Chapter 18 Acid-Base Balance and Blood Gas Analysis
Regulation of the hydrogen ion concentration
6. What is the normal plasma H+ concentration, the normal plasma HCO3− concentration, and the normal arterial pH of blood?
7. How is normal arterial pH maintained?
8. What are the buffering systems, and which system has the greatest contribution to the total buffering capacity of blood?
9. How does the bicarbonate buffering system work? What enzyme facilitates this reaction?
10. How does hemoglobin act as a buffer?
11. How does the ventilatory response work?
12. How does the renal response work?
13. How quickly can the buffering system, ventilation changes, and the renal response work?
Measurement of arterial blood gases
14. What is the relationship between a venous and arterial blood gas drawn from the same patient concurrently?
15. What errors can occur if heparin or air is present in an arterial blood gas sample?
16. What happens if there is a delay in analysis of the blood sample?
17. How does temperature affect the arterial blood gas (ABG)?
18. What does an anesthesia provider using alpha stat during cardiopulmonary bypass do to the ABG?
19. What does an anesthesia provider using pH stat during cardiopulmonary bypass do to the ABG?
Differential diagnosis of acid-base disturbances
20. What is the difference between a primary disturbance and a compensatory disturbance in acid-base status?
21. What adverse responses are associated with severe acidemia?
22. What adverse responses are associated with severe alkalemia?
23. What defines a primary respiratory acidosis or alkalosis?
24. What are the causes of a respiratory acidosis?
25. What is the compensatory response for a respiratory acidosis?
26. What is the treatment for a respiratory acidosis?
27. What are the causes of a respiratory alkalosis?
28. What is the compensatory response for a respiratory alkalosis?
29. What is the treatment for a respiratory alkalosis?
30. What defines a primary metabolic acidosis or alkalosis?
31. How is the anion gap calculated?
32. What are the causes of a metabolic acidosis?
33. What is the compensatory response for a metabolic acidosis?
34. Describe how the Stewart strong ion difference approach works.
35. What is the treatment for a metabolic acidosis?
36. What are the causes of a metabolic alkalosis?
37. What is the compensatory response for a metabolic alkalosis?
38. What is the treatment for a metabolic alkalosis?
39. How can an acute respiratory process be distinguished from a chronic process?
40. How is the Δgap determined?
41. How is the Winter’s formula used?
42. Diagram the algorithm for diagnosing an acid-base disorder.
Answers*
General definitions
1. A physiologic acid-base status optimizes enzyme function, myocardial contractility, and saturation of hemoglobin with oxygen. (334)
2. Bronsted defined an acid as a molecule that can act as a proton [H+] donor, and a base as a molecule that can act as a proton acceptor. In biologic molecules, weak acids or bases are molecules that can reversibly donate H+ or reversibly bind H+. (334)
3. Acidemia is defined as an arterial pH less than 7.35 and alkalemia is defined as an arterial pH greater than 7.45. (334)
4. An acidosis is the underlying process that lowers the pH, whereas an alkalosis is the process that raises the pH. A patient can have a mixed disorder with both an acidosis and an alkalosis, but can only be either acidemic or alkalemic. (334)
5. Base excess is usually defined as the amount of strong acid or strong base required to return 1 L of whole blood exposed in vitro to a PCO2 of 40 mm Hg to a pH of 7.4. The number is supposed to refer to the metabolic component of an acid-base disorder. It is most often used in the operating room as a surrogate marker for lactic acidosis to help determine the adequacy of volume resuscitation. (335)
Regulation of the hydrogen ion concentration
6. At 37° C, the normal plasma H+ concentration is 35 to 45 nmol/L. The normal plasma HCO3− concentration is 24 ± 2 mEq/L, and the normal arterial pH of blood is between 7.36 and 7.44. (335)
7. Normal arterial pH is maintained through three systems: buffers, ventilation changes, and renal response. The ventilatory response involves changes in alveolar ventilation and CO2 concentrations. The renal response involves reabsorption of bicarbonate ions or secretion of hydrogen ions. (335)
8. The buffering systems in blood include bicarbonate, hemoglobin, phosphate, and plasma proteins. The bicarbonate buffering system is the largest contributor and provides 50% of the total buffering capacity of the body. Hemoglobin is responsible for about 35% of the total buffering capacity, and phosphate and plasma proteins account for the remainder. (335)
9. Carbonic anhydrase facilitates the hydration of carbon dioxide in the plasma and in the erythrocytes into H2CO3, which spontaneously dissociates to H+ and HCO3−. The HCO3− that is formed then enters the plasma to function as a buffer, and the H+ that is generated is buffered by hemoglobin. (335)
10. In plasma, hemoglobin exists as a weak acid. It acts as a buffer by binding H+
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