Instructions and clinical scope
Enter the arterial blood gas values and inspired oxygen concentration. Results update automatically. The A-a gradient helps separate hypoxemia due to hypoventilation or low inspired oxygen from hypoxemia due to V/Q mismatch, diffusion limitation, or shunt. Interpretation must be integrated with oxygen delivery method, altitude, pregnancy physiology, imaging, and the clinical picture.
Pregnancy note: normal pregnancy is associated with chronic respiratory alkalosis and lower PaCO2. A PaCO2 that appears normal for a nonpregnant adult may be abnormal in pregnancy. The A-a gradient can still be calculated, but it should not be used alone to rule out pulmonary embolism, pneumonia, edema, ARDS, or other causes of maternal hypoxemia.
Patient and ABG values
Equation assumptions
Equations and interpretation guardrails
Alveolar gas equation
PAO2 = FiO2 x (Patm - PH2O) - (PaCO2 / RQ) A-a gradient = PAO2 - PaO2 Expected A-a gradient estimate = (age + 10) / 4
FiO2 is entered as a percent in the form, then converted to a decimal for the equation. The A-a gradient is most interpretable on room air or when the delivered FiO2 is known. It may increase with age and with higher FiO2.
| Pattern | Common interpretation |
|---|---|
| Hypoxemia with normal A-a gradient | Consider hypoventilation or low inspired oxygen, depending on PaCO2 and clinical setting. |
| Hypoxemia with increased A-a gradient | Consider V/Q mismatch, diffusion limitation, or shunt. Examples include pneumonia, pulmonary edema, ARDS, atelectasis, asthma/COPD exacerbation, pulmonary embolism, or right-to-left shunt. |
| Limited response to supplemental oxygen | Raises concern for clinically important shunt physiology, but interpretation requires clinical context. |
Pregnancy-specific interpretation notes
Pregnancy increases minute ventilation and lowers arterial carbon dioxide tension. Published physiologic reviews describe arterial PCO2 in pregnancy as reaching a plateau around 32 mm Hg, with some studies reporting third-trimester mean PaCO2 near 26 to 27 mm Hg. Therefore, a PaCO2 of 40 mm Hg may represent inadequate ventilation in pregnancy even though it may be reported as normal by a general adult laboratory.
Maternal oxygen saturation is generally expected to remain normal in uncomplicated pregnancy. Persistent SpO2 below 95%, increased work of breathing, chest pain, tachycardia, fever, abnormal lung exam, pulmonary edema, or fetal/maternal instability should prompt clinical evaluation rather than reliance on the calculator alone.
References with detailed links
- StatPearls / NCBI Bookshelf. Alveolar Gas Equation. Describes the alveolar gas equation, clinical use of the A-a gradient, and the expected normal estimate using (age + 10) / 4.
- StatPearls / NCBI Bookshelf. Physiology, Alveolar to Arterial Oxygen Gradient. Reviews PAO2, PaO2, and calculation of the A-a gradient.
- MSD Manual Professional Edition. Equations for calculating alveolar oxygen pressure and alveolar-to-arterial oxygen gradient. Provides formula structure and notes that the gradient increases with age and FiO2.
- Merck Manual Professional Edition. Measurement of Gas Exchange. Discusses gas exchange, V/Q mismatch, and response to supplemental oxygen.
- Sarkar M, Niranjan N, Banyal PK. Mechanisms of hypoxemia. Lung India. 2017;34(1):47-60. Reviews hypoxemia mechanisms including V/Q mismatch and widened A-a gradient.
- LoMauro A, Aliverti A. Respiratory physiology of pregnancy. Breathe. 2015;11(4):297-301. Reviews pregnancy respiratory physiology and lower PCO2 in pregnancy.
- Templeton A, Kelman GR. Third-trimester arterial blood gas and acid base values in normal pregnancy. Obstet Gynecol. 1976;47:347-350. Classic reference for normal third-trimester ABG values.
- Ahmed A, et al. Fetomaternal Acid-Base Balance and Electrolytes during Pregnancy. J Clin Med. 2022;11(8):2160. Reviews acid-base physiology and respiratory alkalosis of pregnancy.
Disclaimer
This calculator is for clinical decision support and educational use. It does not diagnose the cause of hypoxemia and is not a substitute for clinical judgment, ABG verification, assessment of oxygen delivery, imaging, specialist consultation, or local practice standards. Confirm all calculations before clinical use.