FiO2 and A a Gradient Calculator

Inspired Oxygen Fraction FiO2 PaO2 arterial oxygen pressure mmHg PaCO2 arterial carbon dioxide pressure mmHg Atmospheric Pressure Patm mmHg Water Vapor Pressure PH2O mmHg Respiratory Quotient RQ FiO2 and A a Gradient Calculator Explanation and Clinical Context This calculator estimates the inspired oxygen fraction FiO2 together with the alveolar to arterial oxygen gradient which provides an assessment of gas exchange efficiency in the lungs. The alveolar gas equation calculates the theoretical alveolar oxygen level by incorporating FiO2 atmospheric pressure water vapor pressure carbon dioxide pressure and the respiratory quotient. The A a gradient reflects the difference between alveolar oxygen and measured arterial oxygen.

A normal gradient becomes wider with age but a markedly elevated value suggests impaired oxygen diffusion ventilation perfusion mismatch or right to left shunt. This tool supports clinical evaluation in conditions such as hypoxemic respiratory failure pulmonary embolism interstitial lung disease and severe pneumonia. It guides clinicians when interpreting oxygenation impairment and determining escalation of therapy including oxygen supplementation non invasive support or invasive ventilation.

Reference West JB Respiratory Physiology The Essentials. Nunn JF Applied Respiratory Physiology. ATS Clinical Standards on Oxygenation Assessment.