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KEY POINTS
Recognition of acid-base disturbances through interpretation of arterial blood gases is of fundamental importance to the daily clinical practice of critical care.
The carbonic acid-bicarbonate buffer is the most important buffer system. The relation of pH to this buffering system is defined by the Henderson–Hasselbalch equation.
There are four cardinal acid-base disorders: metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis.
The etiologies of metabolic acidosis can be classified by the typical serum anion gap association (elevated or normal anion gap acidosis).
The serum osmole gap is often used as a screening test when methanol or ethylene glycol intoxication is suspected.
Gastric fluid loss, diuretic use, and extracellular volume contraction are among the most common causes of metabolic alkalosis.
The etiologies of metabolic alkalosis can be classified by the expected urine chloride concentration or excretion (normal, high, or low urine chloride).
Common causes of respiratory acidosis are pulmonary disorders (eg, chronic obstructive lung disease, severe pneumonia, aspiration pneumonitis, and smoke inhalation), neurologic injury, neuromuscular and metabolic disorders, and narcotic and sedative agents.
Respiratory alkalosis is common with severe sepsis, hepatic failure, mechanical ventilation, and with drugs such as salicylates and illicit stimulants (eg, cocaine and amphetamine).
Mixed acid-base disorders are not uncommon in critically ill patients and frequently complicate interpretation of arterial blood gases in the intensive care unit setting.
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Recognition of acid-base disturbances through interpretation of arterial blood gases is of fundamental importance to the day-to-day clinical practice of critical care. Knowledge of the underlying basic chemical relationships and established nomenclature is prerequisite to this understanding. Several different paradigms of acid-base relationships have been proposed and are in current clinical use for framing the results of blood gas and related laboratory measurements. These paradigms include the base excess method, the Stewart or physicochemical method, and the bicarbonate-pH-Pco2 method, and they all basically arrive at common endpoints of interpretation.1,2 The bicarbonate-pH-Pco2 method, also known as the physiological method, will be described herein. It is based on the carbonic acid-bicarbonate buffer system, is well established, has an empiric basis, and is in wide clinical use. Starting with a review of acid-base terminology and basic chemical relationships, this chapter will describe the basis of the physiological method for arriving at a diagnosis of simple and mixed acid-base disorders, and show how readily available laboratory testing, in conjunction with clinical information from the history and physical examination, can narrow the differential diagnosis with the goal of pin-pointing the underlying causative process or disease.
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CHEMICAL RELATIONSHIPS
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One definition for an acid is a chemical compound capable of donating a hydrogen ion (ie, a proton). The degree of acidity of a solution can thus be quantified as the hydrogen ion activity, which is closely related to the molar concentration of hydrogen ions. Normal hydrogen ion concentration of plasma averages about 40 nmol/L, ...