ABG INTERPRETATION
Vicky Honkus MSN RN 2005 Education Department
OBJECTIVES: Upon completion of this self learning module, the participant will be able to: 1.
State the normal values for pH, pCO2 and HCO3.
2.
Discuss the basic rules to follow in interpreting ABG results.
2.
Explain how compensation is evaluated.
3.
Correctly interpret the practice ABG sets in this self learning module.
4.
List common causes of acid-base disorders.
2
The chemical balance of body fluids is of enormous clinical importance, since it is a critical factor in all metabolic and cellular functions. The body's own regulatory mechanisms usually retain this balance within normal limits, but certain disease processes, illnesses and treatments can cause imbalances that may even become life threatening, unless appropriate interventions are implemented. Acid base balance refers to the hydrogen ion concentration of all body fluids, which is determined by chemical substances present in the blood and other body fluids. Arterial Blood Gas interpretation is an easy skill to master. It simply requires an understanding of pH, the respiratory component (pCO2), a metabolic component (HCO3 and/or Base Excess), what they represent, and what the normal ranges are. I.
pH In chemistry class we learned that pH describes the concentration of hydrogen ions, and that a pH of 7.0 is perfectly neutral. The acceptable pH range of our blood is 7.35 – 7.45, which is slightly alkaline. The body needs to maintain the pH of its chemical broth within fairly narrow limits for proper body functioning. The body’s own regulatory mechanisms usually maintain this balance, but certain disease processes, illnesses, and treatments can cause imbalances that may become life threatening unless appropriate interventions are implemented. A pH that is less than 7.0 or greater than 7.7 is not compatible with life. The first step in interpreting ABG results is to determine if the pH represents acidosis or alkalosis, or is perfectly normal. For purposes of ABG interpretation, we will consider the middle of the acceptable range (7.4) as neutral for the blood, any value less than 7.4 as acidotic, and any value higher than 7.4 as alkalotic. pH acidotic
Acceptable ranges: alkalotic pH 7.35-7.45
7.35
7.4
7.45 pCO2 35-45 HCO3 22-26 BE
+2 to -2
3
The following are some pH values for you to practice determining whether a pH is acidic or alkalotic. Remember to base your decision on a pH of 4.0, which is neutral for blood. Cover up the answers on the right, and check yourself after your interpretation. pH 7.0 = _______________________________________acidic pH 7.9 = _______________________________________alkalotic pH 7.4 = _______________________________________neutral pH 7.5 = _______________________________________alkalotic pH 7.3 = _______________________________________acidic The pH alone is not sufficient to tell us whether an imbalance is due to a respiratory or metabolic problem. The other reported values enable us to determine the primary problem, and whether any attempts are being made by the body to compensate for the imbalance. II.
Bicarbonate-carbonic acid buffer system The body has several buffer systems that work to maintain the pH within acceptable limits. Of these, the bicarbonate-carbonic acid buffer system is the most important, and is controlled by the lungs and kidneys. Normal (aerobic) metabolism produces two forms of acid: respiratory acid (carbonic acid) and metabolic acids. In order to maintain proper pH balance, the body attempts to maintain a ratio of 20:1 (bicarbonate to carbonic acid.) A wide variety of pulmonary and metabolic problems can create an imbalance, so it is necessary to look carefully at the respiratory and metabolic components of ABG interpretation.
III.
20 : 1 bicarbonate carbonic acid
Respiratory component Carbon dioxide is an end product of metabolism, and when dissolved in blood becomes carbonic acid. Carbonic acid is termed a volatile acid because it dissociates into water and a gas, CO2 which is exhaled by the lungs. The pCO2 or PaCO2 represents the partial pressure of carbon dioxide dissolved in arterial blood, and provides an important measure of the adequacy of a patient’s ventilation. The lungs normally maintain an acceptable balance of CO2 (which is acidotic) in the bloodstream. The normal range for this respiratory component of ABG’s, the pCO2, is 35 – 45 mmHg, and is maintained by ventilation.
