Acids-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, 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 (H2 CO3 /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, 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. Low pH 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.