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I HAVE A PATIENT WITH AN ACID-BASE ABNORMALITY

How do I determine the cause?

CHIEF COMPLAINT

PATIENT image

Mr. L is a 42-year-old man with type 1 diabetes mellitus (DM) who complains of weakness, anorexia, and vomiting. Laboratory studies demonstrate a HCO3 of 6 mEq/L.

His very low HCO3 suggests a significant acid-base abnormality. In addition to evaluating his abdominal pain, exploring his acid-base disorder is critical.

image What is the differential diagnosis of acid-base disorders? How would you frame the differential?

CONSTRUCTING A DIFFERENTIAL DIAGNOSIS

The differential diagnosis of acid-base disorders is extensive (Table 4-1) but can easily be organized into 4 distinct subsets by first determining whether the primary disorder is a (1) metabolic acidosis, (2) metabolic alkalosis, (3) respiratory acidosis, or (4) respiratory alkalosis. The key pivotal feature that allows the clinician to narrow the differential to 1 of these subsets is to first evaluate the pH and then the HCO3 and PaCO2.

Table 4-1.Differential diagnosis of primary acid-base disorders.

Step 1: Determine Whether the Primary Disorder is an Acidosis or Alkalosis by Reviewing the pH

  1. pH < 7.4 indicates the primary disorder is an ...

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