# Alveolar Gas Equation (PAO₂ Calculator) Calculate PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. Defaults: P_atm 760 mmHg (sea level), RQ 0.8. Gives a directional hint vs expected ~100 mmHg. For full A-a gradient interpretation use a-a-gradient-calculator. Derived from StatPearls, LITFL, Cornell PICU, and West. Not medical advice. > Canonical page: https://elysiatools.com/en/tools/alveolar-gas-equation - **Category:** Health - **Keywords:** alveolar gas equation, pao2, alveolar oxygen, fio2, respiratory quotient, altitude, oxygenation, blood gas, abg, pulmonology, critical care ## Overview The Alveolar Gas Equation Calculator estimates alveolar oxygen pressure (PAO₂) from PaCO₂, FiO₂, atmospheric pressure, and respiratory quotient using PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. It uses sea-level pressure and an RQ of 0.8 by default and provides a directional comparison with expected PAO₂ values. ## Inputs - **PaCO₂ (mmHg)** (number): Arterial partial pressure of CO₂ (mmHg) from ABG. - **FiO₂ (inspired oxygen fraction)** (number): Inspired oxygen fraction (0.21 = room air). Range 0.15–1.0. - **Atmospheric Pressure (mmHg, optional)** (number): Atmospheric pressure (mmHg). Default 760 at sea level. Lower at altitude (e.g. Denver ~630). - **Respiratory Quotient (RQ, optional)** (number): Respiratory quotient. Default 0.8 (mixed diet); 1.0 pure carbohydrate; ~0.7 high fat/protein. - **Decimal Places** (number) ## When to use - Estimate PAO₂ from arterial PaCO₂ and inspired oxygen values. - Adjust the calculation for altitude by entering a different atmospheric pressure. - Check how calculated PAO₂ compares with the expected value on room air at sea level. ## How it works - Enter PaCO₂ in mmHg and FiO₂ as a fraction, such as 0.21 for room air. - Optionally replace the default atmospheric pressure of 760 mmHg and RQ of 0.8. - The calculator applies PAO₂ = FiO₂ × (P_atm − 47) − PaCO₂ / RQ. - The JSON result reports PAO₂, inputs, calculation method, and a directional category. ## Use cases - Review arterial blood gas inputs by estimating the expected alveolar oxygen pressure. - Compare room-air PAO₂ at sea level with PAO₂ at reduced atmospheric pressure at altitude. - Explore how FiO₂, PaCO₂, atmospheric pressure, and RQ affect the calculated PAO₂. ## Frequently asked questions ### What does this calculator measure? It estimates alveolar oxygen partial pressure, or PAO₂, in mmHg. ### What FiO₂ should I enter for room air? Enter 0.21 for room air. ### What are the default atmospheric pressure and RQ? The defaults are 760 mmHg for sea level and 0.8 for a mixed diet. ### Can I calculate PAO₂ at altitude? Yes. Enter the local atmospheric pressure in mmHg instead of using the sea-level default. ### Does this replace a complete A-a gradient interpretation? No. This tool focuses on PAO₂ and does not provide a full A-a gradient interpretation. It is not medical advice. ## Related tools - [A-a Gradient (Alveolar-Arterial O₂)](https://elysiatools.com/en/tools/a-a-gradient-calculator): Calculate the alveolar-arterial O₂ gradient to distinguish hypoxemia causes. PAO₂ = FiO₂×(P_atm−47) − PaCO₂/0.8; A-a = PAO₂ − PaO₂. Normal ≈ (Age+10)/4. Elevated → V/Q mismatch, shunt, diffusion impairment. Derived from StatPearls, LITFL, Cornell PICU, and Martin 1999. Not medical advice. - [Alveolar Ventilation Calculator (VA)](https://elysiatools.com/en/tools/alveolar-ventilation-calculator): Calculate VA = (VT − VD) × RR in L/min. VD may be supplied or estimated as ~2 mL/kg body weight. Normal resting ~4–6 L/min; low → hypercapnia, high → hypocapnia. Derived