Arterial Blood Gas Interpretation
QUICK LOOK AT THE CHAPTER AHEAD
The following chapter identifies the normal laboratory values for arterial blood gases. A step-by-step process is used. Examples of acid-base imbalances are also given.
 Figure 36-1 Arterial blood gas interpretation. |
To interpret arterial blood gases for a patient, one must know what the normal values are:
pH | 7.35-7.45 |
pCO2 | 35-45 mm Hg |
HCO3 | 23-27 mEq/L |
There are three steps in determining and imbalance. In the first step, always look at the pH to determine acidosis or alkalosis:
pH < 7.35 = acidosis
pH > 7.45 = alkalosis
In the second step, determine what is causing the imbalance: Is it the CO2 or HCO3?
pCO2 < 35 mm Hg = alkalosis
pCO2 > 45 mm Hg = acidosis
HCO3 < 23 mEq/L = acidosis
HCO3 > 27 mEq/L = alkalosis
In the third step, determine whether compensation is occurring:
pH 7.35-7.40 = compensated acidosis
pH 7.40-7.45 = compensated alkalosis
The first step = look at the pHThe second step = look at which system is causing the imbalance
The third step = is compensation occurring?
EXAMPLES OF ACID-BASE IMBALANCESMetabolic acidosis (uncompensated):
pH = 7.32
pCO2 = 32 mm Hg
HCO3 = 14 mEq/L
An example of this type of imbalance is a patient suffering from chronic renal failure.
Question: The pCO2 isn’t normal. Why doesn’t that result affect the equation?
View Answer
Answer: Always remember to look at the pH value first. The pH of 7.32 indicates acidosis and the only value contributing to acidosis is the HCO3. The low pCO2 is actually a value indicating alkalosis. This arterial blood gas result could be an example of a patient suffering from chronic renal failure.
Metabolic acidosis (compensated):
pH = 7.37
pCO2 = 28 mm Hg
HCO3 = 21 mEq/L
Question: Why is this result considered compensated?
View Answer
Answer: The pH value falls in the compensated range for metabolic acidosis. The other values can still be abnormal with a compensated pH. One of the systems (respiratory) has compensated for the out-of-normal range, the bicarbonate system. Perhaps the pCO2 has decreased through hyperventilation to decrease the amount of H2CO3 in the body.
Metabolic alkalosis (uncompensated):
pH = 7.52
pCO2 = 48 mm Hg
HCO3 = 30 mEq/L
Question: The pCO2 is also abnormal. How does it factor into this blood gas result?
View Answer
Answer: The pH indicates alkalosis, and the only result that is also indicative of alkalosis is the HCO3. The pCO2 is elevated because the body is attempting to hypoventilate to retain acid and compensate for the increased bicarbonate. This arterial blood gas result could be seen in a surgical patient who is losing too much acid through nasogastric suctioning.
Metabolic alkalosis (compensated):
pH = 7.44
pCO2 = 48 mm Hg
HCO3 = 29 mEq/L
Question: When the pH falls within the range of 7.35-7.45 is it always considered compensated even though the other values are out of range?
View Answer
Answer: Yes, as long as the pH is a normal value, the body has compensated for the value that is out of range. This arterial blood gas is similar to the uncompensated metabolic alkalosis result and could be the same patient situation; however, the body has now compensated for the elevated HCO3 by retaining acid (elevated pCO2).
Respiratory acidosis (uncompensated):
pH = 7.33
pCO2 = 55 mm Hg
HCO3 = 23 mEq/L
Question: Is it truly possible for a patient to have such a high pCO2?
View Answer
Answer: Yes, frequently patients with chronic obstructive lung disease have high pCO2 results.
Respiratory acidosis (compensated):
pH = 7.38
pCO2 = 48 mm Hg
HCO3 = 29 mEq/L
Question: Which system has made the pH compensated?
View Answer
Answer: The metabolic system (HCO3), has increased to compensate for the elevated respiratory system (pCO2).
Respiratory alkalosis (uncompensated):
pH = 7.50
pCO2 = 30 mm Hg
HCO3 = 21 mEq/L
Question: What type of patient condition would cause respiratory alkalosis?
View Answer
Answer: Any situation where the patient would hyperventilate and blow off CO2, such as a patient with a hypermetabolic state such as fever or sepsis.
Respiratory alkalosis (compensated):
pH = 7.44
pCO2 = 32 mm Hg
HCO3 = 22 mEq/L
Question: Is this pH considered to be compensated?
View Answer
Answer: Yes, it falls within the normal range of the pH values. However, the pCO2 is low, indicating a respiratory alkalosis. The HCO3 is slightly decreased in an attempt to decrease bicarbonate levels.
Respiratory and metabolic acidosis (mixed disturbance):
pH = 7.30
pCO2 = 50 mm Hg
HCO3 = 19 mEq/L
Question: What is a mixed disturbance?
View Answer
Answer: A mixed disturbance occurs when both systems are out of the normal value range, resulting in an acidosis or alkalosis. An example of a patient with this type of arterial blood gas result could be one with chronic renal failure and chronic obstructive pulmonary disease.
Respiratory and metabolic alkalosis (mixed disturbance):
pH = 7.50
pCO2 = 32 mm Hg
HCO3 = 30 mEq/L
Question: With a mixed disturbance, do all the values need to indicate an acidosis or alkalosis?
View Answer
Answer: Yes, as with this arterial blood gas result. The pCO2 is low, indicating that the patient is losing acid via the respiratory system, possibly from hyperventilation or from pain, and the HCO3 is elevated, indicating the patient is losing acid, perhaps from prolonged nasogastric suction (loss of HCL acid along with gastric secretions).
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