Disorders of Acid Base Balance

Horacio J. Adrogué Nicolaos E. Madias

Maintenance of acid-base homeostasis is a vital function of the living organism. Deviations of systemic acidity in either direction can impose adverse consequences and when severe can threaten life itself. Acid-base disorders frequently are encountered in the outpatient and especially in the inpatient setting. Effective management of acid-base disturbances, commonly a challenging task, rests with accurate diagnosis, sound understanding of the underlying pathophysiology and impact on organ function, and familiarity with treatment and attendant complications [1].

Clinical acid-base disorders are conventionally defined from the vantage point of their impact on the carbonic acid-bicarbonate buffer system. This approach is justified by the abundance of this buffer pair in body fluids; its physiologic preeminence; and the validity of the iso-hydric principle in the living organism, which specifies that all the other buffer systems are in equilibrium with the carbonic acid-bicarbonate buffer pair. Thus, as indicated by the Henderson equation, [H+] = 24 X PaCO2/[HCO3] (the equilibrium relationship of the carbonic acid-bicarbonate system), the hydrogen ion concentration of blood ([H+], expressed in nEq/L) at any moment is a function of the prevailing ratio of the arterial carbon dioxide tension (PaCO2, expressed in mm Hg) and the plasma bicarbonate concentration ([HCO3], expressed in mEq/L). As a corollary, changes in systemic acidity can occur only through changes in the values of its two determinants, PaCO2 and the plasma bicarbonate concentration. Those acid-base disorders initiated by a change in PaCO2 are referred to as respiratory disorders; those initiated by a change in plasma bicarbonate concentration are known as metabolic disorders. There are four cardinal acid-base disturbances: respiratory acidosis, respiratory alka-losis, metabolic acidosis, and metabolic alkalosis. Each can be encountered alone, as a simple disorder, or can be a part of a mixed-disorder, defined as the simultaneous presence of two or more simple acid-base disturbances. Mixed acid-base disorders are frequently observed in hospitalized patients, especially in the critically ill.

The clinical aspects of the four cardinal acid-base disorders are depicted. For each disorder the following are illustrated: the underlying pathophysiology, secondary adjustments in acid-base equilibrium in response to the initiating disturbance, clinical manifestations, causes, and therapeutic principles.

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