from West, LITFL, StatPearls, and Radford 1955. Not medical advice. - [Apgar Score (Newborn 1 & 5 min)](https://elysiatools.com/en/tools/apgar-score): Calculate the Apgar score to assess newborn condition at 1 minute and 5 minutes after birth. Five signs are each scored 0–2 at both time points: Appearance (skin color), Pulse (heart rate), Grimace (reflex irritability), Activity (muscle tone), and Respiration. Each time point totals 0–10. Interpretation: 7–10 reassuring; 4–6 moderately abnormal (may need assistance); 0–3 critically low (immediate resuscitation). The tool reports both the 1-min and 5-min scores, the change (delta), and per-time-point interpretation. A 5-min score, and especially the change from 1 to 5 min, indicates response to resuscitation; persistent low 5-min scores warrant continued resuscitation and reassessment at 10 min. Derived from Apgar 1953, reaffirmed by ACOG/AAP. The Apgar is a physiologic snapshot at a moment in time and is not, by itself, a marker of asphyxia or long-term neurologic outcome. Not medical advice. - [Henderson-Hasselbalch Equation (pH / HCO₃⁻ / PCO₂)](https://elysiatools.com/en/tools/bicarbonate-ph-conversion): Solve any one of pH/HCO₃⁻/PaCO₂ from the other two via pH = 6.1 + log10(HCO₃⁻/(0.03×PaCO₂)). Reports linear Henderson \[H⁺\] cross-check. Derived from Henderson 1908, Hasselbalch 1916, Siggaard-Andersen 1974, LITFL, StatPearls, Berend 2016. Not medical advice. - [Body Adiposity Index Calculator](https://elysiatools.com/en/tools/body-adiposity-calculator): Calculate Body Adiposity Index (BAI) - estimates body fat percentage from hip circumference and height - [Child-Pugh Score (Cirrhosis Severity)](https://elysiatools.com/en/tools/child-pugh-score): Calculate the Child-Pugh score to grade chronic liver disease / cirrhosis severity and estimate perioperative mortality. Five parameters scored 1–3 each (total 5–15): Total bilirubin (<2 mg/dL =1, 2–3 =2, >3 =3) — supports mg/dL or µmol/L input (auto ÷17.1); Serum albumin (>3.5 g/dL =1, 2.8–3.5 =2, <2.8 =3); INR (<1.7 =1, 1.7–2.3 =2, >2.3 =3); Ascites (absent =1, slight/controlled =2, moderate–severe =3); Hepatic encephalopathy (none =1, grade I–II =2, grade III–IV =3). Classes: A (5–6) well-compensated (1-y ~100%, 2-y ~85%); B (7–9) significant compromise (1-y ~80%, 2-y ~60%); C (10–15) decompensated (1-y ~45%, 2-y ~35%). Derived from Pugh et al. 1973 (Br J Surg) and the original Child-Turcotte classification. Not medical advice. - [CURB-65 Score (Pneumonia Severity)](https://elysiatools.com/en/tools/curb-65-score): Calculate the CURB-65 score to assess community-acquired pneumonia (CAP) severity and guide the site of care. Each component scores 1 point: Confusion (new-onset disorientation), Urea > 7 mmol/L (BUN > 20 mg/dL), Respiratory rate ≥ 30/min, Blood pressure (systolic < 90 OR diastolic ≤ 60 mmHg), and Age ≥ 65. Range 0–5. Interpretation: 0–1 low risk (likely outpatient; 30-day mortality ~1.5–2.4%), 2 moderate (consider admission or supervised discharge, ~9.2%), 3–5 high risk (hospital admission; consider ICU for 4–5; mortality rises to ~57% at score 5). Endorsed by the British Thoracic Society. Derived from Lim et al. 2003 (Thorax). Not medical advice. - [Dead Space Fraction (VD/VT, Bohr & Enghoff)](https://elysiatools.com/en/tools/dead-space-fraction): Calculate VD/VT using both the Bohr equation (PACO₂-based) and the Enghoff modification (PaCO₂-based). Normal ~0.20–0.40; >0.60 signals severe dead space in ARDS/PE. Derived from Bohr 1891, Enghoff 1938, Tusman 2012, LITFL, StatPearls. 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