Metabolic Acidosis

When pH drops and bicarbonate is the culprit, the cause matters as much as the numbers — because DKA and renal failure demand very different nursing responses.

Metabolic acidosis occurs when pH falls below 7.35 due to a primary decrease in bicarbonate (HCO₃⁻ < 22 mEq/L) — either from acid accumulation or bicarbonate loss. The anion gap separates causes into two camps. A high anion gap (>12 mEq/L) means unmeasured acids are building up: ketoacids in DKA, lactic acid in shock or sepsis, uremic acids in renal failure, or ingested toxins like methanol or salicylates (mnemonic: MUDPILES). A normal anion gap (hyperchloremic) means bicarbonate is being directly lost — diarrhea is the most common cause, followed by renal tubular acidosis and pancreatic fistulas. Clinical presentation includes Kussmaul respirations (deep, rapid breathing as the lungs try to blow off CO₂), fruity breath in DKA, warm flushed skin from vasodilation, altered LOC progressing to lethargy, and hyperkalemia as hydrogen ions shift into cells and potassium shifts out. Nursing assessment priorities: monitor ABGs and serum electrolytes (especially potassium), assess respiratory pattern and neurological status, maintain IV access, and track intake and output. Potassium may appear elevated on labs but drops rapidly once acidosis is corrected — this rebound hypokalemia is a critical monitoring concern during treatment.

Key Distinctions

Don't confuse metabolic acidosis (low HCO₃⁻, Kussmaul respirations) with respiratory acidosis (high CO₂, shallow or slow breathing) — the breathing patterns are opposite. Students mix up diarrhea (causes metabolic acidosis from bicarb loss) with vomiting (causes metabolic alkalosis from acid loss). The hyperkalemia in acidosis is a shift phenomenon, not true excess — treating the acidosis unmasks the real potassium deficit.

Clinical Pearl

MUDPILES for high anion gap causes: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. If the gap is high, hunt through MUDPILES; if the gap is normal, think GI bicarb loss — diarrhea is the go-to.