Mixed Acid–Base Disorders

Introduction

Failure of compensation is responsible for mixed acid–base disorders such as respiratory acidosis and metabolic acidosis, or respiratory alkalosis and metabolic alkalosis. In contrast, excess compensation is responsible for metabolic acidosis and respiratory alkalosis, or metabolic alkalosis and respiratory acidosis.

• Respiratory and metabolic acidosis can develop in patients with cardiorespiratory arrest, with chronic lung disease and shock, and with metabolic acidosis and respiratory failure.
• The most common mixed acid–base disorder is respiratory and metabolic alkalosis, which occurs in critically ill surgical patients with respiratory alkalosis caused by mechanical ventilation, hypoxia, sepsis, hypotension, neurologic damage, pain, or drugs; and with metabolic alkalosis caused by vomiting or nasogastric suctioning and massive blood transfusions.
• Mixed metabolic acidosis and respiratory alkalosis occur in patients with advanced liver disease, salicylate intoxication, and pulmonary-renal syndromes.
• Metabolic alkalosis and respiratory acidosis can occur in patients with COPD and respiratory acidosis who are treated with salt restriction, diuretics, and possibly glucocorticoids.

Treatment

Treat mixed respiratory and metabolic acidosis by initiating oxygen delivery to improve hypercarbia and hypoxia. Mechanical ventilation can be needed to reduce Paco2 . During initial therapy, give appropriate amounts of alkali to reverse the metabolic acidosis.

• Correct the metabolic component of mixed respiratory and metabolic alkalosis by administering sodium and potassium chloride solutions. Readjust the ventilator or treat the underlying disorder causing hyperventilation to treat the respiratory component.
• Treatment of mixed metabolic acidosis and respiratory alkalosis should be directed at the underlying cause.
• In metabolic alkalosis and respiratory acidosis, pH does not usually deviate significantly from normal, but treatment can be required to maintain Pao2 and Paco2 at acceptable levels. Aim treatment at decreasing plasma bicarbonate with sodium and potassium chloride therapy, allowing renal excretion of retained bicarbonate from diuretic-induced metabolic alkalosis.