Interpretation of ABGs Interpretation of Arterial Blood Gases (ABGs) (Section of Pulmonary & Critical Care Medicine) Introduction:
The following six-step process helps ensure a complete interpretation of every ABG. In addition, you will find tables that list commonly encountered acid-base disorders. Many methods exist to guide the interpretation of the ABG. This discussion does not include some methods, such as analysis of base excess or Stewart’s strong ion difference. A summary of these techniques can be found in some of the suggested articles. It is unclear whether these alternate methods offer clinically important advantages over the presented approach, which is based on the “anion gap.” 6-step approach: Step 1: Assess the internal consistency of the values using the Henderseon-Hasselbach equation: [H+] = 24(PaCO2) If the pH and the [H+] are inconsistent, the ABG is probably not valid.
Step 2: Is there alkalemia or acidemia present? pH < 7.35 acidemia
Step 3: Is the disturbance respiratory or metabolic? What is the relationship between the direction of change in the pH and the direction of change in the PaCO2? In primary respiratory disorders, the pH and PaCO2 change in opposite directions; in metabolic disorders the pH and PaCO2 change in the same direction.
Step 4: Is there appropriate compensation for the primary disturbance? Usually, compensation does not return the pH to normal (7.35 – 7.45).
If the observed compensation is not the expected compensation, it is likely that more than one acid-base disorder is present. Step 5: Calculate the anion gap (if a metabolic acidosis exists): AG= [Na+]-( [Cl-] + [HCO3-] )-12 ± 2
Step 6: If an increased anion gap is present, assess the relationship between the increase in the anion gap and the decrease in [HCO3-]. Assess the ratio of the change in the anion gap (∆AG ) to the change in [HCO3-] (∆[HCO3-]): ∆AG/∆[HCO3-] This ratio should be between 1.0 and 2.0 if an uncomplicated anion gap metabolic acidosis is present. If this ratio falls outside of this range, then another metabolic disorder is present:
It is important to remember what the expected “normal” anion gap for your patient should be, by adjusting for hypoalbuminemia (see Step 5, above.) Table 1: Characteristics of acid-base disturbances
Table 2: Selected etiologies of respiratory acidosis
Table 3: Selected etiologies of respiratory alkalosis
Table 4: Selected causes of metabolic alkalosis
Table 5: Selected etiologies of metabolic acidosis
a Most common causes of metabolic acidosis with an elevated anion gap
Table 6: Selected mixed and complex acid-base disturbances
Suggested additional reading:
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