The alveolar-arterial oxygen gradient (A-a gradient) is a mathematical equation that assesses oxygen delivery by subtracting the partial pressure of oxygen in arterial blood (PaO₂) from the estimated partial pressure of oxygen in the alveolus (PAO₂).

The A-a gradient is used to help identify the cause of hypoxemia and intrapulmonary shunting as (1) ventilated alveoli but no perfusion, (2) unventilated alveoli with perfusion, or (3) collapse of both alveoli and capillaries.

The arterial/alveolar oxygen ratio (a/A ratio) is a calculation that reflects the percentage of PAO₂ that is contained in PaO₂ and can be used to determine if oxygen diffusion is improving.

1. Obtain and analyze an arterial blood sample. This gives the arterial partial pressures of oxygen (PaO₂) and of carbon dioxide (PaCO₂). The barometric pressure (Pb) and water vapor pressure (PH₂O) are also known, as is the fractional concentration of inspired oxygen (FIO₂), which is 0.21 (21%) for room air.

2. From these, derive the alveolar oxygen tension (PAO₂), the a/A ratio, and the alveolar-to-arterial difference for PO₂ (A-aDO₂) by use of formulas.

1. Increased values may be caused by:

   1. Mucus plugs

   2. Bronchospasm

   3. Airway collapse, as seen in:

      1. Asthma

      2. Bronchitis

      3. Emphysema

2. Hypoxemia (increased A-a gradient) is caused by:

   1. Atrial septal defects

   2. Pneumothorax

   3. Atelectasis

   4. Emboli

   5. Edema

Pretest Patient Care

1. Explain the purpose, benefits, and risks of arterial blood sampling.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Values increase with age and increasing O₂ concentration (gradient increases by 5–7 mm Hg [0.6–0.9 kPa] for every 10% increase in oxygen).

Normal

• A-a gradient: <10 mm Hg (<1.33 kPa) at rest (room air): 20–30 mm Hg (2.7–4.0 kPa) at maximum exercise (room air)

• a/A ratio: >75% (0.75)