4
When the patient is moving a normal volume of air in and out of the lungs, the pCO2 will stay within the normal range. Hypoventilation will prevent sufficient removal of CO2 from the bloodstream, causing a respiratory acidosis. Some causes of respiratory acidosis include obstructive lung disease, restrictive lung disease and hypoventilation as a result of oversedation, anesthesia or improper ventilator settings (e.g., tidal volume too low.) Respiratory acidosis could also result from a drug overdose and neuromuscular diseases such as Guillain-Barre syndrome or myasthenia gravis. Hyperventilation, on the other hand, causes CO2 to be “blown off” or removed, causing a respiratory alkalosis. The pCO2 falls below 35, and the acid-load decreases, causing an alkalosis. Some possible causes of respiratory alkalosis include pain, panic attacks, anxiety, pulmonary embolism, pregnancy, and a tidal volume that is too high for a ventilator patient. The second step in ABG interpretation, is to evaluate the pCO2, to determine if it falls within the acceptable range, falls below the lower limit (respiratory alkalosis) or is above the upper limit (respiratory acidosis.) Notice that a high value in pCO2 actually represents an acidosis (retention of CO2), whereas a high value in pH represents an alkalosis. Do not try to memorize, but rather to understand the concept of what the components represent. A high level of pCO2 simply means there is retention of CO2 or hypoventilation. CO2 is acidotic in the blood, and too much causes respiratory acidosis. Practice with the values below, to determine the status for each pCO2 value. Cover over the answers on the right, and then check your interpretation. pCO2 of 22 = __________________________________respiratory alkalosis pCO2 of 32 = ___________________________________respiratory alkalosis pCO2 of 35 = ___________________________________low normal pCO2 of 40 = ___________________________________normal pCO2 of 45 = ___________________________________high normal pCO2 of 50 = ___________________________________respiratory acidosis pCO2 of 60 = ___________________________________respiratory acidosis IV.
Metabolic components HCO3 represents the metabolic component of ABG interpretation, with a normal range of 22-26 mEq/L. Bicarbonate (HCO3) is a weak base that is regulated by the kidneys.
5
When there is a loss of acid in the body, or an excess of base, the HCO3 will be greater than 26, resulting in metabolic alkalosis. Some causes of metabolic alkalosis include loss of stomach acid and potassium from vomiting or gastric suction, and ingestion of large amounts of bicarbonate. Prolonged therapy with potassium-wasting diuretics, steroid therapy, Cushing's disease, and aldosteronism can also deplete potassium, chloride, and hydrogen levels, resulting in metabolic alkalosis. When there is an excess of metabolic acid, or not enough base, the HCO3 will be less than 22, causing metabolic acidosis. Conditions that increase acid-load include diabetic ketoacidosis or prolonged fasting, lactic acidosis, and renal failure. Actual loss of bicarbonate ions through severe diarrhea leads to metabolic acidosis. During cardiac arrest, or when low cardiac output states (as in external cardiac compression) are present, anaerobic metabolism occurs and there is an increase in the [production of lactic acid. Metabolism of lactic acid is normally effected through the Krebs cycle, and oxygen is the essential element for this metabolic process. In the absence of adequate tissue oxygenation, lactic acid cannot be metabolized; its quantity increases and the result is metabolic acidosis. The third step in ABG interpretation is to determine the direction of the metabolic component HCO3, whether it is within normal limits, is high (metabolic alkalosis), or low (metabolic acidosis.) Simply think of HCO3 as a base, and too much causes metabolic alkalosis, whereas too little causes metabolic acidosis. Practice and check your answers below: HCO3 of 22 = ___________________________________low normal HCO3 of 16 = ___________________________________metabolic acidosis HCO3 of 30 = ___________________________________metabolic alkalosis HCO3 of 25 = ___________________________________normal HCO3 of 12 = ___________________________________metabolic acidosis HCO3 of 27 = ___________________________________metabolic alkalosis Base Excess (BE) is another metabolic component that is reported with our blood gas report. This value is a calculation of the circulating buffer/base, and is reported as milliequivalents per liter of base above or below the normal buffer base range. The normal range for BE is –2 to +2. A base excess value of –8 would represent a deficit of base and therefore metabolic acidosis. A base excess value of +10 would represent an excess of base and therefore metabolic alkalosis.
