There are four acid–base disorders, and the fastest way to keep them apart is to stop thinking of them as four things. There are two organs — lungs and kidneys — and each can push the pH one of two ways. That is the whole grid. Below is each disorder as a nurse meets it: what causes it, what it looks like, and what you do, followed by the four patients loaded into a live gas.
What is the difference between respiratory and metabolic acid–base disorders?
Respiratory disorders are CO₂ problems and belong to the lungs. CO₂ is an acid, so retaining it (hypoventilation) causes acidosis and blowing it off (hyperventilation) causes alkalosis. They happen in minutes.
Metabolic disorders are everything else and show up in the bicarbonate. Lose base or gain acid and bicarbonate falls (acidosis); lose acid or gain base and it rises (alkalosis). They come from the gut, the kidneys, the pancreas, a drug, or a failing circulation — anywhere but the lungs.
On the gas, CO₂ moves opposite to pH when it is the cause, and bicarbonate moves with it (ROME). That is the read; the rest of this post is the patient.
The four disorders side by side
| Disorder | Gas | Common causes | What you see | What the nurse does |
|---|---|---|---|---|
| Metabolic acidosis | ↓ HCO₃⁻, ↓ pH | DKA, lactic acidosis (shock, sepsis), kidney failure, salicylates; diarrhea, renal tubular acidosis (normal gap) | Kussmaul breathing, confusion, warm flushed skin, hyperkalemia; nausea | Treat the cause and restore perfusion; watch K⁺; bicarbonate only if pH < 7.1 |
| Metabolic alkalosis | ↑ HCO₃⁻, ↑ pH | Vomiting, NG suction, loop and thiazide diuretics, hypokalemia, excess antacids | Slow shallow breathing, cramps, tingling, dysrhythmias; ↓ K⁺, ↓ Cl⁻ | Stop the loss; 0.9% saline with KCl (chloride fixes it); antiemetic |
| Respiratory acidosis | ↑ PaCO₂, ↓ pH | Hypoventilation: opioids, sedation, COPD, airway obstruction, neuromuscular weakness, chest injury | Drowsiness → coma, headache, low RR, bounding pulse, hyperkalemia | Open the airway, stimulate, bag; naloxone if opioid; O₂ 88–92% in COPD; BiPAP or intubation if worsening |
| Respiratory alkalosis | ↓ PaCO₂, ↑ pH | Hyperventilation: anxiety, pain, fever, hypoxemia, sepsis, early salicylates, ventilator over-breathing | Tachypnea, lightheadedness, perioral tingling, carpopedal spasm (ionized Ca falls) | Treat the driver: oxygen if hypoxemic, analgesia, calm coaching; never a paper bag |
Metabolic acidosis — acid added or base lost
Two kinds, and the anion gap tells them apart. High-gap acidosis is acid being added — ketoacids in DKA, lactate in shock, retained acids in kidney failure, salicylates, methanol. Normal-gap acidosis is bicarbonate being lost — diarrhea, renal tubular acidosis, large volumes of normal saline — with chloride rising to fill the space. The lungs answer within minutes with deep, fast Kussmaul breathing; do not sedate it away, it is the compensation. Watch the potassium: acidosis pushes it out of cells, so the DKA patient reads high and crashes once insulin drives it back in.
Below is DKA. Read it in three calls, then open the anion-gap drawer and see why this one is high-gap. The diarrhea patient is one chip away for the normal-gap comparison.
DKA: read the gas in three calls
Drag either driver
DKA. A patient with type 1 diabetes has abdominal pain, dehydration, and deep rapid respirations.
Kussmaul breathing, fruity breath, dry mucosa, glucose over 250, and a potassium that falls once insulin starts.
Two large-bore IVs, hourly glucose and potassium, strict I&O. Do not slow the breathing — it is the compensation.
Glucose every hour, BMP with K⁺ every 2–4 h, ketones, repeat ABG.
0.9% NaCl bolus, regular insulin infusion, KCl once K⁺ is under 5.3 and urine flows.
Free, no signup. Eight patients, random gases, and a normal-values drill in the full simulator →
Metabolic alkalosis — acid lost or base gained
Vomiting and NG suction throw away hydrochloric acid; loop and thiazide diuretics waste hydrogen, potassium and chloride; hypokalemia itself drives the kidney to hold on to bicarbonate. The lungs try to help by hypoventilating, but they cannot go far — the drive to breathe for oxygen wins — so metabolic alkalosis is rarely fully compensated. The exam-grade fact: most metabolic alkalosis is chloride-responsive. The fix is 0.9% saline with potassium, not bicarbonate and not anything clever. Watch for the tingling and spasm of a falling ionized calcium, and for dysrhythmias if potassium is low. Load the vomiting patient.
Respiratory acidosis — CO₂ retained
Anything that slows or shallows breathing: opioids and sedatives, an airway obstruction, COPD, chest wall injury, neuromuscular disease, a ventilator set too low. The distinguishing question is how fast. A fresh rise (opioid, obstruction) arrives uncompensated with a normal bicarbonate and a drowsy patient — open the airway, stimulate, bag, reverse. A chronic rise (COPD) arrives fully compensated with a high bicarbonate and a patient at their normal — keep oxygen at 88–92%, sit them up, and treat falling alertness as the alarm, not the CO₂. Opioid hypoventilation and COPD are both loaded in the simulator so you can hold the CO₂ still and compare.
Respiratory alkalosis — CO₂ blown off
Hyperventilation from anxiety or pain is the textbook cause, but the causes the exam wants you to rule out first are the dangerous ones: hypoxemia (the patient is breathing fast because they need oxygen), sepsis, pulmonary embolism, and early salicylate toxicity. The tingling and carpopedal spasm come from alkalosis binding calcium to albumin — ionized calcium falls while the total reads normal. Treat the driver. Coach slow breathing; never a paper bag, which can drop the oxygen in a patient who was hypoxemic all along. Load the hyperventilating patient.
What about a gas that fits two boxes?
Then it is two disorders. Salicylate overdose drives breathing (respiratory alkalosis) and adds acid (metabolic acidosis) at the same time; shock with a failing airway is a metabolic and a respiratory acidosis compounding each other. The size of compensation is how you catch them — mixed acid–base disorders and Winter’s formula walks through both.
Free lessons for each: metabolic acidosis, metabolic alkalosis, respiratory acidosis, respiratory alkalosis. The pH–potassium and pH–calcium couplings that run underneath all four are in the six electrolyte relationships.
Practise all four the way the exam asks
Free account, no card. Which disorder, what caused it, and what the nurse does first — per-option rationales on every question.