What is an ABG?
ABG is the abbreviation for Arterial Blood Gas. This is a measure of certain characteristics of arterial blood. The first is the pH. The pH designates the acid-base balance of arterial blood. Ideally, blood pH would be 7.4. However, many variables affect the pH of the blood. If one of these variables forces the pH too far from 7.4, the cells of the body will be unable to function properly. Therefore, the body has two main buffering systems: the respiratory system and the renal system. These systems generally balance each other to provide an optimum environment within the body. They can be thought of as being on either side of a seesaw. When one side moves in one direction the other will move in the opposite direction to maintain balance.
The balancing component of the respiratory system is the dissolved carbon dioxide (CO2) that is produced by cellular processes and removed by the lungs. The balancing component of the renal system is the dissolved bicarbonate (HCO3) produced by the kidneys. The kidneys also help control pH by eliminating hydrogen (H+) ions. The way the two systems interact is through the formation of carbonic acid (H2CO3). Movement through the carbonic acid system is fluid and constant. What this means is that water (H2O) can combine with CO2 and form carbonic acid. If necessary, carbonic acid (H2CO3) can then break up to form hydrogen ions (H+) and bicarbonate (HCO3). This balance works in both directions. By balancing back and forth, pH balance is achieved. The respiratory system balances pH by manipulating the CO2 level. Increasing or decreasing respiratory rate does this. Faster and deeper breathing “blows off” more CO2. Conversely, slower and shallower breathing “retains more CO2. The renal system balances pH by producing HCO3 or by eliminating hydrogen ions (H+).
The renal system will reflect changes in metabolic activity within the body. For example, a patient who becomes hypoxic will undergo anaerobic metabolism, which produces lactic acid. The production of lactic acid will bind or use up available HCO3 and will be manifested by a decrease in the HCO3 level. Therefore, the HCO3 level is an indicator of metabolic acid-base balance.
Balance must always be achieved by the opposite system. If an adult were on one side of a seesaw and a small child on the other, we would expect the child’s side of the seesaw to go up and the adult’s side to go down. We cannot make the child go down by adding another adult to the adult’s side. In the same way, our body regulates pH by using the opposite system to balance pH. So if the pH is out of balance because of a respiratory disorder, it will be the renal system that makes the corrections to balance the body pH. Conversely, if the renal system is to blame for the pH disorder, the respiratory system will have to compensate. This process is called compensation. Compensation may not always be complete. Complete compensation returns the pH balance to normal. There are times when the imbalance is too large for compensation to return the pH to normal. This is called incomplete compensation.
System causing pH imbalance Compensating system
Respiratory (pCO2) Metabolic (HCO3)
Metabolic (HCO3) Respiratory (pCO2)
OK, let’s review what we’ve done so far…
• ABGs measure blood acid-base balance
• Carbon dioxide (CO2) is the respiratory component in acid-base balance
• Bicarbonate (HCO3) is the renal component in the acid-base balance
• CO2 and HCO3 work through carbonic acid to balance pH
• Compensation for pH imbalance comes from the opposite system
• Compensation attempts to bring pH back to normal
There are two sets of information that can be obtained from an ABG. The first is the blood acid-base balance, and the second is blood oxygenation. The measures of blood oxygenation are the oxygen (pO2) and the oxygen saturation (O2 sat). The dissolved oxygen in the blood is called the pO2 and is measured in mmHg. The second measure is the oxygen saturation, which represents the amount of hemoglobin sites with attached oxygen. Oxygen saturation is expressed as a percentage of the total sites that have hemoglobin. The O2 sat can be continually monitored non-invasively with pulse oximetry.
An ABG can detect four main states other than normal. These are: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. Use the "6 Easy Steps to ABG Analysis" to help determine which state exists in your patient.
The Six Steps to ABG Analysis
In order for our analysis to be effective, notes will have to be written next to the results on our lab slip. Alternately, the ABG results can be transcribed onto another paper for analysis (see example one below).
1. The first step in analyzing ABGs is to look at the pH. Normal blood pH is 7.4, plus or minus 0.05, forming the range 7.35 to 7.45. If blood pH falls below 7.35 it is acidic. If blood pH raises above 7.45, it is alkalotic. If it falls into the normal range, label what side of 7.4 it falls on. Lower than 7.4 is normal/acidic, higher than 7.4 is normal/alkalotic. Label it.
2. The second step is to examine the pCO2. Normal pCO2 levels are 35-45mmHg. Below 35 is alkalotic, above 45 is acidic. Label it.
3. The third step is to look at the HCO3 level. A normal HCO3 level is 22-26 mEq/L. If the HCO3 is below 22, the patient is acidotic. If the HCO3 is above 26, the patient is alkalotic. Label it.
4. Next match either the pCO2 or the HCO3 with the pH to determine the acid-base disorder. For example, if the pH is acidotic, and the CO2 is acidotic, then the acid-base disturbance is being caused by the respiratory system. Therefore, we call it a respiratory acidosis. However, if the pH is alkalotic and the HCO3 is alkalotic, the acid-base disturbance is being caused by the metabolic (or renal) system. Therefore, it will be a metabolic alkalosis.
5. Fifth, does either the CO2 or HCO3 go in the opposite direction of the pH? If so, there is compensation by that system. For example, the pH is acidotic, the CO2 is acidotic, and the HCO3 is alkalotic. The CO2 matches the pH making the primary acid-base disorder respiratory acidosis. The HCO3 is opposite of the pH and would be evidence of compensation from the metabolic system.
6. Finally, evaluate the PaO2 and O2 sat. If they are below limits there is evidence of hypoxemia.
Normal Arterial Values Range
O2 Sat. 95-100%
Now let’s try an example:
pH 7.30 acidotic
pCO2 58 acidotic
pO2 50 low
O2 Sat. 80% low
HCO3 26 normal
Step 1. The pH is acidotic
Step 2. The CO2 is acidotic
Step 3. The HCO3 is normal
Step 4. The CO2 matches the pH, therefore the imbalance is respiratory acidosis
Step 5. The HCO3 is normal, therefore there is no compensation
Step 6. The PaO2 and O2 sat are low indicating hypoxemia
The full diagnosis for this ABG is: Uncompensated respiratory acidosis.