Arterial blood gas (ABG) measurements are a crucial diagnostic test in medicine, providing vital insights for diagnosing acid-base disorders and guiding life-saving treatments in critically ill patients. Acid-base disorders frequently manifest in critically ill patients and can also serve as initial indicators of underlying diseases, such as Kussmaul’s breathing presenting in diabetic ketoacidosis or renal failure.

To clearly understand the subject and its approach, this chapter outlines basic terminology, the physiology of pH regulation, compensation in various acid-base disorders, and the diagnostic approach.

Basic terminology used to discuss acid-base disorders and their values in simple acid-base disorders are summarized below (Table 30.1).

pH: pH represents the concentration of free hydrogen ions (H+). It has an inverse relationship with H+ ion concentration. The normal value of pH in arterial blood is 7.4 (7.35–7.45).

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