To the Editor:-Determination of partial pressure of alveolar oxygen (PA^{O}^{2}) is necessary in several pathophysiologic conditions, including evaluation of alveolar-arterial oxygen gradient ([Greek small letter Delta] sub (A-a) O^{2}) and calculation of shunt fraction. The Equation bywhich the PA^{O}^{2}is calculated, the alveolar air Equation 1, [1]is where, FI^{O}^{2}is the inspiratory oxygen fraction, P^{B}is the inspiratory air pressure, P^{H}^{2}O is the alveolar saturated water vapor pressure, PA^{CO}^{2}is the alveolar carbon dioxide tension, and R is the respiratory exchange ratio (V^{CO}^{2/V}^{O}^{2}, normally 0.8).

The alveolar air equation (Equation 1) necessitates a knowledge of P sup *^{H}^{2}O. In most texts of physiology, P sup *^{H}^{2}O 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 *^{H}^{2}O 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 *^{H}^{2}O may also be calculated by the following empirical Equation 2:[3]To facilitate the calculation of P sup *^{H}^{2}O, based on Equation 2, we developed a simple nomogram by which derivation of P^{H}(^{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 *^{H}^{2}O can be found easily at any particular temperature, which ranges from 15 to 45 [degree sign]C. As an example, to find out the P^{H}^{2}O 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 *^{H}^{2}O can be read from the P sup *(^{H})^{2}O 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 P^{B}= 760 mmHg, FI^{O}^{2}= 21%, PA^{CO}^{2}= Pa^{CO}^{2}= 40 mmHg, Pa^{O}^{2}= 103 mmHg, and R = 0.8. Using Equation 1, then PA^{O}^{2}= 105.06 mmHg, and, as a consequence, [Greek small letter Delta] sub (A-a) O^{2}= 2.06 mmHg.

If instead of using the correct value of 31.6 mmHg for P sup *^{H}^{2}O, 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) O^{2}=-1.17 mmHg < 0.

Arterial P^{O}^{2}could never be higher than that of the alveolar pressure, therefore, a zero or a negative [Greek small letter Delta] sub (A-a) O^{2}, in any case, reflects an error. In the aforementioned case, although the calculated value of PA^{O}^{2}differs by only 3% from its actual value, the resultant [Greek small letter Delta] sub (A-a) O^{2}became 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.)