Fluid, Electrolyte & Acid-Base · Topic 6 of 23
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: read the gas in three calls
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A textbook metabolic acidosis with no patient attached yet. Read it in three calls, then load a patient.
Make the three calls first. What to look for, what to do and what to expect ordered fill in for the disorder you find.
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Metabolic acidosis
Too much acid or too little bicarbonate. The HCO₃⁻ falls, the pH falls with it — and the lungs race to blow off CO₂.
mEq/L · normal 22 – 26 · with a pH below 7.35 (normal 7.35 – 7.45)
Deep, rapid breaths — the lungs blowing off CO₂ to offset the acid
Acid pushes K⁺ out of cells → dysrhythmias · once it is corrected, K⁺ can fall fast
Potassium in DKA
Example patientDKA: fluids first. Check K⁺ before insulin — insulin pulls K⁺ into cells and the level can crash. The first K⁺ may read high, but the body is low.
No insulin → ketones flood the blood with acid (H⁺).
What’s happeningNo insulin, the acid and the high glucose → K⁺ shifts out of the cells into the blood.
Hyperkalemia on the first lab is common — but K⁺ can also be normal or low. Either way the body is low on K⁺ (lost in the urine).
Put the patient on a continuous cardiac monitor. Report a rising K⁺ right away — above 6.0, report now. Watch for dysrhythmias.
Illustration · example patient in DKA (the K⁺ numbers are examples). Normal K⁺ 3.5 – 5.0 mEq/L. Follow your DKA protocol.
What causes it
Two kindsAcid gained (high anion gap) or bicarbonate lost (normal gap)
Diarrhea causes acidosis (bicarbonate lost from the gut) · vomiting causes alkalosis (stomach acid lost). MUDPILES lists the high-gap causes: Methanol, Uremia, DKA, Paraldehyde (some versions: propylene glycol), Isoniazid or iron, Lactic acidosis, Ethylene glycol, Salicylates.
What you'll see
SignsLabs & diagnostics
CheckHCO₃⁻ below 22 with pH below 7.35
Winters' formula predicts the compensating PaCO₂: 1.5 × HCO₃⁻ + 8 (± 2). Example patient: 1.5 × 12 + 8 = 26 ± 2 — a PaCO₂ outside 24 – 28 would mean a second, respiratory disorder.
Red flags
Act nowTreatment: fix the cause
Cause decidesTreat the cause — the acid clears when the cause is fixed
Fluids first to restore volume; check K⁺ before the insulin starts.
K⁺ falls fast once insulin runsRestore perfusion — fluids first, vasopressors if needed.
For kidney failure and methanol, ethylene glycol or salicylate poisoning.
Reserved for pH below about 7.1 — most helpful when bicarbonate was lost (normal gap).
Not a routine fixMore detail
Nursing priorities
In orderMetabolic or respiratory acidosis?
ROME — Respiratory Opposite, Metabolic Equal
| pH | pH below 7.35 |
|---|---|
| PaCO₂ | Low (below 35) — the lungs blow off CO₂ |
| HCO₃⁻ | Low HCO₃⁻ (below 22) — the cause |
| Which way | HCO₃⁻ moves the same way as the pH (Metabolic Equal) |
| Breathing | Kussmaul breathing — deep and fast |
| Potassium | High K⁺ — shifts out of cells; can fall fast once corrected |
| Classic causes | DKA, diarrhea, kidney failure, shock |
| Fix | Treat the cause — insulin + fluids, dialysis |
Teach your patient
DischargeMetabolic acidosis: HCO₃⁻ down, pH down, breathing deep and fast. Acid pushes K⁺ out — and K⁺ can drop fast once the acid is fixed, so keep checking it.
Sources · OpenStax Medical-Surgical Nursing 10.4 (also 10.3 and 21.2), OpenStax Fundamentals of Nursing 20.2, Open RN Nursing Fundamentals 15.5, Merck Manual Professional on metabolic acidosis, acid-base disorders, DKA and hyperkalemia, MedlinePlus and the ADA 2024 hyperglycemic-crises consensus report (Umpierrez et al., Diabetes Care). Diagram drawn from Cell membrane detailed diagram (Mariana Ruiz, Wikimedia Commons, public domain), Sodium-potassium pump scheme, ions crossing the bilayer through a transport protein (Mariana Ruiz, Wikimedia Commons, public domain), Insulin receptor conformation change upon binding (Gutmann et al., Wikimedia Commons, CC BY 4.0) and Insulin binding its receptor on the cell membrane (XcepticZP, Wikimedia Commons, public domain). Typical adult values — follow your protocol.
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.
Knowledge Check
3 quick questions on the must-knows for this topic.
Which breathing pattern is classic for metabolic acidosis?
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Go further
- SimulatorAcid–Base & ABG SimulatorBuild a gas. Watch compensation happen. Move CO₂ or bicarbonate, then read the gas in three calls: pH, cause, compensation.
- SimulatorFluid & Electrolyte SimulatorChange one lab. Watch the whole patient respond. Sodium, potassium, calcium, magnesium, pH and volume, all linked — with a live ECG and an IV pole where every bag runs.
- GuideRespiratory vs Metabolic Acidosis and Alkalosis: The 4 Disorders With Causes, Findings and What the Nurse DoesTwo organs, two directions. Respiratory disorders are CO₂ problems that belong to the lungs and happen in minutes; metabolic disorders show up in the bicarbonate and come from everywhere else. Each of the four with its causes, what you see, what the nurse does and what the provider orders — in a table, then patient by patient, with each one loaded in a live gas.
- GuideMixed Acid–Base Disorders: How Winter’s Formula Finds the Second ProblemA mixed disorder is two primary problems in one patient, and it often looks less abnormal than a simple one because they cancel. The test is size: is the “compensating” value the size compensation would be? Winter’s formula for metabolic acidosis, the rules for the other three, the clues that need no calculator, and the salicylate patient whose near-normal pH hides two disorders.
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