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, 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. 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. 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. At a particular absolute temperature T, P sup *H2O may also be calculated by the following empirical Equation 2: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.