ACID-BASE INTERPRETATION

Disorders of acid-base balance can be found in as many as nine of every 10 patients in the ICU, which means that acid-base disoders may be the most common clinical problems you will encounter in the ICU. This chapter present a structured approach to the identification of acid-base disoders based on a set of well-defined rules that can be applied to arterial blood gas (ABG) and serum electrolyte measurement.

BASIC CONCEPTS
The hydrogen ion concentration [H+] in extracellular fliud is determined by the balance between the partial pressure of carbon dioxide (PCO2) an the conceration of bicarbonate (HCO3) in the fliud. This relationship is expressed as follows.

[H+] (nEq/L) = 24 x (PCO2/HCO3)

Using a normal arterial PCO2 of 40 mm Hg and a normal serum HCO3 concentration of 24 mEq/L, the normal [H+] in arterial blood is 24 x (40/24) = 40 nEq/L


Hydrogen Ion concentration and pH

Note that the [H+] in extracellular fliud is expressed in nanoequivalents (nEq) per liter. A nanoequivalent is one-millionth of a miliequivalent, so there are millions more sodium, chloride, and oder ions measured in mEq than there are hydrogen ions. because nanoequivalents represent such as small amount, the [H+] is routinely expressed in pH units, which are derived by taking the negative logarithm of the [H+] in nEq/L.