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The partial pressure of oxygen in the alveoli (PAO₂) can be calculated using the alveolar gas equation: PAO₂ = (PB - PH₂O) × FiO₂ - (PaCO₂ / R), where PB is the barometric pressure, PH₂O is the water vapor pressure, FiO₂ is the fraction of inspired oxygen, PaCO₂ is the arterial carbon dioxide pressure, and R is the respiratory quotient (typically around 0.8).Alveolar Gas Equation
The alveolar gas equation is essential for assessing pulmonary gas exchange and diagnosing hypoxemia. The formula is as follows:\[ PAO₂ = (PB - PH₂O) \times FiO₂ - \left(\frac{PaCO₂}{R}\right) \]- PB: Barometric pressure (usually around 760 mmHg at sea level).
- PH₂O: Water vapor pressure (approximately 47 mmHg at body temperature).
- FiO₂: Fraction of inspired oxygen (e.g., 0.21 for room air, 0.40 for 40% oxygen).
- PaCO₂: Partial pressure of carbon dioxide in arterial blood, measured via arterial blood gas (ABG) analysis.
- R: Respiratory quotient, typically around 0.8.
Example Calculation
For example, if a patient is breathing room air (FiO₂ = 0.21), with a barometric pressure of 760 mmHg, a water vapor pressure of 47 mmHg, and a PaCO₂ of 40 mmHg, the calculation would be:- Calculate the effective pressure of oxygen:
\[ PAO₂ = (760 - 47) \times 0.21 - \left(\frac{40}{0.8}\right) \]\[ PAO₂ = 713 \times 0.21 - 50 \]\[ PAO₂ = 149.73 - 50 \]\[ PAO₂ = 99.73 \, \text{mmHg} \]Importance of PAO₂
Lab Tests GuideAlveolar Gas Equation Calculator - Lab Tests GuideCalculate alveolar oxygen tension (PAO₂) using the alveolar gas equation. Essential for pulmonologists, intensivists, and anesthesiologists to assess pulmonary gas exchange and dia…https://www.labtestsguide.com › alveolar-gas-equation-calculatorRespiratory Therapy ZoneRespiratory Formulas, Calculations, and Equations (2025)Alveolar minute ventilation (VA) calculates the volume of air that reaches the alveoli and participates in gas exchange per minute. It’s important for evaluating how well the lungs…https://www.respiratorytherapyzone.com › respiratory-therapy-formulas-calculationsWikipediaAlveolar gas equation - WikipediaThe alveolar gas equation is the method for calculating partial pressure of alveolar oxygen (pAO2). The equation is used in assessing if the lungs are properly transferring oxygen …https://en.wikipedia.org › wiki › Alveolar_gas_equation
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