Controversy has existed concerning acid-base physiology and the ideal method to assess acid-base disorders for 130 years.1 The two most common methods advocated to analyze acid-base disorders are the traditional bicarbonate-centered method2,3 and the Stewart, or strong ion, method.4,5 The traditional approach teaches that acid-base homeostasis is maintained by respiratory control of the partial pressure of carbon dioxide (Pco2) through changes in alveolar ventilation and control of HCO3– reabsorption and H+ excretion by the kidneys. Peter Stewart proposed that acid-base physiology involves the dynamic interaction of body fluids and multiple chemical species including strong ions (primarily Na+, K+, Ca2+, Mg2+, and Cl–) and weak acids, as well as Pco2 control by the lungs.
Each of these methods has limitations. The traditional bicarbonate-centered model continues to be the most commonly used at the bedside3 but is criticized for failing to identify acid-base abnormalities that are due to alterations in plasma free water or in complex cases of mixed acid-base disorders.6,7 The Stewart method is praised for its accuracy in identifying acid-base disorders but is criticized for the difficulty of application at the bedside.7,8,9,10,11 This chapter does not detail the Stewart method, but we acknowledge its importance and its contribution to underscoring the limitations of the traditional method, which has led to modifications that improve the performance of the traditional method at the bedside.8 For example, using a correction factor for the albumin level (detailed in this chapter), the traditional method performs as well as the Stewart method for identifying complex acid-base abnormalities in critically ill patients.8,9,10,11,12
Many diseases, including those that present an imminent threat to life, produce acid and base (acid-base) disturbances that provide important clues concerning the nature of the underlying illness and suggest immediate therapeutic interventions. Further, ED treatments such as rapid resuscitation of critically ill patients may create unintended acid-base disorders. This chapter describes a practical approach to the clinical evaluation and treatment of acid-base disorders.
MEASUREMENT OF PLASMA ACIDITY
Plasma hydrogen ion concentration ([H+])* is normally 40 nmol/L, corresponding to a pH of 7.4. Because pH is a logarithmic transformation of [H+], the relation of [H+] to pH is not linear for all pH values (Table 15–1). However, for pH values from 7.20 to 7.50, the relation between [H+] and pH is nearly linear; pH changes of 0.01 correspond to approximately 1 nmol/L change in [H+]. This linear relation allows for rapid bedside interpretation of blood gas and electrolyte results.