Arterial Blood Gas
Terminology
Arterial blood gas (ABGs) are drawn from arterial blood and measures the concentration of concentration of oxygen and carbon dioxide in the blood among other things.
Everything we will discuss is centered around the goal of maintaining a normal blood pH. The general term used for pathologies surrounding blood pH is acid-base disorders.
When are ABGs Used
ABGs are drawn when there is concern regarding the pulmonary system, circulatory system or metabolic processes.
These systems affect the major organs of the body and are necessary for maintaining homeostasis and the pH of blood.
They are codependent, and as we will see, they are so intertwined that they compensate for each other when one is dysregulated.
When coordination becomes so dysregulated that they can no longer compensate for each other we see critical illness. ABGs are usually needed in patients who are severely ill.
What do ABGs Measure
An ABG panel will report:
pH: the degree of acidity or alkalinity
PCO2: partial pressure of carbon dioxide i.e. the dissolved carbon dioxide in the blood
PO2: partial pressure of oxygen i.e. the dissolved oxygen in the blood
HCO3–: dissolved bicarbonate in the blood
SaO2: oxygen saturation. The percent of hemoglobin carrying oxygen
There are more components to the ABG panel but the concentration of oxygen, carbon dioxide and bicarbonate primarily affects the pH of blood and will be the focus of this unit.
Acid-Base Disorders:
There are 2 broad categories of acid-base disorders:
Metabolic Disorders
Respiratory Disorders
Each of these can result in either acidosis (low pH) of alkalosis (high pH)
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Primary Problem & Compensation
The respiratory and metabolic system work together to maintain pH of blood. They compensate for each other. When the primary cause of abnormal pH is because of respiratory issues, the metabolic system will try to compensate to restore pH. In this case the respiratory system is the primary problem and the metabolic system compensates.
Likewise, if dysfunction in the metabolic system is causing acidosis or alkalosis, the respiratory system will respond trying to correct pH. In this case the metabolic system is the primary problem and the respiratory system compensates.
Metabolic
Acid-Base Disorders
Metabolic acid-base disorders are oriented about the bicarbonate level. Bicarbonate is basic (alkalotic).
Metabolic acidosis will therefore result when bicarbonate levels are low. Metabolic alkalosis will result when bicarbonate levels are high.
Metabolic acid-base homeostasis is maintained largely by the kidneys. The kidneys are able to regulate the release or uptake of bicarbonate to maintain pH.
The respiratory response will compensate for pH imbalance that is caused by metabolic dysregulation. Respiratory response is fairly rapid, achieved by adjusting the rate of breathing (hyperventilation/hypoventilation).
Respiratory
Acid-Base Disorders
Respiratory acid-base disorders are oriented about the partial pressure of carbon dioxide.
Carbon dioxide is weak acid. Respiratory acidosis will therefore result when CO2 levels are high. Respiratory alkalosis will occur when CO2 levels are low.
Respiratory acid-base homeostasis is maintained largely by the rate of breathing (respiratory rate).
Slowing down the rate of exhalation will keep more carbon dioxide which will increase acidity (low pH). Increasing the rate of exhalation will clear carbon dioxide from the blood decreasing acidity (increase pH).
Metabolic responses will compensate for pH imbalances caused by respiratory dysregulation. This is a slow response, it can takes hours or days for the kidneys to detect and adjust bicarbonate levels.
Blood pH Equation
Bicarbonate and carbon dioxide make up the main pH buffering system in the body. Plasma is 92% water therefore these molecules exist in plasma in solution creating a codependent balance between acids and bases.
This balance is represented by the equation below:
Note that carbon dioxide and bicarbonate are on opposite sides of the equilibrium equation. To maintain equilibrium, if one increases in concentration, the other must decrease and vice versa. This is the basis of the tandem relationship of the kidneys and lungs in acid base disorders.
Normal ABG Values
As discussed above an ABG reports many values but we will focus on the three that are used to determine acid-base disturbances.
Mental hacks to remember these values:
- Notice that the pCO2 range has the same value as the decimal points as the pH values (35 and 45)
- (Bi) means 2. Your value range for (bi)carbonate includes all twos and 6 (22-26).
Read an ABG in 4 Simple Steps
When we read an ABG we want to determine the cause of an acid-base disorder. This is referred to as the primary problem. We can read an ABG in simple in 4 steps:
- Is the pH: acidic or alkalotic?
- Is the pCO2 acidic or alkalotic?
- Is the HCO3 acidic or alkalotic?
- Whichever (pCO2/HCO3) can be described the same as pH is the primary problem.
Primary problem is respiratory if pH and pCO2 can be described the same (acidic/basic)
Primary problem is metabolic if pH and HCO3- can be described the same (acidic/basic)
Clinical Scenarios
Because respiratory will compensate for metabolic and vice versa there are really only 4 basic clinical scenarios that can result.
HCO3 represents metabolic, pCO2 represents respiratory.
If pH is acidic, one value will also be acidic (primary problem) the other will be alkalotic (trying to compensate).
If pH is basic, one value will also be basic (primary problem) the other will be acidic (trying to compensate).
MedCalc offers an ABG analyzer. As always, I will caution you that no calculator assumes your liability in patient care. It is always best that you understand that values you are entering and how to interpret the results.
Example Calculations
These concepts are the foundation of acid-base base disorders. Can it get more complicated? Absolutely!
Compensation by either system is not complete, so it does not “solve” the primary problem. Secondary problems can also coexist with primary problems. We can have mixed acid-base disorders. ABGs can also present differently if the disturbance is acute versus chronic. You will have difficulty understanding these more advanced concepts of acid-base disorders if you do not get a good grasp on the concepts in this unit.
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