Respiratory Acidosis and Alkalosis

Respiratory acid-base disorders are primary disturbances of carbon dioxide tension caused by changes in alveolar ventilation. Respiratory acidosis results from carbon dioxide retention (hypoventilation) and respiratory alkalosis from excessive carbon dioxide removal (hyperventilation); each is compensated by a slower renal adjustment of bicarbonate.

Definition

Respiratory acidosis is a primary rise in the partial pressure of carbon dioxide that lowers pH, and respiratory alkalosis is a primary fall in the partial pressure of carbon dioxide that raises pH; both reflect a mismatch between carbon dioxide production and alveolar ventilation and are compensated by renal changes in bicarbonate.

Scope

The topic covers the definition and causes of respiratory acidosis and alkalosis, the relationship between alveolar ventilation and carbon dioxide tension, the distinction between acute and chronic forms, and the renal (metabolic) compensation that distinguishes them. It is presented as physiology and diagnostic reasoning, not as treatment guidance.

Core questions

• How does alveolar ventilation set the arterial carbon dioxide tension?
• What distinguishes acute from chronic respiratory acid-base disorders?
• How does renal compensation differ in magnitude between acute and chronic states?
• How are mixed respiratory and metabolic disorders detected?

Key concepts

• Alveolar ventilation
• Partial pressure of carbon dioxide
• Hypoventilation and hypercapnia
• Hyperventilation and hypocapnia
• Acute versus chronic disorder
• Renal (metabolic) compensation
• Mixed acid-base disorders

Key theories

Acute versus chronic compensation framework

Distinguishes the small, immediate buffering response to an acute change in carbon dioxide tension from the larger renal adjustment of bicarbonate that develops over days in chronic disorders, with different expected pH and bicarbonate values for each.

Mechanisms

Arterial carbon dioxide tension is inversely proportional to alveolar ventilation for a given carbon dioxide production, so any process that reduces ventilation, central depression, neuromuscular weakness, or airway and lung disease, raises carbon dioxide and produces respiratory acidosis, while excessive ventilation lowers carbon dioxide and produces respiratory alkalosis. Acutely, only tissue and blood buffers respond, so bicarbonate changes little and pH shifts substantially. Over hours to days the kidney adjusts acid excretion and bicarbonate reabsorption, producing a larger compensatory change in bicarbonate that returns pH toward, but not fully to, normal; the expected bicarbonate change is greater in chronic than in acute disorders. Comparing the measured bicarbonate with the value expected for acute versus chronic compensation distinguishes the two and reveals any superimposed metabolic disorder.

Clinical relevance

Respiratory acid-base disorders are central to the assessment of patients with respiratory failure, sedation, and chronic lung disease, and distinguishing acute from chronic forms is part of interpreting blood-gas results. This entry describes the underlying physiology and diagnostic reasoning and is not a source of dosing or individualised management advice.

Evidence & guidelines

The relationship between ventilation and carbon dioxide and the acute-versus-chronic compensation rules are established physiology described consistently across reviews (Berend and colleagues, 2014; Adrogué and Madias, 1998; Hamm and colleagues, 2015). The quantitative compensation expectations are descriptive and not clinical protocols.

History

Mid-twentieth-century blood-gas measurement made it possible to characterise carbon dioxide-driven acid-base disorders directly, and subsequent work established the predictable, time-dependent renal compensation that separates acute from chronic respiratory acidosis and alkalosis.

Key figures

• Horacio J. Adrogué
• Nicolaos E. Madias
• L. Lee Hamm

Seminal works

• adrogue-madias-1998b
• berend-2014

Frequently asked questions

What is the difference between respiratory acidosis and respiratory alkalosis?

Respiratory acidosis is a primary rise in carbon dioxide tension from inadequate ventilation, lowering pH, while respiratory alkalosis is a primary fall in carbon dioxide tension from excessive ventilation, raising pH.

Why does it matter whether a respiratory disorder is acute or chronic?

Because renal compensation takes days to develop, a chronic disorder shows a much larger compensatory change in bicarbonate than an acute one; comparing the measured bicarbonate with the expected value distinguishes them and helps detect mixed disorders.