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Critical Care - Acid–Base Disturbances - Fast Facts | NEJM Resident 360

Acid–base disturbances are common in the critically ill. The NEJM review Physiological Approach to Assessment of Acid–Base Disturbances provides the following algorithms to guide the workup of alkalemia and acidemia and to assess for compensatory responses and etiology:

Assessment of Acidosis

Reference values for the alveolar–arterial (A–a) oxygen tension difference are less than 10 mm Hg in young persons and less than 20 mm Hg in the elderly. ΔAG denotes delta anion gap, PaCO2 partial pressure of arterial carbon dioxide (mm Hg), PaO2 partial pressure of arterial oxygen (mm Hg), and RTA renal tubular acidosis. To convert the values for PaCO2, PaO2, and the alveolar–arterial difference to kilopascals, multiply by 0.1333.
(Source: Physiological Approach to Assessment of Acid–Base Disturbances. N Engl J Med 2014.)

Assessment of Alkalosis

Lactic acidosis****from septic shock is the most common acid–base disturbance in the medical ICU. However, if a patient is acidemic, remember to consider other etiologies, including alternative causes of lactic acidosis and acidosis from diabetic ketoacidosis (DKA), toxic ingestions, and other causes.

The use of IV sodium bicarbonate as a buffer in lactic acidosis is controversial. In the 2021 Surviving Sepsis Campaign guidelines, a weak recommendation was made against the use of sodium bicarbonate overall to improve hemodynamics or reduce vasopressor requirements. However, its use is suggested in a subset of patients with septic shock, severe metabolic acidemia (pH ≤7.2), and AKI (with AKIN score of 2 or 3) as per the results of the BICAR-ICU trial.

Electrolyte Disturbances

Electrolyte disturbances, including hypokalemia; hyperkalemia; hyponatremia; hypernatremia; and hypomagnesemia, are common in the ICU and can occur as a result of both the underlying condition and the use of resuscitation fluids.

• Electrolyte levels should be corrected to normal ranges in the critically ill to prevent arrhythmia, altered mental status, and other complications.

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