Fluids, Electrolytes, Acid-Base Disorders, and Nutrition Support

Normal values

Variables

Arterial Blood

Mixed Venous Blood

pH

7.35–7.45

7.31–7.41

Pco2

35–45

41–51

Po2

80–100

35–40

HCO3

22–26

22–26

Base excess

-2 to +2

-2 to +2

O2 saturation

> 95%

70%–75%

HCO3 = bicarbonate.

Acidemia (pH less than 7.35) versus alkalemia (pH greater than 7.45)

Acidemia and alkalemia refer to serum pH being either low or high, respectively. Acidosis and

alkalosis refer to the metabolic or respiratory processes that led to the abnormal pH. Although the

terms emia and osis are similar, they are different.

For simple acid-base disorders, identify pH, Pco2, and HCO3 in that order. Whichever side of 7.40

the pH is on, the respiratory or metabolic processes that coincide with that pH abnormality are the

primary etiology. If the pH is less than 7.40, an elevated Pco2 (respiratory acidosis) or a decreased

HCO3 (metabolic acidosis) is the primary etiology. If the pH is greater than 7.40, a decreased Pco2

(respiratory alkalosis) or an increased HCO3 (metabolic alkalosis) is the primary etiology. An easy

introductory overview to acid-base disorders by Haber is provided in the references (West J Med

1991;155:146-51).

process (metabolic or respiratory) is inadequate and may be contributing to the acid-base disorder.

As a result, various formulas have been developed to predict what may be considered adequate

compensation. However, these mathematical equations have limitations in their clinical utility

and accuracy (J Trauma Acute Care Surg 2012;73:27-32; Clin J Am Soc Nephrol 2007;2:162-74;

memorize (West J Med 1991;155:146-51).

Mixed acid-base disorders are more nuanced to interpret in patients with non–steady-state

conditions and in those receiving mechanical ventilation, which provides exogenous respiratory

compensation. Some of the more common equations for assessing acid-base disorders are discussed

later in this chapter.