Alveolar vs VentRef

A-a oxygen gradient — The alveolar - arterial (A-a) oxygen gradient is a measure of oxygen transfer across alveolar capillary membrane (“A” denotes alveolar and “a” denotes arterial oxygenation). It is the difference between the amount of the oxygen in the alveoli. PAO2 = Alveolar oxygen tension PaO2 = Arterial oxygen tension A-a oxygen gradient = PAO2 - PaO2. PaO2 is derived from ABG while PAO2 is calculated. PAO2 = (FiO2 x [Patm - PH2O]) - (PaCO2 ÷ R) FiO2 is the fraction of inspired oxygen (0.21 at room air). Patm is the atmospheric pressure (760 mmHg at sea level). PH2O is the partial pressure of water (47 mmHg ). PaCO2 is derived from ABG , and R is the respiratory quotient. (The respiratory quotient is approximately 0.8 at normal physiological state) A-a gradient varies with age and can be estimated from the following equation, assuming the patient is breathing room air. A-a gradient = 2.5 + 0.21 x age in years A-a gradient increases with higher FiO2. PaO2/FiO2 ratio — The PaO2/FiO2 ratio is a measure of oxygen transfer across the alveolar capillary membrane . Normal PaO2/FiO2 ratio is 300 to 500 mmHg, with values less than 300 mmHg indicating impaired gas exchange and values less than 200 mmHg indicates severe hypoxemia . Concept By: Dr. Satish Deopujari Dr Vivek Shivhare Disclaimer: This app is based on equations available publicly in Medical domain.  The information contained on this app is intended for educational purposes only. It does not constitute medical advice. You should consult a physician regarding medical diagnosis or treatment.

VentRef is a focused clinical reference for clinicians managing invasive and noninvasive mechanical ventilation and oxygen therapy, grounded in core respiratory physiology and evidence-based critical care practice. This application is intended for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Calculations and reference values are derived from published peer-reviewed medical literature and textbooks cited in the Sources section of the app. Always seek the guidance of a qualified healthcare professional with any questions regarding medical conditions or clinical decisions, and do not disregard or delay seeking professional advice because of information provided by this app. About Rishi Kumar, MD, FACC ("RK.MD") I'm an adult cardiothoracic anesthesiologist and cardiovascular ICU intensivist practicing in academic medicine. I'm passionate about teaching with free, open-access medical education. Learn more on my site: https://rk.md