6
The base excess can be used to help determine the presence of a metabolic imbalance, whether acidosis or alkalosis. For the rest of this self learning module however, bicarbonate will be the only metabolic component used in interpreting arterial blood gases. Practice interpreting the following results (check your interpretation with answers below): #1 #2 #3 #4 #5 #1: #2: #3: #4: #5:
pH
pCO2
HCO3
7.41 7.5 6.72 7.26 7.52
40 42 40 63 18
24 35 5 25 25
Interpretation
normal metabolic alkalosis metabolic acidosis respiratory acidosis respiratory alkalosis
The acid-base imbalances above involved only one component (respiratory or metabolic.) Sometimes it is possible to have both components reflect the same abnormality. For example, the patient who is in respiratory distress, and not exchanging air sufficiently develops respiratory acidosis due to retention of CO2 (and hypoxemia.) If that same patient’s condition should deteriorate to the point where his cells are not receiving oxygen, and CO2 is building up in the blood, anaerobic metabolism will take place, causing a metabolic acidosis as well. That patient would be said to have a combined respiratory and metabolic acidosis, which is typically seen in the arrest setting. IV.
Compensation Maintenance of acid-base balance is normally accomplished by buffer systems which react to changes in the hydrogen ion concentration of body fluids. The primary buffer system is the bicarbonate (kidneys) - carbonic acid (lungs) buffer system. When there is an imbalance in one parameter, the other tries to compensate for it by causing the opposite imbalance. The goal of this compensation process is to restore the 20:1 ratio, and return the pH back to the acceptable range of 7.35 – 7.45.
⇨ 7
The regulation of respiration is achieved mainly by chemoreceptors in the medulla, that are stimulated or inhibited by an increase or decrease in the carbonic acid and hydrogen ion levels. Normally an increase in carbonic acid or hydrogen ion concentration in the blood stimulates an increase in the rate and depth of ventilations, cause more carbon dioxide to be removed. A decrease in concentration of carbonic acid results in a decrease in rate and depth of breathing, thus retaining more carbon dioxide. The lungs can begin to compensate for metabolic imbalances immediately, but can only partially correct pH deviations (can return pH to normal range, but never to perfectly normal 7.4). The kidneys, on the other hand, take much longer to compensate, sometimes as long as 3-5 days to fully compensate. An increase in hydrogen ion concentration is corrected by increasing the amount of bicarbonate that is returned to the blood, and increasing the amount of acid that is excreted. A decrease in hydrogen ion concentration is corrected by increasing the amount of acid that is retuned to the blood, and increasing the amount of bases, particularly, bicarbonate, that are excreted. The kidneys may be slower to compensate, but they are much more powerful, and can return the pH to perfectly normal 7.4. The fourth and final step in ABG interpretation, then is to determine if compensation is present, and to what extent. .
When compensation is present, we will see two imbalances. The question then becomes, which is the primary problem, and which imbalance is due to compensation. The clue is the pH. If the pH is leaning toward acidosis or alkalosis, then the parameter with the matching imbalance is the primary problem, and the other is due to compensation.
Look at the following examples: One patient has chronic pulmonary problems, and is in a chronic respiratory acidosis. His kidneys will compensate by retaining bicarbonate, creating a metabolic alkalosis to balance his chronic respiratory acidosis.
