Chapter 72: Acid–Base Disorders

INTRODUCTION

• Acid–base disorders are caused by disturbances in hydrogen ion (H⁺) homeostasis, which is ordinarily maintained by extracellular buffering, renal regulation of hydrogen ion and bicarbonate excretion, and ventilatory regulation of carbon dioxide (CO₂) elimination.

GENERAL PRINCIPLES

• Buffering refers to the ability of a solution to resist change in pH after the addition of a strong acid or base. The body’s principal extracellular buffer system is the carbonic acid/bicarbonate (H₂CO₃/HCO₃⁻) system.

• Most of the body’s acid production is in the form of CO₂ and is produced from catabolism of carbohydrates, proteins, and lipids.

• There are four primary types of acid–base disturbances, which can occur independently or together as a compensatory response.

• Metabolic acid–base disorders are caused by changes in plasma bicarbonate concentration (HCO₃⁻). Metabolic acidosis is characterized by decreased HCO₃⁻, and metabolic alkalosis is characterized by increased HCO₃⁻.

• Respiratory acid–base disorders are caused by altered alveolar ventilation, producing changes in arterial carbon dioxide tension (Paco₂). Respiratory acidosis is characterized by increased Paco₂, whereas respiratory alkalosis is characterized by decreased Paco₂.

DIAGNOSIS

• Blood gases (Table 72–1), serum electrolytes, medical history, and clinical condition are the primary tools for determining the cause of acid–base disorders and for designing therapy.

• Arterial blood gases (ABGs) are measured to determine oxygenation and acid–base status (Fig. 72–1). Low pH values (<7.35) indicate acidemia, whereas high values (>7.45) indicate alkalemia. The Paco₂ value helps determine whether there is a primary respiratory abnormality, whereas the HCO₃⁻ concentration helps determine whether there is a primary metabolic abnormality. Steps in acid–base diagnosis and interpretation are described in Tables 72–2 and 72–3.