Activity Title

Interpretation of PaO₂ & PaCO₂ on the ABG

Activity Overview

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

  • Consultant Physician at St. Vincent's University Hospital, Dublin, Ireland
  • Lecturer at the School of Medicine and Medical Science, University College Dublin, Ireland
  • Studied medicine at the University of Cambridge, United Kingdom
  • A natural sciences graduate of Trinity College Dublin, Ireland
  • PhD in Cell Biology from University College London, United Kingdom

Planner: Karen Strahan, PhD (Cantab), Head of Editorial

Planner: Tommy O'Sullivan, CME Manager

Estimated Time to Complete

2 hours

Target Audience

  • Physicians (General Internal Medicine, Family Physicians)
  • Nurses
  • Nurse Practitioners
  • Physician Assistants
  • Paramedics

Learning Objectives

Upon successful completion of this activity, you will be able to:

  • Explain the nature of the normal A-a gradient
  • Explain the mechanisms by which disease affects the A-a gradient
  • Calculate the A-a gradient appropriate to a patients age and level of inspired oxygen
  • Interpret the PaO₂ in terms of the degree of alveolar ventilation and in the presence of oxygen therapy
  • Use appropriate analysis of the PaO₂ to identify unsuspected pathology
  • Avoid clinical error when interpreting the PaO₂ and PaCO₂ on an ABG result
  • Explain the terms, hypoventilation and hyperventilation and explain the difference between type I and type II respiratory failure thereby
  • Allowing you to communicate effectively with colleagues

Activity Content

  • Introduction
  • Control of Respiration
  • The Alveolar Gas Equation
  • VQ Mismatch & the Physiological Shunt
  • The Expected A-a Gradient
  • Elevation of the A-a Gradient in Disease
  • Elevation of the A-a Gradient: ABG Findings
  • Hypoventilation
  • Some Key Terms
  • Case Examples - PaO₂
  • Respiratory Failure
  • Case Examples - Respiratory Failure
  • Quiz
  • Quiz Explanation

Release Date


Expiration Date


Instructions for Participation

Participants must complete the online activity during the valid period as noted above.

Follow these steps:

  1. View videos in sequence
  2. Complete quiz
  3. View and complete post quiz video to support learning outcomes and objectives
  4. Complete the activity evaluation form to provide feedback for continuing education purposes and for the development of future activities
  5. Download the Certificate of Completion

Relevant Financial Disclosures

Planners and faculty for this activity have no relevant financial relationships with commercial interests to disclose.


Albert TJ and Swenson ER. Circumstances when arterial blood gas analysis can lead us astray. Respir care 2016;61(1):119-21.

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

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.


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