Interpretation of PaO₂ & PaCO₂ on the ABG
In this course we teach you how to interpret the arterial partial pressure of oxygen (the PaO₂) in the context of the Alveolar-arterial oxygen gradient (A-a gradient). Our ultimate aim is to show you how analysis of the PaO₂ in the context of the A-a gradient can help you avoid critical errors in clinical practice. In order to adopt this approach to arterial blood gas (ABG) analysis, certain concepts must first be understood. We begin by explaining how in response to CO₂ produced during metabolism, brainstem centres control the level of alveolar ventilation. We explain how this establishes a strict relationship between the fraction of inspired oxygen (FiO₂), the PaO₂, the arterial partial pressure of CO₂ (PaCO₂) and the partial pressure of oxygen in the alveoli (PAO₂). We show you how the relationship between these key parameters is described by the alveolar gas equation. We teach you how to use this equation to calculate the PAO₂ and the actual A-a gradient present in a patient based on values reported on the ABG. We explain the nature of and factors contributing to the normal A-a gradient. We teach you how to calculate the predicted appropriate A-a gradient for a patient of a given age on a given level of inspired oxygen. In a series of case studies, we then illustrate how an understanding of the A-a gradient helps us avoid misdiagnosis in practice and allows us to detect problems in patients on oxygen therapy. We also explain two important, commonly misused terms in respiratory medicine, hypoventilation and hyperventilation. We explain the difference between and the identification of type I and type II respiratory failure.
Planner and Author: Dr. John Seery, MB, PhD
Planner: Karen Strahan, PhD (Cantab), Head of Editorial
Planner: Tommy O'Sullivan, CME Manager
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Planners and faculty for this activity have no relevant financial relationships with commercial interests to disclose.
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Curran-Everett D. A Classic learning opportunity from Fenn, Rahn and Otis (1946): the alveolar gas equation. Teaching with classic papers. Advances in Physiology Education. American Physiological Society. 01 JUN 2006 https://doi.org/10.1152/advan.00076.2005
Dempsey JA and Smith CA. Pathophysiology of human ventilatory control. Eur Respir J 2014;44(2):495-512.
Ganong’s Review of Medical Physiology 25th Edition. Barrett et al (authors). Chapter 36: Control of Respiration. McGraw-Hill.