ABG, acid-base & ventilation
Read the gas. See the relationships.
Review arterial blood gas (ABG) relationships, expected acid-base compensation, oxygenation and adult predicted body weight (PBW). Check the sample and clinical context.
Private by designCalculations run in your browser. Do not enter names, dates of birth, medical-record numbers or other patient identifiers.
01
Interpretation
ABG & acid-base
Expected compensation——
P/F ratio—PaO₂ ÷ FiO₂
Anion gap—Na − (Cl + HCO₃)
Calculated HCO₃⁻—Henderson–Hasselbalch check
Show formulas and interpretation limits
Metabolic acidosis: expected PaCO₂ = 1.5 × HCO₃⁻ + 8, with a ±2 mmHg range.
Metabolic alkalosis: PaCO₂ typically rises 0.6–0.75 mmHg per 1 mEq/L rise in HCO₃⁻ and ordinarily should not exceed 55 mmHg.
Respiratory disorders: both acute and chronic expected HCO₃⁻ ranges are shown because one ABG cannot establish duration.
P/F ratio: Berlin oxygenation bands require PEEP or CPAP ≥5 cmH₂O, and an ARDS diagnosis also requires timing, imaging and origin-of-edema criteria. This tool does not diagnose ARDS.
Reference intervals vary by laboratory and patient context. Review the full clinical picture, sampling method, electrolytes, lactate, history and trends.
02
Adult ventilation
PBW, tidal volume & rate estimate
6 mL/kg PBW—4–8 mL/kg range: —
Current VT / PBW—entered tidal volume
Current minute ventilation—VT × respiratory rate
Proportional RR estimate—RR × current PaCO₂ ÷ target PaCO₂
Show formulas and ventilator safeguards
PBW: male coefficient = 50 + 2.3 × inches over 60; female coefficient = 45.5 + 2.3 × inches over 60. PBW is based on height, not actual body weight.
Proportional RR estimate: new RR = current RR × current PaCO₂ ÷ target PaCO₂. This assumes unchanged tidal volume, dead space, CO₂ production, spontaneous effort, leaks and circuit performance.
The RR result is a mathematical estimate, not an order. Verify pH and trend, plateau and driving pressures, total rather than set rate, auto-PEEP, synchrony, hemodynamics, alarms, device limits, local protocol and the complete clinical picture before changing ventilation.
How the calculations work
ABG interpretation questions
How does the calculator check compensation in metabolic acidosis?
It uses Winter’s formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8, with a ±2 mmHg range. A result outside that range can suggest an additional process, but the full clinical picture and sample must be reviewed.
What does the P/F ratio show?
The P/F ratio is PaO₂ in mmHg divided by FiO₂ expressed as a decimal. For example, PaO₂ 80 mmHg at FiO₂ 40% gives 80 ÷ 0.40 = 200. It is one oxygenation measure and does not by itself diagnose ARDS.
Are entered blood gas values sent to analytics?
No. Calculations run in this browser. Analytics counts an explicit use of the ABG or ventilator tool without sending pH, PaCO₂, HCO₃⁻, PaO₂, FiO₂, patient details or entered settings. Do not enter patient identifiers.
Planning Aid Only
Not a Clinical or HIPAA-Secure Interface. Always double-check your work, the sample and timing, ventilator settings and circuit, and clinically correlate. This tool does not diagnose a disorder or prescribe treatment.
Formulas, examples & assumptions
Content updated October 10, 2026 · Created by Bryan Fleck. Independently verify measurements and scoring assumptions.
Winter’s formula and P/F ratio
For metabolic acidosis, expected PaCO₂ = 1.5 × bicarbonate + 8 ± 2 mmHg. P/F ratio = PaO₂ in mmHg ÷ FiO₂ as a decimal. These are separate checks and do not establish a diagnosis.
Examples: bicarbonate 12 mmol/L gives expected PaCO₂ 26 ± 2 mmHg. PaO₂ 80 mmHg at FiO₂ 40% gives a P/F ratio of 200.
Predicted body weight and tidal volume
Adult PBW (kg) = 50 + 2.3 × (height in inches − 60) for the male equation, or 45.5 + 2.3 × (height in inches − 60) for the female equation. Tidal volume (mL) = selected mL/kg × PBW.
Example: the male equation at 70 inches gives 73 kg PBW; 6 mL/kg gives 438 mL. PBW is based on height and equation selection, not actual weight.
Minute ventilation and proportional rate
Minute ventilation (L/min) = tidal volume (mL) × respiratory rate ÷ 1,000. A proportional rate estimate uses current rate × current PaCO₂ ÷ target PaCO₂, assuming unchanged CO₂ production, dead space and effective tidal volume.
Example: 450 mL at 16 breaths/min gives 7.2 L/min. The proportional rate estimate is an educational relationship; changing ventilation requires clinical assessment.