Chapter 19

This chapter describes a practical approach to the clinical evaluation of acid-base disorders.

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 19-1). 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 very rapid bedside interpretation of blood gas and electrolyte results.

Table 19-1 pH and Hydrogen Ion Concentrations

### Plasma Acid Homeostasis

Plasma [H+] is influenced by the rate of endogenous production, the rate of excretion, and the buffering capacity of the body. Buffers mitigate the impact of large changes in available hydrogen ion on plasma pH.

Buffer systems that are effective at physiologic pH include hemoglobin, phosphate, proteins, and bicarbonate (Figure 19-1).1 One can consider the [H+] to be the result of all physiologic buffers acting on the common pool of hydrogen ions.

###### Figure 19-1.

Schematic representation of hydrogen ion homeostasis.

The familiar Henderson-Hasselbalch equation, shown in this equation,

specifies the interrelationship between carbonic acid, bicarbonate, and pH; any two of these determine the third.

The clinical utility of the Henderson-Hasselbalch equation is limited. However, if all constants are inserted into the Henderson-Hasselbalch equation and the antilog of all its terms is taken, the resulting Kassirer-Bleich equation (Formula 19-1) is of great clinical utility.2

#### FORMULA 19-1

Kassirer-Bleich equation to calculate Pco2, [HCO3], or [H+].

The Kassirer-Bleich equation may be used to calculate the concentration of any component of the bicarbonate buffer system provided the concentrations of the other two components are known. It therefore allows clinicians to determine, for example, what the pH must be when the Pco2 and [HCO3] are known.** Performing such a calculation with values reported by the laboratory permits the clinician to check the internal consistency of the reported data. (In fact, in most hospital laboratories, the [HCO3] reported as part of a blood gas analysis is not measured, but rather calculated from pH and Pco2 measurements.) Note that, when [HCO3...

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