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Atmospheric and Alveolar Pressures 

Atmospheric and Alveolar Pressures
Atmospheric and Alveolar Pressures

James R. Munis

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date: 09 May 2021

What you need to know, either to study altitude physiology or to monitor patients in the operating room or intensive care unit, is how to calculate alveolar oxygen pressure (PAO2) and how to compare that calculated value with the measured arterial oxygen pressure (PaO2). ‘P’ denotes pressure, of course (measured in mm Hg or torr, unless otherwise noted). Small capital ‘A’ denotes alveolar. Lowercase ‘a’ represents arterial. ‘PB’ is barometric pressure. ‘R’ is the respiratory quotient, which is simply the ratio of CO2 produced by the body divided by the amount of O2 consumed. ‘PH2O’ is the vapor pressure of water. FIO2 is the fraction of inspired O2, with 1.0 equivalent to 100% inspired oxygen. PIO2 is the partial pressure of inspired oxygen. This difference (PAO2 -PaO2), also termed AaDO2, gives an estimate of how efficiently the lungs are oxygenating the blood. There are several physiologic causes of hypoxemia. Hypoventilation, lowered PIO2, and lowered PB will not increase AaDO2 . The other 3 will.

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