An arterial blood gas (ABG) is a blood test that measures the acidity (pH) and the levels of oxygen and carbon dioxide in the blood. Blood for an ABG test is taken from an artery whereas most other blood tests are done on a sample of blood taken from a vein. This test is done to monitor several conditions that can cause serious health complications especially to critically ill individuals.
Every day, a lot of nursing and medical students assigned in acute areas encounter ABG results, which they may not necessarily be able to interpret with its knotty aspect. They struggle over the interpretation of its measurements, but they are not especially complicated nor difficult if you understand the basic physiology and have a step by step process to analyze and interpret them.
There may be various tips and strategies to guide you, from mnemonics, to charts, to lectures, to practice, but this article will tell you how to interpret ABGs in the easiest possible way. And once you have finished reading this, you’ll be doing actual ABG analysis in the NCLEX with fun and excitement! Here are the steps:
1. Know the normal values
Know the normal and abnormal ABG values when you review the lab reports. They’re fairly easy to remember: for pH, the normal value is 7.35 to 7.45; 35-45 for paCO2; and 22-26 for HCO3. Remember also this diagram and note that paCO2 is intentionally inverted for the purpose of this method.
2. Determine if pH is under acidosis or alkalosis
Next thing to do is to determine the acidity or alkalinity of the blood through the value of pH. The pH level of a healthy human should be between 7.35 to 7.45. The human body is constantly striving to keep pH in balance.
pH level below 7.35 is acidosis
pH level above 7.45 is alkalosis
3. Determine if acid-base is respiratory or metabolic
Next thing you need to determine is whether the acid base is Respiratory or Metabolic.
paCO2 = Respiratory
HCO3 = Metabolic
4. Remember ROME
Still, it all boils down to mnemonics. The mnemonic RO-ME.
When pH is up, PaCO2 is down = Alkalosis
When pH is down, PaCO2 is up = Acidosis
When pH is up, HCO3 is up = Alkalosis
When pH is down, HCO3 is down = Acidosis
And yes, ABG problems can be solved work using the tic-tac-toe method. All you have to do is make a blank chart similar to this:
6. Mark the Chart
Using the lab result values, mark them on your tic-tac-toe. Let’s begin with this sample problem:
pH: 7.26, paCO2: 32, HCO3: 18
Using the normal values reference chart in the first step, determine where the values should be under in the tic-tac-toe. In the given example, the solution is as follows:
pH of 7.26 is LOW = ACID so place pH under Acid
paCO2 of 32 is LOW = BASE so place paCO2 under Base
HCO3 of 18 is LOW = ACID so place HCO3 under Acid
Your chart should look like this:
7. Match it up
In this step, determine at which column matches up with the pH. In the given example, HCO3 goes with pH. HCO3 is considered Metabolic (shown in step 3), and both are under Acid, so this example implies Metabolic Acidosis.
8. Determine compensation
The last step is to determine if the ABG is Compensated, Partially Compensated, or Uncompensated. Here’s the trick:
If pH is NORMAL, PaCO2 and HCO3 are both ABNORMAL = Compensated
If pH is ABNORMAL, PaCO2and HCO3 are both ABNORMAL = Partially Compensated
If pH is ABNORMAL, PaCO2or HCO3 is ABNORMAL = Uncompensated
Therefore this ABG is METABOLIC ACIDOSIS, PARTIALLY COMPENSATED .
By applying the steps above, interpret the following ABGs:
pH:7.44, PaCO2: 30, HCO3: 21
pH is NORMAL = NORMAL so place pH under Normal
PaCO2 is LOW = BASE so place PaCO2 under Base
HCO3 is LOW = ACID so place HCO3 under Acid
*Since the acidity of the blood is determined by the value of the pH, determine whether the normal pH is SLIGHTLY ACIDIC or SLIGHTLY BASIC. In this example, pH is NORMAL but SLIGHTLY BASIC therefore it is ALKALOSIS.
