A 70-year-old female presents with fever, tachypnea, and AMS. Her ABG
shows pH =7.44, PO 2 =90, PCO 2 =20, HCO 3 −=12. The chemistry panel
shows an increased anion gap. What acid-base disorders are present?
Primary anion gap metabolic acidosis and primary respiratory alkalosis,
consider salicylate toxicity.ACID-BASE DISORDERSInterpreting Blood Gases
BASICDEFINITIONS
■ Acidosis/alkalosis: A process →acidemia/alkalemia
■ Acidemia: pH <7.40
■ Alkalemia: pH >7.40
■ Metabolic disorder: Change in HCO 3 −
■ Respiratory disorder: Change in PCO 2
■ Respiratory or renal compensation: Other system alterations that bring the
blood gas toward a normal pH of 7.40
DIAGNOSIS
Use the history and exam for clues to the disorder.
■ Respiratory status (increased RR suggests respiratory alkalosis; decreased
RR or respiratory failure suggests respiratory acidosis)
■ Dehydration/vomiting =metabolic alkalosis; diarrhea =metabolic acidosis.
■ Past medical history, medication, and toxin exposures
Now look at the blood gas and chemistry.
■ Look at the pH to determine if there is an acidemia or alkalemia.
■ Check the PCO 2 to determine if there is a respiratory acidosis or alkalosis.
■ Examine the HCO 3 −to determine if there is a metabolic acidosis or alkalosis.
■ Calculate the anion gap [Na+−(K++Cl−)]. If >−20, there is an underlying
metabolic gap acidosis.
Classify the disturbance.
■ Respiratory alkalosis (PCO 2 <40) caused by CHF, hypoxia, ↑ICP, toxic
salicylates, and sympathomimetics
■ Respiratory acidosis (PCO 2 >40) caused by respiratory failure, sedatives,
and opiates, common and often chronic in patients with COPD, consid-
ered a sign of respiratory failure in asthmatics
■ Metabolic alkalosis (↑HCO 3 −) caused by hypovolemia, hyperaldostero-
nism and Bartter syndrome.
■ Metabolic acidosis (↓HCO 3 −) causes are subdivided by the presence of an
anion gap.
■ Low gap (anion gap <1) states caused by:
■ Increases in unmeasured serum cations (seen in myeloma)
■ Decreased serum albumin
■ Bromide or iodine poisoning (which are mistaken for chloride in
many labs)
■ Normal anion gap acidosis (variously reported as 3–11, 8–16, or 1–20
mmol/L depending on analyzer) caused by:
■ Diarrhea
■ Renal tubular acidosis
ENDOCRINE, METABOLIC, FLUID, ANDELECTROLYTE DISORDERS