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4: Acid-Base Abnormalities

CHIEF COMPLAINT

PATIENT

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 PaCO_2.

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.