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Clinical Calculator
Alveolar-Arterial (A-a) Oxygen Gradient
Calculates the A-a gradient, P/F ratio, a/A ratio, and respiratory index to assess the degree of pulmonary gas exchange impairment from ABG values.
Patient & ABG Values
Advanced Options — Water Vapor Pressure & Respiratory Quotient
RQ is typically 0.8 on a mixed diet. Values range from 0.7 (fat metabolism) to 1.0 (carbohydrate metabolism). Water vapor pressure is 47 mmHg at normal body temperature (37°C).
Calculated Results
PAO2 — Alveolar O2
— mmHg
Calculated
A-a Gradient
— mmHg
Expected A-a Gradient
— mmHg
2.5 + 0.21 × Age
P/F Ratio
— mmHg
a/A Ratio
—
Respiratory Index
—
Clinical Interpretation
Hypoxemia Severity (PaO2)
Formulas Used
Equations
PAO2 = FiO2 x (Patm - PH2O) - PaCO2 / RQ A-a = PAO2 - PaO2 Exp A-a = 2.5 + 0.21 x Age (primary formula) Age / 4 + 4 (simplified formula) P/F = PaO2 / FiO2(fraction) a/A = PaO2 / PAO2 RI = (PAO2 - PaO2) / PaO2 [= A-a / PaO2]
Reference — Normal Values
| Parameter | Normal Value | Unit |
|---|---|---|
| PAO2 (room air) | ~100 | mmHg |
| PaO2 (room air) | 80–100 | mmHg |
| A-a gradient (young, <30 yrs) | <10 | mmHg |
| A-a gradient (elderly) | <20–25 | mmHg |
| P/F ratio | ≥400 | mmHg |
| a/A ratio | >0.75 | — |
| Respiratory Index | <0.4 | — |
ARDS — Berlin Criteria (P/F Ratio)
Berlin Definition of ARDS — Requires bilateral infiltrates + non-cardiogenic pulmonary edema + PEEP ≥5 cmH2O
Mild ARDS
P/F 200–300
with PEEP ≥5 cmH2O
Moderate ARDS
P/F 100–200
with PEEP ≥5 cmH2O
Severe ARDS
P/F <100
with PEEP ≥5 cmH2O
Medical Disclaimer: This calculator is intended for educational and clinical reference purposes only. All results must be interpreted within the complete clinical context by a qualified healthcare professional. Do not use as a sole basis for clinical decisions. Always consult current clinical guidelines and institutional protocols.
