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Interpretation of PaO₂ & PaCO₂ on the ABG

In this course, you will learn how to interpret arterial oxygenation in the context of the A-a gradient. This will help you avoid serious clinical error. You will learn the meaning of the terms, type I and type II respiratory failure and hypoventilation.

2.0 x AMA PRA Category 1 Credits™
Available on the Pro Plan



In this course, we 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. You will learn 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: Dr Karen Strahan PhD (University of Cambridge), Head of Editorial
Planner: Tommy O'Sullivan, CME Manager

Estimated Time to Complete

2.0 hours

Target Audience

  • 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 communicate effectively with colleagues

Course 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 the quiz
  3. View 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

Acadoodle adheres to the ACCME's Standards for Integrity and Independence in Accredited Continuing Education. Any individuals in a position to control the content of a CE activity, including faculty, planners, reviewers or others are required to disclose all relevant financial relationships with ineligible entities (commercial interests). All relevant conflicts of interest have been mitigated prior to the commencement of the activity.

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


Curran-Everett D. A Classic learning opportunity from Fenn, Rahn and Otis (1946): the alveolar gas equation. Advan Physiol Educ. 2006:30(2):58-62.

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

Dempsey JA and Smith CA. Pathophysiology of human ventilatory control. Eur Respir J. 2014;44(2):495-512.

Sue DY. Treating hypoxemia with supplemental oxygen. Same game, different rules. Ann Am Thorac Soc. 2016;13(12): 2266-71.

Additional Reading

Sue DY. Treating Hypoxemia with Supplemental Oxygen. Same Game, Different Rules. Ann Am Thor Soc. 2016;13(12):2266-71.

ACCME Accreditation Statement

Acadoodle, Ltd is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to provide continuing medical education for physicians.

Credit Designation Statement

AMA Physician’s Recognition Award

Acadoodle, Ltd designates this enduring material activity for a maximum of 2.0 AMA PRA Category 1 Credits™. Physicians should claim only the credit commensurate with the extent of their participation in the activity.

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