What is ABGs Meaning ? | Compensated & Uncompensated

Welcome, nursing students! This tutorial will guide you through interpreting arterial blood gases (ABGs). In this lesson, we will understand what ABGs are, their components, and how to solve an ABG sample with an example.

What is ABGs Meaning ?

Arterial blood gas (ABG) is a test that measures the concentrations of acid, oxygen, and carbon dioxide levels in the arterial blood. It helps to analyze how well an individual’s lungs function by measuring various gases in the blood.

What are the Components of Arterial Blood Gas?

The six components of arterial blood gas include:

1. pH (measurement of acidity/alkalinity)
2. PaCO2 (partial pressure of carbon dioxide)
3. PaO2 (partial pressure of oxygen)
4. SO2 (oxygen saturation)
5. HCO3 (bicarbonate)
6. BE (base excess).

pH (measurement of acidity/alkalinity)

The pH is the hydrogen ions’ concentration and determines body fluids’ acidity or alkalinity. A pH of 7.35 indicates acidosis, and a pH greater than 7.45 indicates alkalosis. The average ABG level for pH is 7.35 to 7.45.

PaCO2 (partial pressure of carbon dioxide)

PaCO2 is the amount of carbon dioxide gas in arterial blood. It measures the ability of the lungs to exchange oxygen and carbon dioxide molecules. A higher level indicates hypercapnia, and a lower level indicates hypocapnia. The average ABG level for PaCO2 is 35-45 mm Hg.

PaO2 (partial pressure of oxygen)

PaO2 is the amount of oxygen in arterial blood. It measures the ability of the lungs to absorb oxygen from inspired air. A higher level indicates better lung efficiency, while a lower level may indicate hypoxia or anemia. The average ABG level for PaO2 is 80-100 mm Hg.

SO2 (oxygen saturation)

The SO2 measures the number of oxygen molecules bound to hemoglobin compared with the total capacity for those molecules. It indicates how efficiently oxygen is transported in the blood. The average ABG level for SO2 is 95%-100%.

HCO3 (bicarbonate)

The HCO3 measures the concentration of bicarbonate ions in the blood, which helps regulate acid-base balance. A higher level indicates metabolic alkalosis, and a lower level may indicate metabolic acidosis. The average ABG level for HCO3 is 22-26 mEq/L.

BE (base excess)

The BE measures the extracellular fluid’s capacity to buffer acid. It indicates the base required to return blood to an average pH level. A higher BE may indicate metabolic alkalosis, while a lower BE may signify metabolic acidosis. The average ABG level for BE is –2 to +2 mEq/L.

Normal Values in Arterial Blood Gas 

It is essential to know the average values to identify any acid-base imbalances. An ABG test helps you determine if anything deviates from these standards. The blood pH is used as a general guide to assess the acidity or basicity of the blood. 

The HCO3 level should be checked when fundamental since the kidneys regulate it. When acidic, PaCO2 (partial pressure of carbon dioxide in arterial blood) should be assessed as this is managed primarily by the lungs. Generally speaking, these are considered normal ABG results:

• P(H): 7.35 – 7.45
• PaCO2: 35 – 45 mmHg
• HCO3: 22 – 26 mEq/L
• SaO2: 95 – 100%
• BE -2 to +2 mmol/L.

Interpreting Arterial Blood Gas Imbalances

Interpreting arterial blood gases is essential for diagnosing respiratory acidosis or alkalosis and metabolic acidosis or alkalosis during acute illness. The concentration of critical components must be evaluated to identify the type of arterial blood gas. The most efficient (and enjoyable) way to interpret these gases is via the tic-tac-toe method outlined below:

The tic-tac-toe method involves drawing a Tic Tac Toe grid with 9 boxes and filling each box with either an HCO3 (bicarbonate) or pH value from the tested sample. Once all nine boxes are filled in, you can compare them to predetermined standards indicating whether the patient has acidosis, alkalosis, or if the model is typical. This method can quickly and accurately identify a patient’s type of acid-base imbalance.

