Alveolar - arterial (A-a) Gradient [iPhone]

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.