Aa Gradient Calculator

A-a Gradient Equation:

\[ A\text{-}a\ Gradient = (FiO_2 \times (P_{atm} - P_{H_2O}) - (PaCO_2 / 0.8)) - PaO_2 \]

FiO2 (Fraction):

fraction

P_atm (mmHg):

mmHg

P_H2O (mmHg):

mmHg

PaCO2 (mmHg):

mmHg

PaO2 (mmHg):

mmHg

A-a Gradient:

Unit Converter ▲

Unit Converter ▼

From:	To:

1. What is A-a Gradient?

The Alveolar-arterial oxygen gradient (A-a gradient) measures the difference between alveolar oxygen concentration and arterial oxygen concentration. It's a valuable tool for assessing gas exchange efficiency in the lungs and diagnosing various pulmonary conditions.

2. How Does the Calculator Work?

The calculator uses the A-a Gradient equation:

\[ A\text{-}a\ Gradient = (FiO_2 \times (P_{atm} - P_{H_2O}) - (PaCO_2 / 0.8)) - PaO_2 \]

Where:

\( FiO_2 \) — Fraction of inspired oxygen (0.21 for room air)
\( P_{atm} \) — Atmospheric pressure (760 mmHg at sea level)
\( P_{H_2O} \) — Water vapor pressure (47 mmHg at 37°C)
\( PaCO_2 \) — Arterial carbon dioxide partial pressure
\( PaO_2 \) — Arterial oxygen partial pressure

Explanation: The equation calculates the alveolar oxygen partial pressure and subtracts the measured arterial oxygen partial pressure to determine the gradient.

3. Importance of A-a Gradient Calculation

Details: A-a gradient is crucial for differentiating between hypoxemia due to ventilation-perfusion mismatch, diffusion defects, or shunts versus hypoventilation. It helps diagnose pulmonary embolism, pneumonia, ARDS, and other respiratory disorders.

4. Using the Calculator

Tips: Enter FiO2 as a fraction (0.21 for room air, 1.0 for 100% oxygen), atmospheric pressure (760 mmHg at sea level), water vapor pressure (47 mmHg), PaCO2 and PaO2 from arterial blood gas analysis.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal A-a gradient?
A: Normal is <10-15 mmHg on room air in young adults, increasing with age (approximately 3 mmHg per decade).

Q2: When is A-a gradient elevated?
A: Elevated in V/Q mismatch, diffusion defects, right-to-left shunts, pulmonary embolism, pneumonia, and interstitial lung disease.

Q3: Why use 0.8 in the equation?
A: The respiratory quotient (R) is assumed to be 0.8, representing the average ratio of CO2 production to O2 consumption.

Q4: Does altitude affect A-a gradient?
A: Yes, atmospheric pressure decreases with altitude, requiring adjustment of P_atm value in the calculation.

Q5: What are limitations of A-a gradient?
A: Affected by FiO2, age, body position, and assumes steady-state conditions. Not reliable with high FiO2 concentrations.