This document provides an overview of arterial blood gas interpretation. It discusses normal values for pH, PaCO2, HCO3, PaO2 and SaO2. It explains acid-base balance and the respiratory and metabolic mechanisms that control pH. A 3-step process is outlined for interpreting ABG results: 1) determine if acidosis or alkalosis based on pH, 2) evaluate the respiratory mechanism using PaCO2, and 3) evaluate the metabolic mechanism using HCO3. Compensation and combined disturbances are also addressed. Case examples are provided to demonstrate interpreting ABG results and diagnosing respiratory vs. metabolic causes of acid-base imbalances.

1. Arterial Blood Gas Interpretation Sherry L. Knowles, RN, CCRN, CRNI Orlando Regional Medical Center 2004

2. Objectives Recognize Signs & Symptoms of Respiratory Failure Understand Ventilation and Perfusion Mechanics Analyze and Interpret Acid Base Disturbances Identify Appropriate Treatments for Abnormal ABG’s

3. Objectives Recognize normal and abnormal values for pH, PaO 2 , PaCO 2 , SaO 2 and HCO 3 . Relate the pH scale to acidosis and alkalosis. Discuss the respiratory and metabolic mechanisms involved in controlling the body's acid-base balance. Interpret basic arterial blood gas values and relate these values to patient conditions. Anticipate appropriate therapies for acid-base correction.

4. Why Do ABG’s? 1) Check oxygenation 2) Check the pH ( acid base balance ) 3) Define the problem 4) Determine the treatment

5. Fundamentals All human cells require oxygen. Breathing (ventilation) brings oxygen in and CO 2 out of the lungs. Oxygen is absorbed into the bloodstream through the alveoli. Hemoglobin molecules carry oxygen to the tissues.

6. Hemoglobin Carries Oxygen Has 4 Binding Sites Hemoglobin + 4 Oxygen = Oxyhemoglobin

7. Hemoglobin Binding Sites When all of 4 sites are occupied, the hemoglobin molecule cannot hold any more. Molecules, other than oxygen, can attach to the oxygen binding sites. If enough hemoglobin binding sites are occupied with molecules other than oxygen, severe tissue hypoxia can result. Hypoxia can occur even in the presence of 100% oxygen. This can be a life-threatening condition.

8. Carboxyhemoglobin (HbCO) Carboxyhemoglobin (HbCO) is a hemoglobin molecule that has carbon monoxide attached where the oxygen should be. Exposure to car exhaust, or other chemicals can cause carbon monoxide (CO) to attach to hemoglobin binding sites, instead of oxygen and thus compete with the oxygen for the limited number of binding sites. The blood will exhibit a cherry red color.

9. Methemoglobin (MetHb) Methemoglobin (MetHb) is produced when exposed to certain poisons or due to a genetic condition that affects the hemoglobin molecule. The hemoglobin molecule is saturated with methemoglobin (MetHb) and competes with oxygen for the hemoglobin binding sites. Methemoglobin (MetHb) changes blood to a brownish color.

10. Oxygen and Hemoglobin CO 2 's affinity for hemoglobin is much less than O 2 's affinity for hemoglobin. When CO 2 and O 2 are both available, hemoglobin will accept oxygen rather than CO 2 . In the oxygen rich environment of the alveoli, hemoglobin carries oxygen. Oxygenated blood then travels through the body.

11. Acid Base Balance Understanding the cause of an acid-base imbalance is the key to treating it. The Respiratory component of acid base balance affects the pH within minutes . The Metabolic component of acid base balance can take days to affect pH.

12. Buffer System Carbonic Acid - Bicarbonate Buffer System CO 2 + H 2 O <--> H 2 CO 3 <--> ( HCO 3 - ) + ( H+ ) carbon dioxide + water <--> carbonic acid <--> bicarbonate + hydrogen ion Note: The two headed arrows indicate that the process is reversible

13. CO 2 When combined with water, carbon dioxide becomes carbonic acid ( H2CO 3 ) Carbon dioxide is an acid when dissolved in water. Carbon dioxide is a product of metabolism. As long as cells are functioning, CO 2 is produced.

14. HCO 3 Bicarbonate = HCO 3. HCO 3 increases in response to high CO 2. Metabolic changes take days to affect pH.

15. Basic ABG Components pH PaCO 2 HCO 3 PaO 2

16. pH The pH scale ranges from 1 to 14. pH 7 is Neutral Low pH is Acid . High pH is Alkaline .

17. pH Normal pH is maintained by balancing the H2CO3 (carbonic acid) and HCO3- (bicarb) Normal blood pH = 7.35-7.45 pH < 7.35 = acidosis Ph > 7.45 = alkalosis

18. PaCO 2 CO2 has several forms in the blood. Like oxygen, some is dissolved directly into the plasma. The PaCO2 is the measurement of the partial pressure of carbon dioxide dissolved in the plasma. It is measured in mm Hg (millimeters of mercury). The rest is found in the red blood cells on a hemoglobin molecule.

19. PaO 2 About 3% of the body's oxygen is dissolved in the plasma. PaO 2 is a measurement of the partial pressure of oxygen dissolved in the plasma only. It is measured in mm Hg. The PaO 2 does not tell us about the body's total oxygen content, but it does indicate how much oxygen was available in the alveoli to dissolve in the blood.