Goals of Arterial Blood Gas Analysis

For this guide, we have set three (3) goals that we need to accomplish when interpreting arterial blood gases. The goals are as follows:

1. Based on the given ABG values, determine if values interpret ACIDOSIS or ALKALOSIS.
2. Second, we need to determine if values define METABOLIC or RESPIRATORY.
3. Lastly, we need to determine whether the compensation is: FULLY COMPENSATED, PARTIALLY COMPENSATED, or UNCOMPENSATED.

We need to keep these goals in mind as they’ll come up later in the steps for the ABG interpretation technique.

There are eight (8) steps simple steps you need to know if you want to interpret arterial blood gas (ABGs) results using the tic-tac-toe technique.

Step 1: Memorize the Normal values.

The first step is to familiarize yourself with the standard and abnormal ABG values when you review the lab results. They are easy to remember:

• For pH, the normal range is 7.35 to 7.45
• For PaCO2, the normal range is 35 to 45
• For HCO3, the normal range is 22 to 26

The recommended way of memorizing it is by drawing the diagram of average values above. Please write it down together with the arrows indicating ACIDOSIS or ALKALOSIS. Note that PaCO2 is intentionally inverted for the Tic-Tac-Toe method.

Step 2: Draw a tic-tac-toe grid with 9 boxes

Once you’ve memorized the average values and the diagram, create a blank on your tic-tac-toe grid and label the top row ACIDOSIS, NORMAL, and ALKALOSIS. Based on their values, we need to determine in which column we’ll place pH, PaCO2, and HCO3 in the grid.

Step 3: Fill in the boxes with pH and HCO3 values from the tested sample

The third step of this technique is to determine the blood’s acidity or alkalinity with the given pH value as our determining factor. Remember in step #1 that the normal pH range is from 7.35 to 7.45.

• If the blood pH is between 7.35 to 7.39, the interpretation is NORMAL but SLIGHTLY ACIDOSIS; place it under the NORMAL column.
• If the blood pH is between 7.41 to 7.45, the interpretation is NORMAL but SLIGHTLY ALKALOSIS; place it under the NORMAL column.
• Any blood pH below 7.35 (7.34, 7.33, 7.32, and so on) is ACIDOSIS; place it under the ACIDOSIS column.
• Any blood pH above 7.45 (7.46, 7.47, 7.48, and so on) is ALKALOSIS; place it under the ALKALOSIS column.

Step 4: Analyze the pattern of pH and HCO3 to identify acidosis or alkalosis (top row, bottom row, left column, right column)

For this step, we need to interpret if the value of PaCO2 is within the NORMAL range, ACIDIC, or BASIC and plot it on the grid under the appropriate column. Remember that the normal range for PaCO2 is from 35 to 45:

• If PaCO2 is below 35, place it under the ALKALOSIS column.
• If PaCO2 is above 45, place it under the ACIDOSIS column.
• If PaCO2 is within its normal range, place it under the NORMAL column.

Step 5: Determine if HCO3 is under NORMAL, ACIDOSIS, or ALKALOSIS

Next, we need to interpret if the value of HCO3 is within the NORMAL range, ACIDIC, or BASIC, and plot it under the appropriate column in the tic-tac-toe grid. Remember that the normal range for HCO3 is from 22 to 26:

• If HCO3 is below 22, place it under the ACIDOSIS column.
• If HCO3 is above 26, place it under the ALKALOSIS column.
• If HCO3 is within its normal range, place it under the NORMAL column.

Step 6: Solve for goal #1: ACIDOSIS or ALKALOSIS

Now, we will start solving for our goals. Looking at the tic-tac-toe grid, determine whether in what column the pH is placed and interpret the results:

• If pH is under the ACIDOSIS column, it is ACIDOSIS.
• If pH is under the ALKALOSIS column, it is ALKALOSIS.
• If pH is under the NORMAL column, determine whether the value is leaning towards ACIDOSIS or ALKALOSIS and interpret accordingly.