8
A diabetic patient goes into ketoacidosis and develops Kussmaul breathing, causing him to blow off CO2, creating a respiratory alkalosis to help balance his pH. Below are some examples of ABG results with compensation: pH PCO2 HCO3 Interpretation 30 70 7.30 Respiratory acidosis with partial (mild compensation by the kidneys. The kidneys metabolic have only begun to compensate, because the pH (respiratory alkalosis) acidosis) (acidosis) is not back to acceptable limits yet. The pH is alkaline, so the primary problem is 15 20 7.43 alkalosis. Respiratory alkalosis with nearly (within (metabolic complete compensation by the kidneys. (respiratory acceptable acidosis) alkalosis) range, but not perfectly normal 7.4) The pH is acidotic, so the primary problem is 10 5 7.36 (respiratory (metabolic acidosis. Metabolic acidosis with (within compensation by the kidneys. alkalosis) acidosis) acceptable range, but not perfectly normal 7.4) 7.4 60 30 The pH is perfectly normal in this case, with 2 (perfectly (respiratory (metabolic very distinct imbalances. The primary problem normal) acidosis) alkalosis) has to be respiratory acidosis, because only the kidneys can compensate completely, and bring the pH back to perfectly normal. This respiratory problem is probably chronic, since it takes the kidneys several days to completely compensate for a respiratory problem. So this is respiratory acidosis with complete compensation by the kidneys. BELOW ARE SOME PRACTICE ABG RESULTS FOR YOU TO DO, AND THE ANSWERS ARE AT THE END OF THIS SELF LEARNING MODULE. Practice: #1
pH 7.52 = ___________________________________ pCO2 40 = ____________________________________ HCO3 35 = ____________________________________ Interpretation_______________________________________________
9
#2
pH 7.25 = ___________________________________ pCO2 60 = ____________________________________ HCO3 27 = ____________________________________ Interpretation_______________________________________________
#3
pH 7.25 = __________________________________ pCO2 40 = ___________________________________ HCO3 12 = ___________________________________ Interpretation______________________________________________
#4
pH 7.55 = _________________________________ pCO2 20 = ___________________________________ HCO3 26 = ___________________________________ Interpretation______________________________________________
#5
pH 7.29 = _________________________________ pCO2 20 = __________________________________ HCO3 18 = __________________________________ Interpretation______________________________________________
#6
pH 7.48 = _________________________________ pCO2 50 = ___________________________________ HCO3 34 = ___________________________________ Interpretation_______________________________________________
#7
pH 7.5 = __________________________________ pCO2 20 = __________________________________ HCO3 30 = __________________________________ Interpretation_____________________________________________
#8
pH 7.18 = _________________________________ pCO2 60 = _________________________________ HCO3 26 = _________________________________ Interpretation_____________________________________________
#9
pH 7.29 = ________________________________ pCO2 60 = __________________________________ HCO3 35 = __________________________________ Interpretation_____________________________________________ 10
#10
pH 7.48 = _________________________________ pCO2 20 = __________________________________ HCO3 34 = __________________________________ Interpretation______________________________________________
#11
pH 7.43 = ________________________________ pCO2 35 = __________________________________ HCO3 23 = __________________________________ Interpretation _____________________________________________
#12
pH 7.4 = _________________________________ pCO2 60 = __________________________________ HCO3 30 = __________________________________ Interpretation:_____________________________________________
ABG studies are very helpful to reveal numerous underlying problems, and provide clues to a patient’s ventilatory status, hypoxemic state, and tissue oxygenation. This is an important skill to have and is invaluable when looking at the clinical picture.
Answers to practice ABG's: #1
pH 7.52 = alkalosis pCO2 40 = normal HCO3 35 =metabolic alkalosis Metabolic alkalosis (no compensation)
#2
pH 7.25 = acidosis pCO2 60 = respiratory acidosis HCO3 27 = slight metabolic alkalosis Respiratory acidosis, and kidneys are beginning to compensate (retain bicarb. Note that the pH has a long way to go before it comes back into normal range, so compensation has just begun.)
