Mr. L is a 42-year-old man with type 1 diabetes mellitus (DM) who complains of weakness, anorexia, abdominal pain, 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.
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 be easily 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. ||Download (.pdf) Table 4-1. Differential diagnosis of primary acid-base disorders.
|ACIDOSES pH < 7.4 |
|Metabolic Acidoses HCO3− < 24 mEq/L ||Respiratory Acidoses PaCO2 > 40 mm Hg |
Carbonic anhydrase inhibitors
Early renal failure
Lung diseases (most common)
CO or cyanide poisoning
Regional obstruction to blood flow
| || |
Brain: Stroke, intoxication, sleep apnea
Spinal cord: trauma, ALS, polio
Nerve: Guillain-Barre syndrome
Neuromuscular junction: Myasthenia gravis
Chest wall: flail chest, muscular dystrophy
Toxins and miscellaneous
| || |
|ALKALOSES pH > 7.4 |
HCO3− > 24 mEq/L
| || |
PaCO2 < 40 mm Hg
|Vomiting or NG drainage || ||Hypoxemia |
Increased mineralocorticoid activity
Excessive licorice ingestion
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Interstitial lung disease
| || || |
Step 1: Determine Whether the Primary Disorder is an Acidosis or Alkalosis by Reviewing the pH.
pH < 7.4 indicates the primary disorder is an acidosis.
pH > 7.4 indicates the primary disorder is an alkalosis.