What are the causes of respiratory alkalosis?

Causes of Respiratory Alkalosis

Respiratory alkalosis results from hyperventilation that eliminates CO2 faster than metabolic production, and the causes can be systematically categorized into psychiatric/behavioral, pulmonary, central nervous system, iatrogenic, and compensatory mechanisms. 1

Psychiatric and Behavioral Causes

• Anxiety reactions, panic disorders, and hyperventilation syndrome are among the most common causes encountered in emergency settings, characterized by abnormal breathing patterns with impressive hyperventilation, increased respiratory frequency, and decreased PaCO2. 1
• These conditions produce true hyperventilation with respiratory alkalosis where increased respiratory frequency eliminates CO2 below the normal range. 2
• Hyperventilation syndrome is a diagnosis of exclusion after ruling out organic causes, and patients may present with a broad spectrum of somatic symptoms stemming from low PaCO2 and increased sympathetic adrenergic tone. 3, 4
• Panic disorder commonly presents with clustering of suffocating, smothering, and air hunger sensations, and may occur even in the absence of reduced ventilatory capacity due to excessive ventilatory drive or increased sensitivity to CO2. 5

Pulmonary Causes

• Hypoxemia stimulates peripheral chemoreceptors leading to increased ventilation, and pulmonary disorders that trigger this response include pneumonia, pulmonary embolism, and pulmonary edema. 1
• In acute pulmonary edema, patients hyperventilate to compensate for hypoxemia, leading to respiratory alkalosis despite significant respiratory distress and increased work of breathing. 6
• Pulmonary hypertension causes respiratory alkalosis as a compensatory mechanism to reduce pulmonary vascular resistance through hyperventilation that reduces PaCO2 and increases pH. 6
• High altitude-induced hypobaric hypoxia leads to increased respiratory rate and tidal volume, promoting respiratory alkalosis while simultaneously causing pulmonary vasoconstriction. 6

Central Nervous System Causes

• Stimulation of the respiratory center in the medulla can cause hyperventilation, with common CNS causes including head injury, cerebrovascular accidents, and central nervous system infections. 1
• Direct stimulation of respiratory centers in the medulla drives hyperventilation as seen in various neurological conditions. 3

Iatrogenic Causes

• Mechanical ventilation with excessive settings and inappropriate ventilator management in critical care settings can cause respiratory alkalosis. 1
• Gas leaks in ventilator systems causing loss of positive end-expiratory pressure and autocycling can deliver large ventilator breaths in patients with low respiratory impedance, resulting in severe hyperventilation and respiratory alkalosis. 7
• Excessive oxygen therapy can disrupt compensatory mechanisms in patients with chronic respiratory conditions. 1

Compensatory Mechanisms

• Physiological hyperventilation occurs as a compensatory response to metabolic acidosis to maintain pH near normal, as recognized by the American Thoracic Society. 1
• In heart failure with Cheyne-Stokes breathing, respiratory alkalosis may be a compensatory mechanism that protects the failing heart from decompensation and should not be aggressively suppressed. 1, 6

Drug-Related Causes

• Respiratory dyskinesia from antipsychotic medications can lead to respiratory alkalosis, often undiagnosed, and includes orofacial dyskinesia, dysphonia, and dyspnea. 5
• Salicylate toxicity causes respiratory alkalosis through direct stimulation of respiratory centers in the medulla, though this can progress to mixed acid-base disorders in severe cases. 8

Critical Management Pitfalls

• Avoid disrupting compensatory respiratory alkalosis in patients with metabolic disorders; if assisted ventilation is necessary, target appropriate oxygen saturation (88-92% in COPD patients, 94-98% in most others) to avoid worsening the underlying condition. 1, 2
• In mechanical ventilation, failure to match the patient's compensatory hyperventilation can lead to severe acidemia, as the mechanical ventilator cannot replicate the extreme minute ventilation achieved by endogenous respiratory drive in conditions like salicylate toxicity. 8
• Distinguish true hyperventilation (anxiety, panic) from rapid shallow breathing in COPD exacerbations, which appears like hyperventilation but actually represents ineffective ventilation leading to CO2 retention. 2