In this case PaCO2 goes with pH. PaCO2 is considered Respiratory (shown in step 3), and both are under Basic, so this example implies Respiratory Alkalosis. The HCO3 is also abnormal. When pH is NORMAL and PaCO2 and HCO3 are both ABNORMAL, it indicates FULL COMPENSATION.
Therefore this ABG is RESPIRATORY ALKALOSIS, FULLY COMPENSATED.
Try this problem next:
pH 7.1, PaCO2 40, HCO3 18
pH is LOW = ACID so place pH under Acid
PaCO2 is NORMAL = NORMAL so place PaCO2 under Normal
HCO3 is LOW = ACID so place HCO3 under Acid
In this case HCO3 goes with pH. HCO3 is considered Metabolic (shown in step 3), and both are under Acidic, so this example implies Metabolic Acidosis. The PaCO2 is normal. When pH is ABNORMAL, and when either one of PaCO2 or HCO3 is ABNORMAL, it indicates UNCOMPENSATION.
Therefore this ABG is METABOLIC ACIDOSIS, UNCOMPENSATED.
Try it out with these quizzes
Want to try out the Tic-Tac-Toe method? Here are some sample ABG Analysis quizzes:
Gil Wayne, RN
Presentation on theme: "Arterial Blood Gases Made Easy Arterial Blood Gases."— Presentation transcript:
1 Arterial Blood Gases Made Easy Arterial Blood Gases
2 Purpose of ABG Assess degree to which lungs are able to provide adequate oxygen & remove CO2 & degree to which the kidneys are able to reabsorb or excrete HCO3
3 You Must Know What is Normal to be Able to Know What is Abnormal pH = 7.35 to 7.45 PaC02 = 35 to 45 mm Hg PaO2 = 80 to 100 mm Hg HCO3 = 22 – 26 mEq/l
4 What You Must Look at to Interpret ABGs
5 Look at Your pH Is it normal? Is it high ? Is it low?
6 Examples pH = 7.36 pH = 7.23 pH = 7.47 A high pH indicates alkalosis A low pH indicates acidosis
7 Look at Your PaCO2 Is it normal ? Is it high ? Is it low ? This is the respiratory component An abnormality in the PaCO2 will indicate a respiratory problem
8 Examples PaCO2 = 40 mm Hg PaCO2 = 23 mm Hg PaCO2 = 48 mm Hg A high PaCO2 indicates acidity A low PaCO2 indicates alkalosis
9 PaCO2 Lungs will increase or decrease ventilation to remove the appropriate amount of CO2 Lung compensation begins quickly
10 Now Look at Your HCO3 Is it normal ? Is it high ? Is it low ? This is the metabolic component An abnormality in the HCO3 indicates a metabolic problem
11 Examples HCO3 = 25 mEq/l HCO3 = 19 mEq/l HCO3 = 32 mEq/l A low HCO3 indicates acidity A high HCO3 indicates alkalosis
12 HCO3 The kidneys excrete Hydrogen (acid) & retain bicarbonate (base) to help maintain pH Renal compensation is slow
13 Let’s Look at the 4 Situations that Can Occur
14 Acidosis Develops when: Excess accumulation of acid Decreased amount of alkali Can be respiratory or metabolic
15 Metabolic Acidosis ABG: Low pH (below 7.35) Decreased HCO3 (below 22) PaCo2 will be normal Remember both the pH & HCO3 will be low
16 Metabolic Acidosis Caused by too much acid in the body or loss of bicarbonate Diarrhea (loss of HCO3) Diabetic ketoacidosis Renal failure
17 Respiratory Acidosis ABG: Low pH (below 7.35) Increased PaCO2 (above 45) HCO3 will be normal Remember the pH will be low & PaCO2 will be elevated (opposite)
18 Respiratory Acidosis Caused by acid buildup due to lungs not eliminating CO2 Anything that decreased respirations can cause respiratory acidosis Chronic respiratory disease CNS depression
