Fluids, Electrolytes, Acid-Base Disorders, and Nutrition Support
Primary Disorder
Pco2 (mm Hg)
Anticipated Serum Bicarbonate (mEq/L)
Respiratory acidosis
Acutea
> 42
HCO3 should increase by 1 mEq/L for every
10 mm Hg increase in Pco2 above 40 mm Hg
Chronica
HCO3 should increase by 4–5 mEq/L for every
10 mm Hg increase in Pco2 above 40 mm Hg
Respiratory alkalosis
Acutea
< 38
HCO3 should decrease by 2 mEq/L for every
10 mm Hg decrease in Pco2 below 40 mm Hg
Chronica
HCO3 should decrease by 4–5 mEq/L for every
10 mm Hg decrease in Pco2 below 40 mm Hg
aCompensation is different for acute versus chronic respiratory disorders because it takes about 2 days for the kidneys to adapt to a persistent change in respiratory status.
Information from: Berend K, de Vries APJ, Gans ROB. Physiological approach of acid-base disturbances. N Engl J Med 2014;371:1434-45.
Common causes include pulmonary edema, pulmonary embolism, pneumonia, CNS depression,
cardiac arrest, stroke, spinal cord injury, excessive sedation/analgesia, and overfeeding with PN/EN.
Common causes include uncontrolled pain, nicotine and drug withdrawal, agitation, pneumonia,
stimulant drugs, salicylate toxicity (due to direct respiratory stimulation), and head injury.
Use of the serum anion gap (AG)
Used to determine the etiology for the metabolic acidosis. AG is the difference between major
cations and anions in blood (trying to detect whether there is an abundance of unmeasured anions).
If an AG is present, then a metabolic acidosis is present, regardless of pH or HCO3.
AG = Na – (Cl + HCO3)
Normal range is about 3–14 mEq/L. Some clinicians will include serum potassium when calculating
cations (and the normal AG will need to be adjusted), but this is uncommon.
concentration (grams per deciliter) from normal should be multiplied by 2–2.5 and added to the
anions (chloride and bicarbonate).
Albumin adjusted AG = Na - Cl - HCO3 - (2.5 x [4 - serum albumin]).
phorus (milligrams per deciliter) can be multiplied by 0.5 and added to anions, and lactate can
also be included but is not common in routine clinical practice. Using this method (and including
serum potassium), the adjusted AG (or sometimes called the strong ion gap when referring to the
physicochemical methodology for interpreting acid-base disorders) should be close to 0 (± 2) if the
patient does not have an AG acidosis.