all its elegance by Rahn and Fenn (6) and subsequently reviewed by others (2, 5). The O2-CO2 diagram plots PCO2 on the y-axis against PO2 on the x-axis. Any set of values of PO2 and PCO2, whether in atmospheric air, tracheal (inspired) air, alveolar air, mixed expired air, arterial blood, or venous blood, can be plotted on this diagram. It will be appreciated at the end of this discussion that 1) there are certain physical limits to what combinations of O2 and CO2 can exist in the alveolus and, therefore, the blood; and 2) even if arterial blood gases are the only available laboratory parameters, one can make reasonable assumptions about alveolar gas composition and assess if a patient has a pure ventilatory defect or a diffusion defect; however, it is difficult to assess mixed disorders without direct measurement of alveolar gas partial pressures. To begin, let us review the composition of atmospheric air, inspired air, and alveolar air (Fig. 1). Dry atmospheric air is composed of 21% O 2, with the rest N2 and negligible CO2. Therefore, PO2 in dry atmospheric air at sea level is 21% of the atmospheric pressure (Patm) of 760 mmHg, which is 159 mmHg. As air passes through the upper airways during inspiration, it is saturated with water vapor, the partial pressure of which at body temperature, pressure, saturated (BTPS) is 47 mmHg. The addition of water vapor dilutes inspired air, thereby reducing both the partial pressure of nitrogen and PO2 proportionally. The PO2 in inspired air is therefore 149 mmHg [21% of (760 47) mmHg] and is henceforth represented as PIO2 . While the inspired air in the conducting zone is composed of N2 ,O 2, and water vapor, a fourth gas, namely, CO2, is added to alveolar air. Since the alveoli are open to the atmosphere and during either phase of respiration alveolar pressure is only slightly different from Patm, it is safe to assume that alveolar pressure at all times is almost equal to Patm or the total pressure of inspired air (760 mmHg). The partial pressures of nitrogen (564 mmHg) and water vapor (47 mmHg) are almost the same in inspired and alveolar air. The remaining 149-mmHg pressure is due to O 2 alone in the inspired air (PI O2 ), whereas in the alveolus, the combined pressures of O 2 (PA O2 ) and CO 2 (PA CO2 ) must be close (but not necessarily equal 1 ) to 149 mmHg. Since O 2 diffuses out of the alveolus, it is obvious that removal of O2 and, therefore, reduction of its partial pressure compensates for the addition of CO2. The crude relationship is therefore as follows: