Section 15: Renal

Acid-Base Disorders

INTRODUCTION

Acid-base disturbances occur frequently in the acutely ill hospitalized patient. Many physicians are intimidated by the complexity of acid-base analysis, as multiple, partially offsetting disorders can be challenging to diagnose. However, in the hospital setting, especially the critically ill, it is crucial that patients with acid-base disturbances be quickly identified and the abnormality be accurately interpreted. Swift intervention to treat the underlying causes is often necessary to avoid the often lethal consequences of severe acid-base disturbances.

NORMAL ACID-BASE HOMEOSTASIS

A typical Western diet generates about 15,000 mmol of volatile acids, in the form of CO₂, and ~70 mmol (1 mmol/kg) of fixed acid each day. CO₂ is excreted by normal respiration. Fixed acids are buffered by intra- and extracellular buffers. New buffers, predominantly HCO₃^–, must be continuously produced to replace buffers consumed in titrating fixed acids. Complex acid-base homeostasis mechanisms, which include chemical buffering in conjunction with the excretion of CO₂ by the respiratory system and new HCO₃^– production by the kidneys, normally maintain the blood pH between 7.35 and 7.45.

Acid-base homeostasis requires complementary functions of the respiratory and the renal systems. The kidneys play a central role by reabsorbing filtered bicarbonate, approximately 4000 mmol/d, and generating new bicarbonate. The central nervous system and the respiratory systems control the arterial CO₂ tension (PaCO₂). Respiratory CO₂ elimination is determined by alveolar ventilation. Kidneys generate new bicarbonate through the process of net acid excretion. Under normal conditions, net acid excretion balances fixed acid production. A disturbance of either respiratory CO₂ elimination or the balance between fixed acid generation and renal net acid excretion results in an acid-base disorder.

SIMPLE ACID-BASE DISORDERS

These disorders involve only a single acid-base disorder, and include metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. A key distinguishing feature is that the pH is always abnormal, as the compensation is never complete. Simple acid-base disorders can be ...