1. ABG Interpretation

2. Objectives:
Upon completion of this lecture the audience will be able to:
Understand the purpose of ABG testing
Describe what ABG is measuring
List normal ABG values
Differentiate between acidosis and alkalosis
Have a better understanding of ABG interpretation

3. What is an ABG?
An arterial blood gas (ABG) is a blood test taken from an artery, that measures the amount of oxygen and carbon dioxide that is found in the blood. The purpose of this measurement is to determine the lungs effectiveness in moving oxygen and carbon dioxide into and out of the bloodstream

4. blood gas machines
The blood gas machines in most labs actually measure the pH ,the pCO2 and the pO2. The [HCO3-] and the base difference are calculated values using the Henderson-Hasselbalch equation

5. When Should an ABG be Ordered?
There are 4 major reasons to draw an ABG and they are:
Assessment of oxygenation capacity – determine cause of pleuritic chest pain or rule out Pulmonary Embolism
Assessment of oxygen pressure to guide therapy – prevention of vision problems in premature infants and monitoring risk of pleural disruption (Pneumothorax) in such disease processes as ARDS
Assessment of respiratory adequacy – oxygen and carbon dioxide measurement to assist with assessment of ventilation rate, depth and pressure
Assessment of acid-base balance – disease identification and determination of metabolic status Priestley, M. and Huh, J. (Updated 2008 February 12)

6. What does an ABG measure?
Partial pressure of oxygen (Pao2).
Partial pressure of carbon dioxide (PaCo2).
pH – The pH is the measurement of hydrogen ions (H+) found in the bloodstream.
Bicarbonate (HCo3).
Oxygen Content (o2CT) and Oxygen Saturation (o2 Sat).
Base excess (BE) Dugdale, III, D. (Updated 2008 August 10).

7. Normal Arterial Blood Gas Values
PH – 7.45
PaCO – 45 mm Hg
PaO – 95 mm Hg
HCO – 26 mEq/ L
O2 Saturation 95 – 99%
BE /- 1

8. PH > 7.45
PaCO2 < 35mm Hg
HCO3 > 26 mEq/L
PH < 7.35
PaCO2 >45mmHg
HCO3 < 22 mEq/L

9. A Step by Step Approach to Interpreting ABG’s:
Step 1: Determine primary abnormality Step 2: Determine is it Respiratory or Metabolic Step 3: Determine if the Acidosis/Alkalosis is Partially or , Completely Compensated Canham, E. and Beuther, D. (©2007).

10. METABOLIC ACIDOSIS
Metabolic acidosis is a clinical disturbance characterized by an increase in plasma acidity.
Reflected by a decrease in [HCO3 -] and a compensatory decrease in pCO Pagana, K. D. & Pagana, T. J. (© 2007).

11. Nursing management of Metabolic Acidosis
1. The initial therapeutic goal for patients with severe acidemia is to raise the systemic pH above , which dysrhythmias become less likely andcardiac contractility and responsiveness to catecholamines will be restored.
2. Correct any underlying disorder (control diarrhea, etc).
3. Treatment with bicarbonate should be reserved for severe metabolic gap acidosis.
If the pH <7.20, correct with sodium bicarbonate.
4. Replace with one-half the total amount of bicarbonate over Morris CG, Low h and reevaluate.

12. METABOLIC ALKALOSIS
Metabolic alkalosis is a primary increase in serum bicarbonate (HCO3 -) concentration. This occurs as a consequence of a loss of H+ from the body or a gain in HCO3 Pagana, K. D. & Pagana, T. J. (© 2007).

13. Nursing management of Metabolic Alkalosis
The management of metabolic alkalosis depends primarily on the underlying etiology and on the volume status of the patient
If Chloride-responsive alkalosis:
if it is volume depletion treat it with isotonic sodium chloride
if it is occurs in the setting of edematous treat it with potassium chloride to avoid volume overload
2. If Chloride-resistant :Treat underlying problem
Primary hyperaldosteronism: aldosterone antagonist
Cushing syndrome: diuretics should correct the alkalosis until surgical therapy
3. All metabolic alkalosis (specialized):
Hydrochloric acid: Intravenous HCl is indicated in severe metabolic alkalosis (pH >7.55) or when sodium or potassium chloride cannot be administered because of volume
Dialysis: Both peritoneal dialysis and hemodialysis can be used with certain modifications of the dialysate to correct metabolic alkalosis Priestley, M. and Huh, J. (Updated 2008 February 12).

14. RESPIRATORY ACIDOSIS
Respiratory acidosis is a clinical disturbance due to alveolar hypoventilation. Pagana, K. D. & Pagana, T. J. (© 2007).

15. Nursing management of Respiratory Acidosis
The treatment of respiratory acidosis is primarily directed at correcting the underlying disorder
Infusion of sodium bicarbonate is rarely indicated. This measure may be considered after cardiopulmonary arrest with an extremely low  pH (< ).
Treatment also should be aimed at assisting or increasing ventilation.
Drug therapy aimed at reversing the effects of certain sedative drugs may be helpful in the event of an over dosage. Priestley, M. and Huh, J. (Updated 2008 February 12).

16. RESPIRATORY ALKALOSIS:
Respiratory alkalosis is a clinical disturbance due to alveolar hyperventilation. Alveolar hyperventilation leads to a decreased partial pressure of arterial carbon dioxide (PaCO2), or partial pressure of carbon dioxide (PCO2). Pagana, K. D. & Pagana, T. J. (© 2007).

17. Nursing management of Respiratory Alkalosis
Respiratory alkalosis itself is rarely life threatening. Therefore, emergent treatment is usually not indicated unless the pH level is > 7.5. Because respiratory alkalosis usually occurs in response to some stimulus.
The tidal volume and respiratory rate may be decreased in mechanically ventilated patients. Priestley, M. and Huh, J. (Updated 2008 February 12).

18. Examples for Practice 1. pH 7.51 pCO2 40 HCO3- 31

19. The answer
1. pH is high, Bicarb is high, Co2 is normal and not attempting to correct the problem so this metabolic alkalosis is uncompensated.
2. pH is low, Bicarb is low, Co2 is low and attempting to correct the problem (but has not completely helped) so this is partially compensated metabolic acidosis.
3. pH, Bicarb and Co2 are within normal ranges so this is a normal ABG

20. Conclusion
Don’t get overwhelmed, identify the primary (or even just an obvious) disorder and build from that.
Now did you ever think that you would be able to interpret a triple acid/base disorder?
If you use this method to tease out the disturbances, you will NOT get stumped.
You can then use these interpretations to better understand the patient and possibly entertain diagnoses that you might not have considered using your differential lists for the various acid/base disorders!

21. Referances
Canham, E. and Beuther, D. (©2007). Interpreting Arterial Blood Gases. American College of Chest Physicians, Pulmonary and Critical Care Update [On-line information]. Available online at through Accessed February 2010.
Dugdale, III, D. (Updated 2008 August 10). Blood gases. MedlinePlus Medical Encyclopedia [On-line information] Available online at Accessed February 2010.
Morris CG, Low J; Metabolic acidosis in the critically ill: part 1. Classification and pathophysiology. Anaesthesia Mar;63(3):
Pagana, K. D. & Pagana, T. J. (© 2007). Mosby's Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp
Priestley, M. and Huh, J. (Updated 2008 February 12). Respiratory Failure, eMedicine [On-line information] Available online at through Accessed February 2010.