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All You Really Need to Know to Interpret Arterial Blood Gases
by Lawrence Martin, M.D.

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Preface to 2nd Edition

Click on underlined chapters for full text excerpts

In the field of blood gas interpretation, not much has changed since the first edition of this book (1992). The basic physiology is the same of course, and the way the test is run is also little changed. A new edition is nonetheless justified for several reasons.

First, there is the need to incorporate additional information on non-invasive "blood gas" data, principally pulse oximetry measurement of SpO2 and end-tidal PCO2 (PetCO2) monitoring. Both tests have now become routine in the management of patients, the PetCO2 mainly in the intensive care arena. These tests are largely responsible for a nationwide decline in the number of arterial samples analyzed for blood gas data. Indeed, it is now commonplace to manage stable, mechanically-ventilated patients almost entirely without arterial blood draws, using only non-invasive blood gas measurements. Expanded coverage of this topic is included within the respective chapters on PCO2 and oxygen saturation.

A new edition also provides opportunity to better explain some important physiology. For example, in discussions with students and physicians, I often encounter confusion about the crucial differences between PaO2, SaO2 and oxygen content. The confusion is over how they relate to each other, and to factors such as anemia, carboxyhemoglobin and methemoglobin. This subject has been re-written and greatly expanded for clarity and emphasis. Likewise, the section on anion gap and electrolytes has been expanded, with several examples provided on use of the "bicarbonate gap" to uncover mixed acid-base disorders.

I have also added a third "Putting it Altogether" chapter, on clinical evaluation and test ordering. The two "Putting it Altogether" chapters in the first edition gave blood gas data to interpret, but did not ask "When is a blood gas necessary?" and "Can some other test substitute?" The new chapter presents 16 brief patient scenarios, most without any blood gas data, and you are asked to determine what gas exchange tests should be obtained: e.g., complete arterial blood gas measurements?; co-oximetry alone?; pulse oximetry alone?; none?

In the first edition I omitted discussion of venous blood gases since, in my opinion, this is not information most clinicians really need to know. Yet many students and clinicians want to learn about this topic, even though they are seldom called on to interpret mixed venous blood gases. For them I have added a chapter, "Venous Blood Gases: Beyond All You Really Need to Know."

Other changes to this edition include more figures, expansion of the pre- and post-blood gas quizzes, an enlarged bibliography, and an internet address for our Pulmonary Medicine web site (see below).

As always, emphasis is on basic gas exchange physiology useful in the care of patients. You will need a pencil to get the most out of this book, but a calculator is not necessary (though it may speed problem solving). If you do the exercises as they are presented, you cannot help but learn "All You Really Need to Know to Interpret Arterial Blood Gases." I will go one step further: if you read carefully through each chapter and work on all the problems and exercises, you should acquire sufficient expertise to teach the subject.

Lawrence Martin, M.D.


Table of Contents

How to Use This Book for Maximum Benefit

Arterial Blood Gas Pre-Test

ABG Quik-Course on Blood Gas Interpretation

Chapter 1. What is Meant by Interpreting Arterial Blood Gases? One Blood Sample, Two Sets of Tests

Chapter 2. Three Physiologic Processes, Four Equations

Chapter 3. PaCO2 and Alveolar Ventilation

Chapter 4. PaO2 and the Alveolar-arterial PO2 Difference

Chapter 5. PaO2, SaO2 and Oxygen Content (Section from Chapter 5: Differences between PaO2, SaO2, and CaO2)

Chapter 6. SaO2, Hemoglobin binding and Pulse Oximetry

Chapter 7. pH, PaCO2, HCO3-, Electrolytes and Acid-base Status (Section from Chapter 7: Anion Gap and Bicarbonate gap)

Chapter 8. Primary and Mixed Acid-Base Disorders (Section from Chapter 8: How to Diagnose Mixed Acid-Base Disorders)

Chapter 9. Putting It All Together: Cases with Blood Gas Data to Interpret

Chapter 10. Putting It All Together: Free Text Interpretations of Blood Gas Data

Chapter 11. Putting It All Together: Clinical Evaluation and Test Ordering

Chapter 12. Venous Blood Gases: Beyond "All You Really Need to Know"

Chapter 13. Pitfalls in Blood Gas Interpretation

Appendix A. ABG Post-test

Appendix B. Answers to Pre- and Post-tests

Appendix C. Symbols and Abbreviations

Appendix D. Glossary

Appendix E. Bibliography

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