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Home Books Symptom to Diagnosis: An Evidence-Based Guide, 3e

4: Acid-Base Abnormalities

CHIEF COMPLAINT

PATIENT image

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.

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 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.
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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

Anion gap

Ketoacidoses

   DKA

   Alcoholic

   Starvation

Nonanion gap

Diarrhea

RTA

Carbonic anhydrase inhibitors

Dilutional1

Early renal failure

Lung diseases (most common)

   COPD

   Asthma

   Pulmonary edema

   Pneumonia

Lactic acidosis

   Hypoxia

   Shock

      Septic

      Hypovolemic

      Cardiogenic

      Anaphylactic

   CO or cyanide poisoning

   Regional obstruction to blood flow

   Seizures

Pleural disease

   Effusions

   Pneumothorax

Neuromuscular disease

   Brain: Stroke, intoxication, sleep apnea

   Spinal cord: trauma, ALS, polio

   Nerve: Guillain-Barre syndrome

   Neuromuscular junction: Myasthenia gravis

   Chest wall: flail chest, muscular dystrophy
Uremia

Toxins and miscellaneous

   Salicylate2

   Methanol3

   Ethylene glycol

   Rhabdomyolysis

   D-Lactic acidosis
ALKALOSES pH > 7.4

Metabolic Alkaloses

HCO3− > 24 mEq/L

Respiratory Alkaloses

PaCO2 < 40 mm Hg
Vomiting or NG drainage Hypoxemia

Diuretics

Hypokalemia

Increased mineralocorticoid activity

   Primary hyperaldosteronism

   Hypercortisolism

   Excessive licorice ingestion

Pulmonary disorders

   Pneumonia

   Asthma

   Pulmonary edema

   Pulmonary embolism

   Interstitial lung disease

   Mechanical ventilation

Extrapulmonary disorders

   Anxiety

   Pain

   Fever

   Pregnancy

   CNS insult

   Drugs4

   Cirrhosis

1Following very large volume saline administration.

2Salicylate intoxication typically causes a primary respiratory alkalosis (due to CNS stimulation of the respiratory center) and a primary anion gap metabolic acidosis (with accumulation of lactate and ketones).

3Methanol and ethylene glycol ingestion may occur after ingestion of antifreeze, de-icying solutions, and other organic solvents

4Drugs include salicylates, nicotine, and catecholamines.

ALS, amyotrophic lateral sclerosis; CNS, central nervous system; CO, carbon monoxide; COPD, chronic obstructive pulmonary disease; DKA, diabetes ketoacidosis; NG, nasogastric; RTA, renal tubular acidosis.

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 acidosis.

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

Step 2: Determine ...

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