What is the A-a gradient?

The alveolar-arterial (A-a) gradient measures the difference between alveolar oxygen and arterial oxygen. A normal gradient is approximately (age/4) + 4 mmHg. An elevated A-a gradient indicates a problem with oxygen transfer in the lungs, such as V/Q mismatch, shunt, or diffusion impairment.

What is the P/F ratio?

The P/F ratio (PaO2/FiO2) is a quick assessment of oxygenation. A normal P/F ratio is 400–500. A ratio <300 meets criteria for Acute Respiratory Distress Syndrome (ARDS), with <200 classified as moderate and <100 as severe ARDS. It is one of the Berlin criteria for ARDS diagnosis.

How do I estimate oxygen tank duration?

Tank duration depends on the tank size (E-cylinder, H-cylinder), the remaining pressure, and the flow rate in liters per minute. The formula is: duration (minutes) = (pressure × cylinder factor) ÷ flow rate. Our calculator handles the math for common cylinder sizes.

Pulmonology Calculators

Q: How do I estimate oxygen tank duration?

Tank duration depends on the tank size (E-cylinder, H-cylinder), the remaining pressure, and the flow rate in liters per minute. The formula is: duration (minutes) = (pressure × cylinder factor) ÷ flow rate. Our calculator handles the math for common cylinder sizes.

Respiratory and pulmonology tools including A-a gradient, FiO2 conversion, P/F ratio, oxygen tank duration, SpO2-to-PaO2 conversion, and the Asthma Control Test. Essential for managing respiratory conditions and ventilator settings.

This category currently includes 9 tools, including FiO₂ Conversion, P/F Ratio, and O₂ Tank Duration.

These resources are built for clinicians, trainees, and medically informed patients who need fast bedside calculations. Use the results as decision support and pair them with full clinical context and local guidelines.

All Pulmonology Tools (9)

FiO₂ Conversion

Convert nasal cannula L/min to FiO2% instantly. Full reference chart for 1–6 L/min, face mask, non-rebreather, and HFNC. Includes P/F ratio and A-a gradient inputs.

P/F Ratio

Calculate the P/F ratio to classify ARDS severity by Berlin criteria. Mild: 200–300 (27% mortality). Moderate: 100–200 (32%). Severe: <100 (45%). Normal P/F is 400–500.

O₂ Tank Duration

Estimate oxygen cylinder duration by tank size, PSI, and flow rate (L/min). Includes D, E, G, H/K, and M tanks for EMS transport, inpatient transfer, and home oxygen planning.

STOP-BANG

Screen for obstructive sleep apnea risk using the validated STOP-BANG questionnaire (0-8 points).

Pack-Year Calculator

Calculate pack-years from cigarettes per day and years smoked. Useful for documenting smoking burden and lung cancer screening discussions.

ACT Score

Calculate the Asthma Control Test score to assess asthma control over the past 4 weeks.

COPD GOLD

Classify COPD severity using 2024 GOLD spirometric stages (GOLD 1–4) and ABE exacerbation grouping. Guides inhaler step-up therapy, pulmonary rehabilitation, and oxygen therapy decisions.

mMRC Dyspnea Scale

Grade baseline breathlessness from 0 to 4 with the modified Medical Research Council (mMRC) dyspnea scale.

BODE Index

Estimate COPD mortality risk with the BODE index using BMI, airflow obstruction, dyspnea grade, and 6-minute walk distance.

Frequently Asked Questions

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