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Mr. L is a 42-year-old man who complains of weakness, anorexia, abdominal pain, and vomiting. Laboratory studies demonstrate a HCO3 of 6 mEq/L.

Image not available.What is the differential diagnosis of acid-base disorders? How would you frame the differential?
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Listed below are the steps to analyze an acid-base disorder.

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Step 1: Generate Clinical Hypotheses

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  1. Each clinical scenario suggests a few possible acid-base disorders.

  2. The first step considers those possibilities before analyzing the laboratory results.

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Step 2: Check the pH

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  1. pHs < 7.4 indicates the primary disorder is an acidosis.

  2. pHs > 7.4 indicates the primary disorder is an alkalosis.

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Step 3: Determine Whether the Primary Disorder Is Due to a Metabolic or Respiratory Process

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  1. Check HCO3 and PaCO2

  2. CO2 + H2O ⇔ H2CO3 ⇔ HCO3 + H+; therefore

  3. HCO3 changes drive pH as follows:

    1. Increased HCO3 drives the reaction to left: This consumes H+ which raises the pH, resulting in a metabolic alkalosis.

    1. Decreased HCO3 drives the reaction to the right: This increases H+ which lowers the pH, resulting in a metabolic acidosis. This occurs in two situations:

      1. Processes that produce H+ ion (and consume HCO3) (ie, ketoacidosis, lactic acidosis)

      1. Processes that lose HCO3 (ie, diarrhea)

  4. PaCO2 changes drive pH as follows:

    1. Increased PaCO2 drives reaction to right: This increases H+ which lowers pH, resulting in a respiratory acidosis.

    1. Decreased PaCO2 drives reaction to left: This decreases H+ which raises pH, resulting in a respiratory alkalosis.

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Step 4: Calculate Whether Compensation Is Appropriate

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  1. The acid-base system attempts to maintain homeostasis. Alterations in one system (respiratory or metabolic) trigger compensatory changes in the other system to minimize the impact on pH.

  2. Formulas predict the expected degree of compensation (Table 4–1).

  3. Compensation that is greater or less than expected suggests that an additional disease process is affecting the compensating system.

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Table 4–1. Compensation in Acid-Base Disorders.1,2

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