Home | Business News | Browse by Publication | J | Journal of Respiratory Diseases

Arterial blood gas analysis: a 3-step approach to acid-base disorders: when should you suspect a mixed acid-base disturbance?

Article Excerpt
ABSTRACT: The foundation of arterial blood gas (ABG) analysis consists of determining whether the patient has acidosis or alkalosis; whether it is a respiratory or metabolic process; and, if respiratory, whether it is a pure respiratory process. If the patient's pH and PC[O.sub.2] are increased or decreased in the same direction, the process is metabolic; if one is increased while the other is decreased, the process is respiratory. In a number of clinical situations, pulse oximetry is preferred to ABG analysis. However, pulse oximetry may not be accurate in patients who are profoundly anemic, hypotensive, or hypothermic. While venous blood gas (VBG) analysis does not provide any information about the patient's oxygenation, it can help assess the level of acidosis or alkalosis. VBG analysis may be particularly useful in patients with diabetic or alcoholic ketoacidosis.

KEY WORDS: Oxygen saturation, Pulse oximetry, Acid-base disorders

**********

Correct interpretation of an arterial blood gas (ABG) value can quickly help identify a wide range of illnesses. Most physicians recall learning the Henderson-Hasselbalch equation and how to calculate pH based on the presence of hydrogen ions. However, applying this knowledge in practice on a day-to-day basis can be challenging for even the most learned physician.

In this article, we provide a simple, 3-step approach to evaluate blood gases quickly and correctly, thus allowing valuable information to be rapidly available for evaluating and treating patients.

THE 3-STEP APPROACH

Our 3-step approach can be summarized as follows:

* Step 1: Does the patient have acidosis acidosis or alkalosis?

* Step 2: Is the acidosis/alkalosis a respiratory or metabolic process?

* Step 3: If it is respiratory acidosis or alkalosis, is it a pure respiratory process or is there also a metabolic component?

Step 1

If the patient's pH is 7.35 or lower, an acidosis is present. A pH of 7.45 or higher indicates that the patient has an alkalosis.

Step 2

If the PC[O.sub.2] drives the pH in the opposite direction, there is a primary respiratory process (as PC[O.sub.2] increases, pH decreases, and vice versa). If the PC[O.sub.2] and pH move in the same direction, a metabolic process is occurring. (1,2)

Step 3

In a pure respiratory process, for every 10 mm Hg change in PC[O.sub.2], the pH should move in the opposite direction by 0.08 ([+ or -] 0.02). For example: if the patient's PC[O.sub.2] is 30 mm Hg (a decrease of 10 mm Hg), the pH should be 7.48 (an increase of 0.08). Or, if the PC[O.sub.2] is 60 mm Hg (an increase of 20 mm Hg), the pH should be 7.24 (a decrease of 0.16, or 0.08 for each 10 mm Hg rise in [PCO.sub.2)]

If this rule is violated, a second metabolic process is also present. This is often referred to as a mixed process. Step 3 essentially compares the "should be" pH with the actual pH--that is, whether the pH is what it should be based on the rule that for each 10 mm Hg change in PC[O.sub.2] , the pH should change by 0.08. This pH can always be [+ or -] 0.02 of the predicted formula's value.

If the patient's pH is not what it should be, is it higher or lower? If it is higher, there must be a concomitant metabolic alkalosis. If the patient's pH is lower, there must be a concomitant metabolic acidosis.

EXAMPLES

Applying this 3-step approach to each case will correctly and quickly identify the underlying acid-base disorder. (1,2)

Example 1

The patient's pH is 7.56, PC[O.sub.2] is 20 mm Hg, and P[O.sub.2] is 110 mm Hg.

* Step 1: The patient's pH is greater than 7.45, indicating that an alkalosis is present.

* Step 2: The PC[O.sub.2] and pH are moving in opposite directions, so it is a respiratory process.

* Step 3: The PC[O.sub.2]...