The alveolar gas equation

The alveolar gas equation is used to estimate the Pao2 of a perfect alveolus with varying fractions of inspired oxygen and it states that 

Note that the SVP of water in the airways is subtracted from the atmospheric pressure before multiplying by the Fio2. This is because the fractional concentration of 02 only applies to the portion of inhaled mixture that is dry gas. 

The Paco2 is assumed to be in equilibrium with arterial C02 tension (Paco2) and this number will either be given or will be assumed to be within the normal range. 

The value of R varies according to which energy substrates make up the predominant part of the diet. With a normal diet, it is assumed to have a value of 0.8 pure carbohydrate metabolism gives a value of 1.0. 

Note that there is no difference between the ideal alveolar value and the normal arterial Pao2 of 13.3 kPa. In practice a difference of up to 2 kPa is allowable owing to ventilation-perfusion (V/Q) mismatch and shunt. 

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In order to evaluate the ventilatory status of a patient the alveolar oncygevL tension (Pj 02) must be known. It can be determined from the "alveolar gas equation" (equation 5),... 

The value for Cc o2 cannot be calculated in this way very easily as a sample is technically difficult to take without a catheter in the pulmonary vein. It is, therefore, assumed to be in equilibrium with the Pao2, which, in turn, is given by the alveolar gas equation. 

State the first principles, for example the Bohr equation considers a single tidal exhalation comprising both dead space and alveolar gas. 

As arterial C02 tension is practically identical to alveolar C02 partial pressure, it can be used as a surrogate measurement. This is desirable as measuring arterial C02 tension involves only a simple blood gas analysis. The term Paco2, therefore, becomes Paco2 and so the equation is often written as... 

The level of carbon dioxide is itself proportional to the amount of gas passing in and out of the alveoli (alveolar ventilation) according to the following equation ... 

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