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.
|What is the differential diagnosis of acid-base disorders? How would you frame the differential?|
Listed below are the steps to analyze an acid-base disorder.
Step 1: Generate Clinical Hypotheses
Each clinical scenario suggests a few possible acid-base disorders.
The first step considers those possibilities before analyzing the laboratory results.
pHs < 7.4 indicates the primary disorder is an acidosis.
pHs > 7.4 indicates the primary disorder is an alkalosis.
Step 3: Determine Whether the Primary Disorder Is Due to a Metabolic or Respiratory Process
Check HCO3− and PaCO2
CO2 + H2O ⇔ H2CO3 ⇔ HCO3− + H+; therefore
HCO3− changes drive pH as follows:
Increased HCO3− drives the reaction to left: This consumes H+ which raises the pH, resulting in a metabolic alkalosis.
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:
Processes that produce H+ ion (and consume HCO3−) (ie, ketoacidosis, lactic acidosis)
Processes that lose HCO3− (ie, diarrhea)
PaCO2 changes drive pH as follows:
Increased PaCO2 drives reaction to right: This increases H+ which lowers pH, resulting in a respiratory acidosis.
Decreased PaCO2 drives reaction to left: This decreases H+ which raises pH, resulting in a respiratory alkalosis.
Step 4: Calculate Whether Compensation Is Appropriate
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.
Formulas predict the expected degree of compensation (Table 4–1).
Compensation that is greater or less than expected suggests that an additional disease process is affecting the compensating system.
Table 4–1. Compensation in Acid-Base Disorders.1,2 |Favorite Table|Download (.pdf)
Table 4–1. Compensation in Acid-Base Disorders.1,2
|Primary Disorder||Duration||Expected Compensation|
|Metabolic acidosis||Acute/Chronic||PaCO2↓ 1.2 mm Hg per 1 mEq/L ↓ HCO3− (To a minimum PaCO2 of 10-15 mm Hg)|
|Metabolic alkalosis||Acute/Chronic||PaCO2↑ 0.7 mm Hg per 1 mEq/L ↑ HCO3−|
|Respiratory acidosis||Acute||HCO3− ↑ 1 mEq/L per 10 mm Hg ↑ PaCO2|
|Chronic||HCO3− ↑ 3.5 mEq/L per 10 mm Hg ↑ PaCO2|
|Respiratory alkalosis||Acute||HCO3− ↓ 2 mEq/L per 10 mm Hg ↓ PaCO2|
|Chronic||HCO3− ↓ 4 mEq/L per ...|