ABG Interpretation
ABG Interpretation

A-a Gradient



  • The alveolar-arterial (A-a) gradient is a comparison of the partial pressure of O₂ in the alveoli and in arterial blood.
      • Normal Range

      • 5 - 25
    • Calculating the A-a Gradient

    • The A-a gradient is calculated as the alveolar partial pressure of oxygen (PAO₂) minus the arterial partial pressure of oxygen (PaO₂):
  • A-a gradient = PAO₂ - PaO₂
  • The alveolar value is calculated, based on the patient's inspired FiO₂ and the PCO₂ from their blood gas result, while the arterial value is the PaO₂ from the patient's blood gas result.
    • Calculating the Alveolar Pressure of Oxygen

    • To calculate the A-a gradient, first calculate the alveolar partial pressure of oxygen (PAO₂), using the alveolar gas equation:
  • PAO₂ = ( FiO₂ × ( Patmos - PH₂O ) ) -
    • FiO₂: the fraction of inspired oxygen that the patient is inspiring - 0.21 on room air
    • Patmos: atmospheric pressure - 760mmHg at sea level
    • PH₂O: the water vapour pressure - 47mmHg at 37°
    • PCO₂:  the partial pressure of carbon dioxide, from the patient's ABG result
    • R: the respiratory quotient - 0.8
  • This can be simplified further, assuming that the patient is normothermic and at sea level:
  • PAO₂ = ( FiO₂ × ( 760 - 47 ) ) -
  • To produce the following formula:
  • PAO₂ = ( FiO₂ × 713 ) -

Elevated A-a Gradient

  • An elevated A-a gradient indicates that the partial pressure of O₂ is higher in the alveoli than in arterial blood, indicating a V/Q mismatch.
    • Causes of Elevated A-a Gradient

    • Dead space ventilation - pneumonia, asthma, COPD, pulmonary embolismVentilation without perfusion
    • Left to right shunt - pulmonary oedema, ARDS, pneumoniaPerfusion without ventilation
    • Alveolar hypoventilation - pulmonary fibrosis, interstitial lung disease
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 Andersen L, Mackenhauer J, Roberts J, Berg K, Cocchi M, Donnino M. Etiology and Therapeutic Approach to Elevated Lactate Levels. Mayo Clin Proc. 2013;88:1127-1140.
Beasley R, McNaughton A, Robinson G. New look at the oxyhaemoglobin dissociation curve. The Lancet. 2006;367:1124-1126.
 Bellomo R. Bench-to-bedside review: lactate and the kidney. Critical Care 2002;6(4):1. Berend K, de Vries A, Gans R. Physiological Approach to Assessment of Acid-Base Disturbances. N Engl J Med. 2014;371:1434-1445. Brenner BE. Alveolar-arterial oxygen gradients. Ann Emerg Med. 1980;9:648-648. Brindley PG, Butler MS, Cembrowski G, Brindley DN. Falsely elevated point-of-care lactate measurement after ingestion of ethylene glycol. Canadian Medical Association Journal 2007;176(8):1097-9. Donnino MW, Carney E, Cocchi MN, Barbash I, et al. Thiamine deficiency in critically ill patients with sepsis. Journal of critical care 2010;25(4):576-81. Gore DC, Jahoor F, Hibbert JM, DeMaria EJ. Lactic acidosis during sepsis is related to increased pyruvate production, not deficits in tissue oxygen availability. Annals of surgery 1996;224(1):97. Kraut JA, Madias NE. Lactic acidosis. N Engl J Med. 2014; 371: 2309-2319. Levraut J, Ciebiera JP, Chave S, Rabary O, et al. Mild hyperlactatemia in stable septic patients is due to impaired lactate clearance rather than overproduction. Am J RespirCrit Care Med. 1998; 157(4 Pt 1):1021-6. Levy B, Gibot S, Franck P, Cravoisy A, et al. Relation between muscle Na+ K+ ATPase activity and raised lactate concentrations in septic shock: a prospective study. The Lancet 2005;365(9462):871-5. Marino PL. Marino's the ICU Book. Fourthition. ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins; 2014. McCarter FD, Nierman SR, James JH, Wang L, et al. Role of skeletal muscle Na+–K+ ATPase activity in increased lactate production in sub–acute sepsis. Life sciences 2002;70(16):1875-88. Moreau R, Hadengue A, Soupison T, Kirstetter P, et al. Septic shock in patients with cirrhosis: hemodynamic and metabolic characteristics and intensive care unit outcome. Critical care medicine 1992;20(6):746. Perriello G, Jorde R, Nurjhan N, Stumvoll M, et al. Estimation of glucose-alanine-lactate-glutamine cycles in postabsorptive humans: role of skeletal muscle. American Journal of Physiology-Endocrinology and Metabolism 1995;269(3):E443-50. Phypers B, Pierce JT. Lactate physiology in health and disease. Continuing education in Anaesthesia, critical care & pain. 2006 Jun 1;6(3):128-32. Stacpoole PW. Lactic acidosis. Endocrinol Metab Clin North Am 1993 Jun; 22(2) 221-45.
Tunney P, Chinnan NK. Serum Lactate in Intensive Care: Practical Points and Pitfalls. inflammation. 2016;6:7.
 Vary TC. Sepsis-induced alterations in pyruvate dehydrogenase complex activity in rat skeletal muscle: effects on plasma lactate. Shock 1996;6(2):89-94. Venkatesh B, Morgan T, Garrett P. Measuring the lactate gap. The Lancet 2001;358(9295):1806.