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.
19-1 pH and Hydrogen Ion Concentrations |Favorite Table|Download (.pdf)
19-1 pH and Hydrogen Ion Concentrations
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.
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
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,