The following parts of All You Really Need to Know to Interpret Arterial Blood Gases are on this web site. They are copied from the printed version, available from Williams & Wilkins (see "How To Order")
Author and Publisher Information
Preface and Table of Contents
Section: "How To Use This Book"
Pre-Test (Questions & Answers)
How To Order The Book
All You Really Need to Know to Interpret Arterial Blood Gases
by LAWRENCE MARTIN, M.D., FACP, FCCP
Chief, Division of Pulmonary and Critical Care Medicine Mt. Sinai Medical Center, Cleveland
Associate Professor of Medicine
Case Western Reserve University School of Medicine
Ph.: 216-421-3708
Fax: 216-421-6952
e-mail to martin@lightstream.net
Copyright 1992, Published by Williams & Wilkins Co.
Baltimore, MD
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DEDICATION
There are a few basic tests used in the care of patients. A basic test is one that is applicable to a broad group of patients, provides invaluable information quickly, can be repeated as often as necessary, and is not dependent on patient effort for accuracy. My short list of such essential tests (in alphabetical order):
1. arterial blood gases
2. chest x-ray
3. complete blood count (CBC)
4. electrocardiogram
5. Gram's stain for bacteria
6. serum electrolytes, BUN and glucose
7. urine analysis.
No doubt, the better we understand information provided by these few tests, the better we can care for our patients. CT scans, echocardiograms, perfusion scans, Doppler studies, enzyme assays, spirometry, and other tests of specific organ function (e.g., thyroid, liver, pancreas) certainly have their place, and are at times crucial to diagnosis. However the seven tests listed above, along with the medical history and physical examination, form a foundation for managing virtually all inpatients and a great many chronically ill outpatients.
The newest test on this list to become routinely available is arterial blood gases. The first arterial puncture was performed in 1912 by Hurter, a German physician. In 1919 arterial blood gas analysis was first used as a diagnostic procedure. Employing Hurter's radial artery puncture technique, W.C. Stadie measured oxygen saturation in patients with pneumonia and showed that cyanosis of critically ill patients resulted from incomplete oxygenation of hemoglobin (Stadie 1919).
Over the next 40 years blood gas measurements were more of a laboratory research tool than a test available for everyday patient care. Techniques for measuring blood gases required specialized apparatus and were difficult to perform. It was not until the 1950s that electrodes were developed that could rapidly and reproducibly measure PaO2, PaCO2 and pH.
In 1953 Leland Clark invented the platinum oxygen electrode, a prototype that evolved into the first modern blood gas electrode (Clark 1953, Clark 1956). Development of commercially viable pH and PCO2 electrodes soon followed and by the mid-1960s several university centers were able to provide pH, PaCO2 and PaO2 measurements on arterial blood, albeit using cumbersome and non-automated equipment. In 1973 the first commercially available, automated blood gas machine was introduced (ABL1 from Radiometer), and this was soon followed by machines from other companies (Severinghaus 1986). Today virtually every acute care hospital provides rapid and automated blood gas testing 24 hours a day, 7 days a week.
As performed by electrodes on a single sample of arterial blood, the ABG test now has competition: non-invasive measurements. Particularly popular, and replacing the need for some arterial sample-based tests, are pulse oximeters for measuring oxygen saturation and end-tidal gas analysis for PCO2. In neonates and small children, skin electrodes for measuring PO2 and PCO2 have found wide application.
Even more exciting is the new technology for measuring blood gases continuously, using tiny fiberoptic sensors that fit inside the blood vessel. Although an invasive technique, optical sensing promises to add a new dimension to monitoring changes in pH, PaCO2 and PaO2.
Whatever the technology, the important thing is the information and its proper clinical application. All blood gas technologies are designed to provide information on oxygenation, ventilation and/or acid-base balance through one or more measurements. Teaching you how to interpret and wisely use blood gas values no matter how they are obtained is the goal of this book.
This is not a physiology textbook. I have left out some aspects of blood gas physiology that, while interesting, are not crucial to learning basic blood gas interpretation; examples include the shunt equation, the carbon dioxide dissociation curve, and the Fick equation for oxygen uptake. Nor is this a compendium of all clinical situations. Omitted are discussions of blood gases during the neonatal period, mixed venous oxygen measurements, and blood gas alterations during hyperbaric therapy. The bibliography provides several references where one can find discussion of these and other specialized topics.
