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Alveolar Gas Equation Tool

  • Fraction of inspired oxygen FiO2 percent
  • Arterial carbon dioxide PaCO2 in mmHg
  • Respiratory quotient RQ default 0.8
  • Barometric pressure Patm in mmHg default 760
  • Water vapor pressure PH2O in mmHg default 47
  • Optional arterial oxygen PaO2 in mmHg to calculate A a gradient
  • Alveolar Gas Equation Explanation and Clinical Context
    The alveolar gas equation estimates alveolar oxygen partial pressure PAO2 using inspired oxygen fraction FiO2, barometric pressure Patm, water vapor pressure PH2O, arterial carbon dioxide PaCO2 and respiratory quotient RQ. The formula used here is PAO2 equals FiO2 times Patm minus PH2O minus PaCO2 divided by RQ. Normal PAO2 depends on FiO2 and altitude. At sea level with FiO2 of 21 percent and typical values the calculated PAO2 is often around 100 mmHg in healthy lungs.

    This calculation is clinically useful to estimate expected alveolar oxygen and to compute the alveolar to arterial oxygen difference A a gradient when arterial oxygen PaO2 is available. Elevated A a gradient suggests impaired gas exchange due to diffusion limitation ventilation perfusion mismatch or right to left shunt. Use this tool as an aid not as a substitute for clinical judgment and integrate with patient context and other tests.

    Reference
    West JB. Respiratory Physiology The Essentials. 10th edition or latest edition. The alveolar gas equation and its clinical applications are described in standard respiratory physiology texts.

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