Arterial Blood Gas Analysis Made Easy with Tic-Tac-Toe Method

  • October 13, 2020/

Interpretation of arterial blood gases (ABGs) is a crucial skill that a lot of student nurses and medical practitioners need to learn. In this guide, we’ll help you understand the concepts behind arterial blood gas and teach you the easiest and most fun way to interpret ABGs using the tic-tac-toe method.

What is arterial blood gas? 

An arterial blood gas is a laboratory test to monitor the patient’s acid-base balance. It is used to determine the extent of the compensation by the buffer system and includes the measurements of the acidity (pH), levels of oxygen, and carbon dioxide in arterial blood. Unlike other blood samples obtained through a vein, a blood sample from an arterial blood gas (ABG) is taken from an artery (commonly on radial or brachial artery).

What are the components of arterial blood gas? 

There are six components of arterial blood gas (ABGs):

pH

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

PaCO2 (Partial Pressure of Carbon Dioxide)

PaCO2 or partial pressure of carbon dioxide shows the adequacy of the gas exchange between the alveoli and the external environment (alveolar ventilation). Carbon dioxide (CO2) cannot escape when there is damage in the alveoli, excess CO2 combines with water to form carbonic acid (H2CO3) causing an acidotic state. When there is hypoventilation in the alveolar level (for example, in COPD), the PaCO2 is elevated, and respiratory acidosis results. On the other hand, when there is alveolar hyperventilation (e.g., hyperventilation), the PaCO2 is decreased causing respiratory alkalosis. For PaCO2, the normal range is 35 to 45 mmHg (respiratory determinant).

PaO2 (Partial Pressure of Oxygen)

PaO2 or partial pressure of oxygen or PAO2 indicates the amount of oxygen available to bind with hemoglobin. The pH plays a role in the combining power of oxygen with hemoglobin: a low pH means there is less oxygen in the hemoglobin. For PaO2, the normal range is 75 to 100 mmHg

SO2 (Oxygen Saturation)

SO2 or oxygen saturation, measured in percentage, is the amount of oxygen in the blood that combines with hemoglobin. It can be measured indirectly by calculating the PAO2 and pH Or measured directly by co-oximetry. Oxygen saturation, the normal range is 94–100%

HCO3 (Bicarbonate)

HCO3 or bicarbonate ion is an alkaline substance that comprises over half of the total buffer base in the blood. A deficit of bicarbonate and other bases indicates metabolic acidosis. Alternatively, when there is an increase in bicarbonates present, then metabolic alkalosis results.

BE (Base Excess)

BE. Base excess or BE value is routinely checked with HCO3 value. A base excess of less than –2 is acidosis and greater than +2 is alkalosis. Base excess, the normal range is –2 to +2 mmol/L

Normal Values in Arterial Blood Gas 

To determine acid-base imbalance, you need to know and memorize these values to recognize what deviates from normal. The normal range for ABGs is used as a guide, and the determination of disorders is often based on blood pH. If the blood is basic, the HCO3 level is considered because the kidneys regulate bicarbonate ion levels. If the blood is acidic, the PaCO2 or partial pressure of carbon dioxide in arterial blood is assessed because the lungs regulate the majority of acid. The normal ABG values are the following:

  • For  pH, the normal range is 7.35 to 7.45
  • For PaCO2, the normal range is 35 to 45 mmHg (respiratory determinant)
  • For PaO2, the normal range is 75 to 100 mmHg
  • For HCO3, the normal range is 22 to 26 mEq/L (metabolic determinant)
  • Oxygen saturation, the normal range is 94–100%
  • Base excess, the normal range is –2 to +2 mmol/L

Interpreting Arterial Blood Gas Imbalances

Interpreting arterial blood gases is used to detect respiratory acidosis or alkalosis, or metabolic acidosis or alkalosis during an acute illness. To determine the type of arterial blood gas the key components are checked. The best (and fun) way of interpreting arterial blood gas is by using the tic-tac-toe method below:

Goals of ABG analysis

For the purpose of 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 the compensation if it 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.

Steps in ABG analysis using the tic-tac-toe method

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

1. Memorize the normal values. 

The first step is you need to familiarize yourself with the normal 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 normal values above. Write it down together with the arrows indicating ACIDOSIS or ALKALOSIS. Note that PaCO2 is intentionally inverted for the purpose of the Tic-Tac-Toe method.

2. Create your tic-tac-toe grid. 

The third step of this technique is to determine the acidity or alkalinity of the blood with the given value of the pH 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, 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.

Please use the diagram below to help you visualize whether the normal value is ACIDOSIS or ALKALOSIS.

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.

5. Determine if HCO3 is under NORMAL, ACIDOSIS, or ALKALOSIS.