Chart — ABG & Acid-Base
Acid-Base Disorder Causes
The four primary acid-base disorders and what causes each — respiratory and metabolic acidosis and alkalosis — with the high- versus normal-anion-gap split for metabolic acidosis and the RT-relevant triggers.
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
There are four primary acid-base disorders, sorted by whether the driving change is in the respiratory system (PaCO₂) or the metabolic system (HCO₃⁻), and whether the result pushes the blood toward acidemia or alkalemia. A respiratory disorder is one the lungs cause — too little or too much CO₂ blown off — while a metabolic disorder reflects a change in bicarbonate from the kidneys, the gut, or added acid or base. Reading an ABG well means naming the primary disorder, checking whether the expected compensation has occurred, and, for any metabolic acidosis, splitting the differential by the anion gap. The table below pairs each disorder with its primary change, its common causes, and the triggers a respiratory therapist sees most often at the bedside and on the ventilator.
The Four Primary Disorders
| Disorder | Primary change | Common causes | RT-relevant triggers |
|---|---|---|---|
| Respiratory acidosis | PaCO₂ high (hypoventilation) | COPD, severe asthma, opioid/sedative overdose, neuromuscular disease, CNS depression, chest-wall or obesity restriction, respiratory muscle fatigue | Failure to wean, oversedation, splinting, exhaustion — support ventilation |
| Respiratory alkalosis | PaCO₂ low (hyperventilation) | Anxiety or pain, hypoxemia, sepsis, pulmonary embolism, early salicylate toxicity, pregnancy, fever, CNS disease, excessive mechanical ventilation | Over-ventilation on the ventilator — reduce the set minute ventilation; treat the hypoxemia or underlying cause |
| Metabolic acidosis — HIGH anion gap | HCO₃⁻ low, anion gap > 12 | Lactic acidosis, diabetic and other ketoacidosis, uremia, toxic alcohols, salicylates (MUDPILES) | Shock and hypoperfusion, sepsis, DKA — expect Kussmaul (compensatory) hyperventilation |
| Metabolic acidosis — NORMAL anion gap | HCO₃⁻ low, anion gap 8-12, chloride high | Diarrhea, renal tubular acidosis, large-volume normal saline, acetazolamide (HARDASS) | GI bicarbonate losses, TPN, resuscitation fluids |
| Metabolic alkalosis | HCO₃⁻ high | Vomiting or nasogastric suction, diuretics, hypokalemia, volume contraction, excess alkali, hyperaldosteronism | NG suction and diuretics — the alkalosis blunts respiratory drive; correct potassium and volume |
Clinical Notes
In any metabolic acidosis, always split the differential by the anion gap. A high anion gap means unmeasured acid has been added (the MUDPILES causes), while a normal-gap acidosis reflects a loss of bicarbonate with a compensatory rise in chloride (the HARDASS causes). See the lab-diagnostics Anion Gap chart for the full MUDPILES versus HARDASS detail and the urine anion gap that further separates renal from GI bicarbonate losses.
A single primary disorder should drive compensation in a predictable direction and magnitude. When the compensation falls outside its expected window — too much or too little — a mixed disorder is present, and more than one process is acting on the blood at once.
Metabolic alkalosis is worth a second look at the bedside because it blunts respiratory drive: the body hypoventilates to retain CO₂ as compensation, which can slow weaning and confound a ventilator trial. Correct the potassium and volume contraction that drive the alkalosis rather than treating the gas alone.
Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Acid-base balance.
- Berend K, de Vries APJ, Gans ROB. Physiological approach to assessment of acid-base disturbances. N Engl J Med. 2014;371(15):1434-1445.
- National Board for Respiratory Care. Therapist Multiple-Choice Examination Detailed Content Outline. NBRC; 2024. Patient data evaluation and recommendations.