In this step, we can accomplish goal #1 of determining ACIDOSIS or ALKALOSIS.

Step 7: Solve for goal #2: METABOLIC or RESPIRATORY

Looking back again on the tic-tac-toe grid, determine if pH is under the same column as PaCO2 or HCO3 to accomplish our goal #2 of deciding if the ABG is RESPIRATORY or METABOLIC. Interpret the results as follows:

• If pH is under the same column as PaCO2, it is RESPIRATORY.
• If pH is under the same column as HCO3, it is METABOLIC.
• If pH is under the NORMAL column, determine whether the value is leaning towards ACIDOSIS or ALKALOSIS and interpret accordingly.

Step 8: Solve for goal #3: COMPENSATION

Lastly, we must determine the compensation to accomplish our goal #3. Interpret the results as follows:

• It is FULLY COMPENSATED if pH is normal.
• It is PARTIALLY COMPENSATED if all three (3) values are abnormal.
• It is UNCOMPENSATED if PaCO2 or HCO3 is normal and the other is abnormal.

Application and Examples

Let’s solve the ABG interpretation with the examples below:

Practice Problem 1: Metabolic Acidosis, Partially Compensated

pH=7.26 | PaCO₂=32 | HCO₃=18

Answer:

1. Remember the average values.
2. Make your tic-tac-toe grid.
3. pH of 7.26 is ABNORMAL, and under ACIDOSIS, so we place pH under ACIDOSIS.
4. PaCO2 of 32 is ABNORMAL and under ALKALOSIS, so we place PaCO2 under ALKALOSIS.
5. HCO3 of 18 is ABNORMAL and under ACIDOSIS, so we place HCO3 under ACIDOSIS.
6. pH is under ACIDOSIS; therefore, solving for goal #1, we have ACIDOSIS.
7. pH is on the same column as HCO3; therefore, solving for goal #2, we have METABOLIC.
8. All three values are ABNORMAL; therefore, solving for goal #3, we have a PARTIALLY COMPENSATED ABG.

The answer to Practice Problem #1: Metabolic Acidosis, Partially Compensated

Practice Problem 2: Respiratory Alkalosis, Fully Compensated

pH=7.48 | PaCO2=50 | HCO3=25

Answer:

1. Remember the average values.
2. Make your tic-tac-toe grid.
3. The pH of 7.48 ABNORMAL and under ALKALOSIS, so we place pH under ALKALOSIS.
4. PaCO2 of 50 is ABNORMAL and under ACIDOSIS, so we place PaCO2 under ACIDOSIS.
5. HCO3 of 25 is NORMAL, so we place HCO3 under NORMAL.
6. pH is under ALKALOSIS; therefore, solving for goal #1, we have ALKALOSIS.
7. pH is on the same column as PaCO2; therefore, solving for goal #2, we have RESPIRATORY.
8. Since all values are either NORMAL or ABNORMAL but not both, therefore solving for goal #3, we have a FULLY COMPENSATED ABG.

The answer to Practice Problem #2: Respiratory Alkalosis, Fully Compensated.

Practice Problem 3: Metabolic Acidosis, Uncompensated

pH=7.25 | PaCO2=37 | HCO3=15

Answer:

1. Remember the average values.
2. Make your tic-tac-toe grid.
3. The pH of 7.25 ABNORMAL and under ACIDOSIS, so we place pH under ACIDOSIS.
4. PaCO2 of 37 is NORMAL, so we place PaCO2 under NORMAL.
5. HCO3 of 15 is ABNORMAL and under ACIDOSIS, so we place HCO3 under ACIDOSIS.
6. pH is under ACIDOSIS; therefore, solving for goal #1, we have ACIDOSIS.
7. pH is on the same column as HCO3; therefore, solving for goal #2, we have METABOLIC.
8. Since PaCO2 is NORMAL, but HCO3 is ABNORMAL, thus solving for goal #3, we have an UNCOMPENSATED ABG.

The answer to Practice Problem 3: Metabolic Acidosis, Uncompensated.