Calcul Gradient Alvéolo Artériel

A-a Gradient Calculator

A-a Gradient Equation:

\[ A\text{-}a\ Gradient = (FiO_2 \times (760 - 47) - \frac{PaCO_2}{0.8}) - PaO_2 \]

A-a Gradient:

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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 helps assess the efficiency of oxygen transfer from alveoli to pulmonary capillaries.

2. How Does the Calculator Work?

The calculator uses the A-a Gradient equation:

\[ A\text{-}a\ Gradient = (FiO_2 \times (760 - 47) - \frac{PaCO_2}{0.8}) - PaO_2 \]

Where:

\( FiO_2 \) — Fraction of inspired oxygen (0.21 for room air)
\( PaCO_2 \) — Arterial partial pressure of carbon dioxide (mmHg)
\( PaO_2 \) — Arterial partial pressure of oxygen (mmHg)
760 — Atmospheric pressure at sea level (mmHg)
47 — Water vapor pressure at 37°C (mmHg)
0.8 — Respiratory quotient (assumed value)

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 causes of hypoxemia. Normal gradient suggests hypoventilation, while increased gradient indicates ventilation-perfusion mismatch, diffusion impairment, or shunt.

4. Using the Calculator

Tips: Enter FiO2 as fraction (0.21 for room air, 1.0 for 100% oxygen), PaCO2 and PaO2 in mmHg. All values must be positive and within physiological ranges.

5. Frequently Asked Questions (FAQ)

Q1: What is a normal A-a gradient?
A: Normal A-a gradient is age-dependent: approximately (Age/4) + 4 mmHg. For young adults, normal is < 10-15 mmHg on room air.

Q2: What causes increased A-a gradient?
A: Increased gradient occurs in pulmonary embolism, pneumonia, ARDS, pulmonary edema, interstitial lung disease, and right-to-left shunts.

Q3: When is A-a gradient normal despite hypoxemia?
A: Normal gradient with hypoxemia suggests hypoventilation (e.g., drug overdose, neuromuscular disorders).

Q4: How does altitude affect A-a gradient?
A: At altitude, atmospheric pressure decreases, requiring adjustment of the 760 mmHg value in the calculation.

Q5: What are limitations of A-a gradient?
A: Assumes normal respiratory quotient and may be less accurate with high FiO2 or in critically ill patients with complex physiology.