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Acid-base disorders are disturbances in the body's pH homeostasis and are encountered across virtually every clinical setting from the ICU to the outpatient clinic. A systematic approach to interpretation is essential for accurate diagnosis and appropriate management.
The body maintains arterial pH between 7.35 and 7.45 through three integrated buffer systems: bicarbonate (primary extracellular), phosphate (intracellular and renal), and protein (including hemoglobin). Respiratory compensation acts within minutes; renal compensation takes hours to days. Primary disorders include metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis, each with characteristic compensatory responses.
Clinical reasoning begins with arterial blood gas interpretation: assess pH, determine primary disorder, evaluate compensation using Winter's formula or expected rules, and calculate anion gap for metabolic acidoses. Elevated anion gap acidosis (MUDPILES mnemonic) requires different evaluation than normal anion gap (hyperchloremic) acidosis. Mixed disorders are common in critically ill patients.