Rather than produce an encyclopedia that covers everything lightly, I have tried to create a work that will, first and foremost, teach the important aspects in depth and be clinically useful. The vast majority of people who use arterial blood gases in the care of patients should find in this book all they "really need to know."
Lawrence Martin, M.D.
Pre-Test (Questions & Answers)
Introduction to Quik-Course
Chapter 1. What is Meant by Interpreting Arterial Blood Gases? One Blood Sample, Two Machines
Chapter 2. Three Physiologic Processes, Four Equations
Chapter 3. PaCO2 and Alveolar Ventilation
Chapter 4. PaO2 and the Alveolar-arterial PO2 Difference
Chapter 5. SaO2 and Oxygen Content
Chapter 6. pH, PaCO2, HCO3- and Acid-baseStatus
Chapter 7. Putting It All Together: Cases
Chapter 8. Putting It All Together: Free Text Interpretations
Chapter 9. Pitfalls in Blood Gas Interpretation
Chapter 10. Quik-Course on Blood Gas Interpretation
Appendix A. Post-test
Appendix B. Answers to Pre- and Post-tests
Appendix C. Symbols and Abbreviations
Appendix D. Glossary
Appendix E. Bibliography
Index
1. Get a pencil.
This is a very practical book about an important laboratory test, arterial blood gases. The book's emphasis is on interpreting blood gases in the clinical setting. Real patients and real clinical situations are presented along the way.
Don't read this book without a pencil in hand. You won't need a calculator and paper is optional; the necessary arithmetic can be done in the book itself. But go get a pencil. Without applying pencil to paper -- answering the multiple-choice questions and doing some simple calculations before checking your answers -- you won't be forced to think about the problems presented. And you won't get the maximum value out of the book.
Answers to the numbered questions (e.g., 2-1, 2-2) are at the end of each chapter. Questions without numbers are answered in the paragraphs immediately following; they are signified by a ? just before the list of possible answers.
QUESTIONS:
Numbered: Answered at end of chapter
Not numbered: Answered in subsequent
paragraphs
You should answer all questions with a pencil as they are encountered, then check your answers. Follow this advice and you cannot help but learn the fundamentals of blood gas interpretation. If you skip the pencil you won't know whether the information has registered or whether you have really learned what's important. Applying pencil to paper is the only reliable way to learn what the book attempts to teach you.
So go get a pencil.
2. Take the Pre-test, then check your answers (Appendix B).
3. Read the Introduction to Quik-Course, page xxi. Then, either review the Quik-Course (Chapter 10) or begin with Chapter 1, page 1.
4. Read the chapters at your own pace, always stopping to answer each question with your pencil.
5. Make sure you understand all the questions and answers in a given chapter before proceeding to the next chapter.
6. Check the list of symbols or the glossary for any unfamiliar terms (Appendices C and D).
7. Take the Post-test after completing all the chapters (Appendix A).
8. Write me with any corrections, disagreements, suggestions for improvements, etc. If you have a personal computer, and want additional instruction in blood gas interpretation and respiratory failure management, consider the computer programs offered on the last page of this book.
Lawrence Martin, M.D.
Chief, Division of Pulmonary and
Critical Care Medicine
Mt. Sinai Medical Center
One Mt. Sinai Drive
Cleveland, Ohio 44106
Ph.: 216-421-3708
Fax: 216-421-6952
e-mail to martin@lightstream.net
Take this Pre-test now. If you answer over 90% of the 35 items correctly, give this book to a friend. You probably don't need it.
Directions: For each of the following seven statements or questions, there may be none, one, or more than one correct response. Circle the correct response(s) before checking the answers in Appendix B.
1. Normal range for PaCO2 is 35-45 mm Hg. A change in PaCO2 from normal to 28 mm Hg means the subject
a) is hyperventilating.
b) has excess alveolar ventilation for the amount of CO2 production.
c) must have hypoxia, anxiety, and/or metabolic acidosis.
d) must be breathing faster than normal.
e) must have acute respiratory alkalosis.
2. The arterial PO2 is predicted to be reduced to some extent from
a) anemia.
b) ventilation-perfusion (V-Q) imbalance with an increase in the number of low V-Q units.
c) increased PCO2, while the subject is breathing room air.
d) carbon monoxide poisoning.
e) altitude.
