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alveolar arterial gradient and ratio

Definition (1):
  • A-a gradient: The difference between the alveolar and arterial partial pressure of oxygen.
  • A-a ratio: The arterial oxygen concentration divided by the alveolar oxygen concentration.
(Patho)physiology:
  • PAO2 = PiO2 - PACO2/R = [FiO2 × (Patm – PH2O) – (PaCO2/R)]; measured in units of mm Hg (1,2).
    • Normal adult values in nonsmokers are <15 mm Hg on room air (FiO2 = 0.21). Higher FIO2 values result in an increased A-a gradient (1).
    • For every 10% increase in FiO2, the A-a gradient increases by 5–7 mm Hg. This effect is caused by the loss of regional hypoxic vasoconstriction in the lungs (1).
    • PACO2 is assumed to be equal to PaCO2, because of the ease of exchange of carbon dioxide (2).
    • The atmospheric pressure (Patm) at sea level is 101 kPa or 758 mmHg (2).
    • The water pressure (PH2O) is 6.2 kPa or 46.5 mmHg as inspired air is fully saturated by the time it reaches the carina (2).
    • R = 0.8 for a "normal" diet and approaches 1.0 as proportion of carbohydrates consumed increases (2).
  • Hypoxia with increased A-a gradient and a/A ratio: results from an increase in the venous admixture secondary to blood passing through the lung without being properly oxygenated (in addition to the physiologic admixture). Examples of this include:
    • V/Q mismatch: Discrepancy between the alveolar ventilation and capillary perfusion
    • Pulmonary shunt: Perfusion of the alveolar unit without ventilation, due to pathologic processes. Atelectasis describes alveolar deflation or fluid collection of the alveolar unit. Deflation can result from airway obstruction, mucus or blood plugging, inadequate tidal volumes due to pain, or positioning changes; other causes include endobronchial intubation, pneumothorax, collapse of emphysematous blebs, and one lung ventilation. Fluid collection can result from pulmonary edema, pneumonia, or adult respiratory distress syndrome (ARDS).
    • Intracardiac shunt: Venous blood is diverted from the pulmonary circulation directly into the systemic circulation. Examples include atrial or septal defects, pulmonary atrioventricular (AV) malformations, and cyanotic congenital heart disease.
    • Diffusion defects: Observed when the alveolar oxygen and carbon dioxide tensions are normal, but oxygen uptake by the alveolar capillaries is abnormal or impaired. Examples include pulmonary fibrosis, interstitial lung inflammation, and interstitial edema.
  • Hypoxia with normal A-a gradient or a/A ratio: can be seen in situations where the alveolar oxygen (or carbon dioxide) is affected, but oxygen uptake by the capillaries is not impaired. The decreased arterial oxygen concentration reflects the decreased alveolar concentration.
    • Hypoxic delivery: Anesthesia machine or ventilator malfunction, or high altitude
    • Hypoventilation: Respiratory depression from drugs, stroke in the pontine area, respiratory muscle fatigue (such as myasthenia gravis), or obesity–hypoventilation syndrome. The increase in carbon dioxide decreases the oxygen partial pressure (concentration) within the alveoli.
References:
  1. 5-Minute Anesthesiology Consult: Chapter 227: Alveolar Arterial Gradient and Ratio. 2013: full text.
  2. ABC of oxygen: assessing and interpreting arterial blood gases and acid-base balance. BMJ. 1998;317:1213: full text | pdf.