Predicted alveolar po2 equation using only respiration rate, tidal volume and dead space volume2/21/2024 ![]() ![]() ![]() ![]() Increasing altitude decreases the atmospheric pressure thus, for any given FiO2, there is a lower PO2 in the atmosphere and a lower PAO2 in alveoli. In clinical practice, the full alveolar gas equation does not convey relevant increased accuracy and the abbreviated equation discussed above is sufficient in calculating the PO2 in alveoli. Thus, some physicians and scientists suggest using the detailed form of the equation. Low FiO2 conditions could violate the steady state. The equation will only be valid if the assumptions upon which it was constructed remain true. The derived alveolar gas equation is based on the assumption of a steady state condition. If the rise in CO2 is significant, it can displace oxygen molecules which will cause hypoxemia.Īs atmospheric pressure reduces with increased altitude, the alveolar gas equation helps to calculate the PAO2 within the alveoli. This is significant to appropriately identify the developed hypoxemia from decreased atmospheric pressure and subsequently treat with appropriate supplemental oxygen levels. With the help of the alveolar gas equation, the partial pressure inside the alveoli can be calculated.Ĭarbon dioxide is a very important variable in the equation. The PO2 in alveoli can change significantly with variations in blood and alveolar carbon dioxide levels. The arterial PO2 can be determined by obtaining an arterial blood gas. The function of the alveolar gas equation is in calculating alveolar-arterial O2 gradient (A-a gradient).Ī-a gradient increases 5 to 7 for every 10% increase in FiO2. Each has an important clinical significance and can help explain different physiological and pathophysiological states. The 3 major variables of the equation are the atmospheric pressure, amount of inspired oxygen, and levels of carbon dioxide. RQ = amount of CO2 produced/amount of oxygen consumed Indirect calorimetry can provide better measurements of RQ by measuring the VO2 (oxygen uptake) and VCo2 (carbon dioxide production). RQ is different for carbohydrates, fats, and proteins (average value is around 0.82 for the human diet). The value of the RQ can vary depending upon the type of diet and metabolic state. PaCO2 is partial pressure of carbon dioxide in alveoli (in normal physiological conditions around 40 to 45 mmHg). Patm is the atmospheric pressure (at sea level 760 mm Hg), PH2O is partial pressure of water (approximately 45 mm Hg). ![]()
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