Why Get Arterial Blood Gases?
6 WHY GET ARTERIAL BLOOD GASES?
Alden H. Harken M.D.
1. Is breathing really overrated?
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It may be. A Japanese yoga master survived just fine breathing once per minute for an hour (see reference 1)!
2. Mr. O’Flaherty has just undergone an inguinal herniorrhaphy under local anesthesia. The recovery room nurse asks permission to sedate him. She says that he is confused and unruly and keeps trying to get out of bed. Is it safe to sedate Mr. O’Flaherty?
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No. A confused, agitated patient in the recovery room or surgical intensive care unit (SICU) must be recognized as acutely hypoxemic until proved otherwise.
3. Mr. O’Flaherty is moved to the SICU, and at 2:00 a.m. the SICU nurse calls to report that he has a Po2 of 148 mmHg on facemask oxygen. Is it okay to roll over and go back to sleep?
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No. More information is needed.
4. You glance at the abandoned cup of coffee sitting on your well-worn copy of Surgical Secrets. What is the Po2 of that cup of coffee?
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148 mmHg.
5. How can Mr. O’Flaherty and the coffee have the same Po2?
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The abandoned coffee presumably has had time to equilibrate completely with atmospheric gas. At sea level, the barometric pressure is 760 mmHg. To obtain the partial pressure of oxygen in the coffee, subtract water vapor pressure (47 mmHg) and multiply by the concentration of oxygen (20.8%) in the atmosphere:
PO2 = (760-47) x 20.8% – 148 mmHg
6. What is the difference between Mr. O’Flaherty’s and the coffee’s Po2?
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Nothing. Both represent the partial pressure of oxygen in fluid. A complete set of blood gases is necessary.
7. What constitutes a complete set of blood gases?
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* Po2
* Pco2
* pH
* Hemoglobin saturation
* Hemoglobin concentration
8. If Mr. O’Flaherty and the coffee have the same Po2, how would Mr. O’Flaherty do if he were exchange-transfused with coffee?
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Badly.
9. Why?
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Although the oxygen tensions are the same, the amount of oxygen in blood is vastly greater.
10. How does one quantitate the amount of oxygen in blood?
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Arterial oxygen content (CaO2) is quantitated as mL of oxygen/100 mL of blood. (Watch out: Almost all other concentrations traditionally are provided per mL or per L-not per 100 mL.) Because mL of oxygen is a volume in 100 mL of blood, these units frequently are abbreviated as vol %.
11. Why is blood thicker than coffee (or wine)?
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Because hemoglobin binds a huge amount of oxygen. A total of 10 g of fully saturated hemoglobin (hematocrit about 30%) binds 13.4 mL of oxygen, whereas 100 mL of plasma at a Po2 of 100 mmHg contains only 0.3 mL of oxygen.
12. Does the position of the oxyhemoglobin dissociation curve make any difference?
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* An increase in Pco2
* An increase in hydrogen ion concentrations (not pH)
* An increase in temperature
All shift the oxyhemoglobin curve to the right; that is, oxygen is released more easily in the tissues. Within physiologic limits, however, Mae West probably said it best: “There is less to this than meets the eye.”
KEY POINTS: MEDIATORS OF OXYHEMOGLOBIN DISSOCIATION CURVE
Right Shift Left Shift
1. Increase in Pco2
2. Increase in [H+], lower pH
3. Increase in temperature stored
4. Increase in 2,3-DPG
1. Decrease in [H+], higher pH
2. Higher altitudes/elevation
3. Decrease in 2,3-DPG (e.g., at 4 wk blood maintains no DPG)
13. If Cao2 or ultimately systemic oxygen delivery (cardiac output x Cao2) is what the surgeon really wants to know, why does the nurse report Mr. O’Flaherty’s Po2 instead of his Cao2 at 2:00 a.m.?
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No one knows.
14. What is the fastest and most practical method of increasing Mr. O’Flaherty’s Cao2?
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Transfusion of red blood cells. The patient’s Cao2 is increased by 25% with transfusion from a hemoglobin concentration of 8 to 10 g/dL. The patient’s arterial oxygen content is affected negligibly by an increase in arterial Po2 from 100 to 200 mmHg (hemoglobin is fully saturated in both instances).
15. What is a transfusion trigger?
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The hematocrit at which a patient is automatically transfused. This is not a useful concept. The NIH Consensus Conference, drawing data from Jehovah’s Witnesses, patients with renal failure, and monkeys concluded that it is not necessary to transfuse a patient until the hematocrit is 21%. Traditional surgical dogma mandates a hematocrit >30%. When the patient is in trouble, however, authorities in surgical critical care encourage transfusion to a hematocrit of 45% to optimize systemic oxygen delivery.
16. What governs respiratory drive?
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Pco2 and pH are inextricably intertwined by the Henderson-Hasselbalch equation. By juggling this equation in the cerebrospinal fluid (CSF) of goats, it is clear that CSF hydrogen ion concentration (not Pco2) controls respiratory drive. This distinction is not clinically important, however. What is important is that if a person becomes acidotic either with diabetic ketoacidosis or by running up a flight of stairs, minute ventilation (VE) is increased.
17. How tight is respiratory control? Or, if you hold your breath for 1 minute, how much do you want to breathe?
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A lot (unless you are a yoga master approaching nirvana).
18. After 60 seconds of apnea, what happens to Paco2?
Show answer
It increases only from 40 to 47 mmHg. Tiny changes in Pco2 (and pH) translate into a huge respiratory stimulus. Normally, respiratory compensation for metabolic acidosis is tight.
19. Define base excess.
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Base excess is a poor man’s indicator of the metabolic component of acid-base disorders. After correcting the Pco2 to 40 mmHg, the base excess or base deficit is touted as an indirect measure of serum lactate. Although many parameters directing volume resuscitation in shock are more practical and direct (see Chapter 3), base deficit has been advertised as helpful. The base excess or deficit is calculated from the Sigaard-Anderson nomogram in the blood gas laboratory. Normally, there is no base excess or deficit. Acid-base status is “just right.”
References
BIBLIOGRAPHY
1. Dekerle J, Baron B, Dupont L, et al: Maximal lactate steady state, respiratory compensation threshold, and critical power. Eur J Appl Physiol 89:280-288, 2003.
2. Miyamura M, Nishimura K, Ishida K, et al: Is a man able to breathe once a minute for an hour? The effect of yoga exercises on blood gases. Jpn J Physiol 52:313, 2002.
3. Tada T, Hashimoto F, Matsushita Y, et al: Study of life satisfaction and quality of life of patients receiving home oxygen therapy. J Med Invest 50:55-63, 2003.
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Abernathy’s Surgical Secrets, Updated Edition (Book w / Student Consult)
Author / s: Harken Alden H., Abernathy Charles, Moore Ernest Eugene
Year: 2004
Pages: 473
Publishers: Elsevier Mosby; 5th Bk & Acc edition
ISBN: 0323034160