#3
pH 7.25 = acidosis pCO2 40 = normal HCO3 12 = metabolic acidosis Metabolic acidosis (no compensation)
11
#4
pH 7.55 = alkalosis pCO2 20 = respiratory alkalosis HCO3 26 = normal Respiratory alkalosis (no compensation)
#5
pH 7.29 = acidosis pCO2 20 = respiratory alkalosis HCO3 18 = metabolic acidosis Metabolic acidosis with some compensation by the lungs (not very effective; has not returned pH to acceptable range yet)
#6
pH 7.48 = alkalosis pCO2 50 = respiratory acidosis HCO3 34 = metabolic alkalosis Metabolic alkalosis with almost complete compensation by the lungs
#7
pH 7.5 = alkalosis pCO2 20 = respiratory alkalosis HCO3 30 = metabolic alkalosis Combined respiratory & metabolic alkalosis
#8
pH 7.18 = acidosis pCO2 60 = respiratory acidosis HCO3 26 = normal Respiratory acidosis
#9
pH 7.29 = acidosis pCO2 60 = respiratory acidosis HCO3 35 = metabolic alkalosis Respiratory acidosis with some compensation from kidneys
#10
pH 7.48 = alkalosis pCO2 20 = respiratory alkalosis HCO3 34 = metabolic alkalosis Combined respiratory and metabolic alkalosis
#11
pH pCO2 HCO3 Normal
#12
pH 7.4 = perfectly normal pCO2 60 = respiratory acidosis HCO3 30 = metabolic alkalosis Two imbalances with complete compensation, so it is difficult to tell which is the problem and which is the compensation. However, since only the kidneys
7.43 = normal 35 = normal 23 = normal
12
are strong enough in compensation to bring the pH back to perfectly normal, the primary problem has to be respiratory acidosis, with complete compensation by the kidneys.
Now, print a copy of the revised Evaluation Form, which is the last page of this module. Then, exit this module, and on the Old Glory list of modules, select & print the "Instructions for Taking Module Exam in Healthstream." To print the evaluation form, scroll to the last page and make sure your cursor is on that page. Go to File, and select Print. Select Current Page, and then OK.
REFERENCES: Dennison R. Pass CCRN!. Second edition. 2000:221-222. Guyton A. Human Physiology and Mechanisms of Disease. 1992:233-240. Kirksey, KM, et al. An Easy Method for Interpreting the Results of Arterial Blood Gas Analysis. Critical Care Nurse. 21(5):49-54. Lanford A. Name that Acid Base. Critical Care Nurse. March/April 1981:10-12. McCance K, Huether S. Pathophysiology: The Biologic Basis for Disease in Adults and Children. 1994:110-120. Mims, BC. Interpreting ABG's. RN. March 1991:42-47.
13
SELF LEARNING MODULE EVALUATION Professional Title:
RN
LPN
RT
OTHER
Date:__________
NAME: (PRINT)____________________________________ REQUIRED LICENSE # ____________________________ INFO Self Learning Module: ____ABG Interpretation___________ Score on Module Post-Test:_____________________ Circle your choice: (1 = Strongly Disagree, 2 = Disagree, 3 = Agree, 4 = Strongly Agree) 1.
Overall, was the program worthwhile?
1
2
3
4
2.
Did the program meet your personal objectives?
1
2
3
4
3.
Regarding content: Was the information presented clearly? Was the information pertinent to your professional needs?
1 1
2 2
3 3
4 4
4.
5. 6.
Was the information that was presented:
New and Useful
New and Necessary
Review but Useful Review but Necessary
Was the information presented at the level of your understanding? 1 2 3 4 Comments:_______________________________________________________________ _____________________________________________________________________
7.
Recommendations for future topics:____________________________________________ _____________________________________________________________________
8.
Actual time spent completing this self learning module: ____________________________ SELF STUDY MODULE ATTESTMENT STATEMENT
I _______________________________ attest that I have read and understand the information presented in the ______ABG Interpretation_______________________ self learning module. Signature:________________________________________ Employee #:_____________________________
Date:_________________
License # (REQUIRED):_____________