3. To obtain a reasonable idea of the acid-base state of a patient's blood, you would need to know the
a) pH and PaCO2.
b) pH and PaO2.
c) PaCO2 and PaO2.
d) PaCO2 and HCO3-.
e) pH and SaO2 (%saturation of hemoglobin with oxygen).
4. Which of the following statements regarding acid-base balance (is) are correct?
a) HCO3- increases with acute elevation of PaCO2, before any renal compensation takes place.
b) A patient can have metabolic acidosis and metabolic alkalosis at the same time.
c) "Base excess" is normally zero +/- 2 mEq/L.
d) An elevated "anion gap" is presumptive evidence for metabolic acidosis unless proven otherwise.
e) In theory, the bicarbonate value calculated from the Henderson-Hasselbalch equation and the "CO2" value measured in the chemistry lab as part of routine electrolyte measurements should be identical.
5. The following information is accurate and/or useful when determining a patient's arterial oxygen content (CaO2).
a) Each 100 ml of hemoglobin can combine with 1.34 ml of oxygen.
b) Normal CaO2 is between 16 and 22 mgm/dl.
c) Normally, dissolved oxygen constitutes less than 2.0% of the CaO2.
d) Normally, mixed venous oxygen content at rest is about 25% less than CaO2.
e) A 10% decrease in SaO2 will produce the same percentage decrease in CaO2 as will a 10% decrease in hemoglobin content.
6. Arterial blood gas data (pH, PaCO2, PaO2, SaO2) are related in some simple but important ways. Which of the following are valid relationships?
a) Alveolar PO2 is related to PaCO2 by the alveolar gas equation: as PaCO2 goes up, alveolar PO2 goes down.
b) PaO2 is inversely related to blood pH: as pH goes up, PaO2 goes down.
c) If PaCO2 increases while HCO3- remains unchanged, pH always goes down.
d) PaO2 is related to SaO2 on a linear scale (i.e., a straight-line relationship).
e) The SaO2 is related to hemoglobin-bound arterial oxygen content on a linear scale (i.e., a straight-line relationship).
7. There are some 'truisms' in terminology and physiology for proper blood gas interpretation. They include which of the following?
a) "Hyperventilation" and "hypoventilation" are clinical terms, and are not diagnosed by arterial blood gases.
b) The alveolar-arterial PO2 difference increases with age and with increase in the fraction of inspired oxygen.
c) The arterial PO2 cannot go above 100 mm Hg while breathing room air at sea level.
d) A continuously negative alveolar-arterial PO2 difference is incompatible with life.
e) If arterial pH is normal, the patient cannot have a clinically significant acid-base disorder.
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Answers to Pre-test questions
1. a and b are correct
incorrect:
c) patient may have other reasons to hyperventilate, such as asthma exacerbation, fever, any acute parenchymal infiltrate
d) patient may be breathing deeper than normal
e) patient could have metabolic acidosis
2. b, c and e are correct
incorrect:
a) anemia reduces oxygen content, not PaO2
d) carbon monoxide reduces SaO2 and CaO2, not PaO2
3. a and d are correct
incorrect:
b, c, and e; you need at least two of the three variables in the Henderson-Hasselbalch equation to obtain a reasonable idea of a patient's acid-base balance
4. a, b, c and d are correct
incorrect:
e) In theory, the bicarbonate calculated in the blood gas lab should be 1 to 2 mEq/L less than the "CO2" calculated in the chemistry lab, since the latter value includes the quantity contributed by PaCO2
5. c, d and e are correct
incorrect:
a) each gram of hemoglobin can combine with 1.34 ml of oxygen
b) Normal CaO2 is between 16 and 22 ml O2/dl
6. a, c and e are correct
incorrect:
b) arterial PO2 and pH are not directly related by any equation
d) arterial PO2 is related to SaO2 by the O2 dissociation curve, which has a sigmoid configuration
7. b and d are correct
incorrect:
a) "hyperventilation" and "hypoventilation" should only be used clinically as they relate to the PaCO2
c) people with normal lungs can increase arterial PO2 above 100 mm Hg with hyperventilation
e) a patient can have profound acid-base imbalance yet normal pH from opposing acid-base disorders (e.g., combined metabolic acidosis and metabolic alkalosis)
HOW TO ORDER
All You Really Need To Know To Interpret Arterial Blood Gases, by Lawrence Martin, M.D.
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