19 Alkalosis Develops when: Excess accumulation of bicarbonate Loss of acid
20 Metabolic Alkalosis ABG: Increase in pH (greater than 7.45) Increased HCO3 (greater than 26) PaCO2 will be normal Remember both the pH & the HCO3 will be elevated
21 Metabolic Alkalosis Loss of acid or increase in HCO3 Vomiting or NG drainage (loss of Hydrogen) Excessive use of antacids
22 Respiratory Alkalosis ABG: Increase in pH (greater than 7.45) Decrease in PaCO2 (less than 35) HCO3 will be normal Remember the pH will be high & PaCO2 will be low (opposite)
23 Respiratory Alkalosis Caused by too much CO2 being excreted by the lungs Hyperventilation
24 Interpreting ABGs
25 1 st Step Look at pH Is it normal (7.35 to 7.45) Does it reflect acidosis ( less than 7.35) Does it reflect alkalosis (greater than 7.45) Label accordingly
26 Next Look at PaCO2 This is the respiratory component Is it normal Does it reflect alkalosis (less than 35) Does it reflect acidosis (greater than 45) Label accordingly
27 Next Look at the HCO3 This is the metabolic component Is it normal Does it reflect acidosis (less than 22) Does it reflect alkalosis (greater than 26) Label accordingly
28 Let’s Put It All Together
29 Problems pH of 7.33 PaCO2 of 40 mmHg HCO3 of 20 mEq/L What does this indicate Break it down
30 pH = acidosis PaCO2 = normal HCO3 = acidosis An abnormal HCO3 indicates a metabolic problem
31 Metabolic Acidosis Way to go!!
32 Problem pH of 7.59 PaCO2 of 29 mm Hg HCO3 of 24mEq/L What does this indicate Break it down
33 pH = alkalosis PaCO2 = alkalosis HCO3 = normal An abnormal PaCO2 indicates a respiratory problem
34 Respiratory Alkalosis Great Job!!
35 Problem pH of 7.25 PaCO2 of 61 mmHg HCO3 of 26 mEq/L What does this indicate Break it down
36 pH = acidosis PaCO2 = acidosis HCO3 = normal An abnormal PaCO2 indicates a respiratory problem
37 Respiratory Acidosis You are doing a great job !!
38 Problem pH of 7.51 PaCO2 of 44mmHg HCO3 of 56 What does this indicate Break it down
39 pH = alkalosis PaCO2 = normal HCO3 = alkalosis An abnormal HCO3 indicates a metabolic problem
40 Metabolic Alkalosis You have become an ABG pro!!
41 You Are Doing Great
42 Compensation Occurs as the body begins to correct the acid base imbalance pH will be normal or near normal if total compensation pH will be abnormal if partial compensation
43 Compensation Both the PaCo2 & HCO3 will be abnormal Respiratory imbalances are compensated for by the renal system Metabolic imbalances are compensated for by the respiratory system
44 Example pH of 7.27 PaCO2 of 27 mm Hg HCO3 of 10 mEq/l Note that both the PaCO2 & the HCO3 are low
45 Let’s Break it Down Low pH = acidosis Low PaCO2 = alkalosis Low HCO3 = acidosis HCO3 corresponds with the pH This is a metabolic problem Metabolic acidosis with partial compensation
46 Problems pH of 7.52 PaCO2 of 47 mmHg HCO3 of 36 mEq/L What does this indicate Break it down
47 pH = alkalosis PaCO2 = acidosis HCO3 = alkalosis Which 2 go together
48 Metabolic Alkalosis with Partial Compensation You are doing great!!
49 Problems pH of 7.45 PaCO2 of 50 mmHg HCO3 of 33 meq/L What does this indicate Break it down
50 pH = normal PaCo2 = acidosis HCO3 = alkalosis Your pH leans toward the alkalosis side
51 Fully Compensated Metabolic Alkalosis You have done great!!