To the Editor:-Determination of partial pressure of alveolar oxygen (PAO2) is necessary in several pathophysiologic conditions, including evaluation of alveolar-arterial oxygen gradient ([Greek small letter Delta] sub (A-a) O2) and calculation of shunt fraction. The Equation bywhich the PAO2is calculated, the alveolar air Equation 1, [1]is where, FIO2is the inspiratory oxygen fraction, PBis the inspiratory air pressure, PH2O is the alveolar saturated water vapor pressure, PACO2is the alveolar carbon dioxide tension, and R is the respiratory exchange ratio (VCO2/VO2, normally 0.8).
The alveolar air equation (Equation 1) necessitates a knowledge of P sup *H2O. In most texts of physiology, P sup *H2O is designated to be 47 mmHg. [1]This value, however, is a function of alveolar (body) temperature and varies markedly from approximately 13 mmHg at 15 [degree sign]C to approximately 72 mmHg at 45 [degree sign]C. [2]The values for P sup *H2O at different temperatures are readily available in handbooks of physical chemistry and in texts of anesthesia and respiratory physiology. [2]At a particular absolute temperature T, P sup *H2O may also be calculated by the following empirical Equation 2:[3]To facilitate the calculation of P sup *H2O, based on Equation 2, we developed a simple nomogram by which derivation of PH(2) O sup * at different temperatures can be performed easily within a few seconds. The accuracy of this nomogram (Figure 1) is sufficient for routine clinical practice. The corresponding P sup *H2O can be found easily at any particular temperature, which ranges from 15 to 45 [degree sign]C. As an example, to find out the PH2O sup * at 30 [degree sign]C, the corresponding point to the 30 [degree sign]C on the temperature axis (left side values) should be located first. Then, at the same ordinate, the value for the desired P sup *H2O can be read from the P sup *(H)2O axis (right side values), which, in this case, is approximately 31.6 mmHg. Assuming the following scenario, the importance of this simple correction could be evident.
Assume a body temperature of 30 [degree sign]C, the P sup *H(2) O, as was found out earlier, is therefore 31.6 mmHg. Now assume PB= 760 mmHg, FIO2= 21%, PACO2= PaCO2= 40 mmHg, PaO2= 103 mmHg, and R = 0.8. Using Equation 1, then PAO2= 105.06 mmHg, and, as a consequence, [Greek small letter Delta] sub (A-a) O2= 2.06 mmHg.
If instead of using the correct value of 31.6 mmHg for P sup *H2O, the usual value of 47 mmHg is utilized, the result then becomes PA (O)2= 101.83 mmHg, and, subsequently, [Greek small letter Delta] sub (A-a) O2=-1.17 mmHg < 0.
Arterial PO2could never be higher than that of the alveolar pressure, therefore, a zero or a negative [Greek small letter Delta] sub (A-a) O2, in any case, reflects an error. In the aforementioned case, although the calculated value of PAO2differs by only 3% from its actual value, the resultant [Greek small letter Delta] sub (A-a) O2became negative and, therefore, meaningless.
Farrokh Habibzadeh, M.D.
Mahboobeh Yadollahie, M.D.
National Iranian Oil Company Outpatient Polyclinics; Shiraz, Iran;habibzaf@pearl.sums.ac.ir
(Accepted for publication May 22, 1998.)