20. SaO 2 The remainder of the body's oxygen is carried attached to hemoglobin molecules. SaO 2 , or oxygen saturation , measures the degree to which oxygen is bound to hemoglobin. Sa0 2 is expressed as a percentage.

21. Ventilation Controls CO 2 levels CO 2 = Ventilation High CO 2 = Hypoventilation Low CO 2 = Hyperventilation

22. Respiratory Mechanism Respiratory Mechanism (depth and rate of breathing) controls CO2. CO 2 in solution is an acid. Higher PaCO2 causes acidosis (lower pH), or neutralizes alkalosis. Lower PaCO2 causes alkalosis (raises pH.), or neutralizes acidosis.

23. Metabolic Mechanism Bicarbonate = Alkaline = HCO 3. HCO 3 increases in response to high CO 2. Metabolic changes take days to affect pH.

24. Compensation “ Compensation&quot; is the body's normal response to normalize pH By neutralizing the opposite acid base mechanism. Example: If the pH is high because of respiratory alkalosis (low CO 2 ): Alkaline HCO 3 - will decrease to neutralize the pH . In this case, the abnormal bicarb is not a metabolic problem; it is a metabolic solution to a respiratory problem. It is important to determine which is the cause and which is the effect. If you treat the compensatory abnormality, you make the pH even more abnormal.

25. Normal ABG Values pH = 7.35-7.45 7.4 (+/- 0.5) PaCO 2 = 35-45 40 (+/- 5) HCO 3 = 22-26 24 (+/- 2) PaO 2 = 80-100 90 (+/- 10) SaO 2 = 94-100 97 (+/- 3) Norms Quick Reference

26. Steps to ABG Interpretation 1) Determine Acidosis or Alkalosis . 2) Evaluate the Respiratory Mechanism 3) Evaluate the Metabolic Mechanism

27. STEP 1 Step 1. Use pH to determine Acidosis or Alkalosis . Normal or Compensated = Acidosis = Alkalosis = 7.35-7.45 < 7.35 > 7.45

28. STEP 1 Step 1. Use pH to determine Acidosis or Alkalosis . ph < 7.35 7.35-7.45 > 7.45 Acidosis Normal or Compensated Alkalosis

29. STEP # 2 Step 2. Use PaCO 2 to look at the Respiratory Mechanism PaCO2 < 35 35 -45 > 45 Tends toward alkalosis Causes high pH Neutralizes low pH Normal or Compensated Tends toward acidosis Causes low pH Neutralizes high pH

30. STEP 3 Step 3. Use HCO 3 to look at the Metabolic Mechanism HCO 3 < 22 22-26 > 26 Tends toward acidosis Causes low pH Neutralizes high pH Normal or Compensated Tends toward alkalosis Causes high pH Neutralizes low pH

31. Interpretation High pH Low pH Alkalosis Acidosis High HCO 3 Low PaCO2 High PaCO2 Low HCO 3 Metabolic Respiratory Respiratory Metabolic

32. Compensation “ Compensation&quot; is the body's normal response to normalize pH By neutralizing the opposite acid base mechanism. Example: If the pH is high because of respiratory alkalosis (low CO 2 ): Alkaline HCO 3 - will decrease to neutralize the pH. In this case, the abnormal bicarb is not a metabolic problem; it is a metabolic solution to a respiratory problem. It is important to determine which is the cause and which is the effect. If you treat the compensatory abnormality, you make the pH even more abnormal.

33. Normal ABG Values Norms Quick Reference pH = 7.35-7.45 7.4 (+/- 0.5) PaCO 2 = 35-45 40 (+/- 5) HCO 3 = 22-26 24 (+/- 2) PaO 2 = 80-100 90 (+/- 10) SaO 2 = 94-100 97 (+/- 3)

34. Combined Disturbances A “ Combined Disturbance ” occurs when the PaCO 2 and HCO3- both alter the pH in the same direction. A high PaCO2 and low HCO3- (acidosis). Low PaCO2 and high HCO3- (alkalosis). RARE

35. OxyHemoglobin Dissociation Curve This curve describes the relationship between available oxygen and amount of oxygen carried by hemoglobin. Oxygen-Hemoglobin affinity changes with: variation in pH *CO2 *temperature *2,3,-DPG Once the PaO2 reaches 60 mm Hg the curve indicates that there is little change in saturation above this point. So, PaO2 of 60 or more is usually considered adequate. At PaO2 of less than 60 even small changes will greatly reduce the SaO 2.

36. Question 1 If the pH is 7.23, the PaCO 2 is 50, and the HCO 3 is 24 what is the likely diagnosis? RESPIRATORY ACIDOSIS

37. Question #2 If the pH is 7.49, the PaCO 2 is 25, and the HCO 3 is 22 what is the likely diagnosis? RESPIRATORY ALKALOSIS

38. Question # 3 If the the pH is 7.56, the PaCO 2 is 39, and the HCO 3 is 38, what is the likely diagnosis? METABOLIC ALKALOSIS

39. Question # 4 If the pH is 7.35, the PaCO 2 is 25, and the HCO 3 is 9, what is the likely diagnosis? COMPENSTATED METABOLIC ACIDOSIS

40. Question # 5 If the pH is 7.30, the PaCO 2 is 25, and the HCO 3 is 9, what is the likely diagnosis? PARTIALLY COMPENSTATED METABOLIC ACIDOSIS