What are the common causes of respiratory alkalosis?

Causes of Respiratory Alkalosis

Respiratory alkalosis occurs when alveolar ventilation exceeds carbon dioxide production, resulting in PaCO₂ <35 mmHg and pH >7.45, and is most commonly caused by pulmonary disorders, anxiety-related hyperventilation, CNS lesions, and compensatory mechanisms in metabolic acidosis. 1, 2

Pulmonary and Hypoxic Causes

Pulmonary disorders are primary triggers that stimulate peripheral chemoreceptors and drive increased ventilation 1:

• Pneumonia, pulmonary embolism, and pulmonary edema directly stimulate chemoreceptors, causing hyperventilation as patients attempt to compensate for hypoxemia 1
• Pulmonary hypertension induces hyperventilation as a compensatory mechanism to reduce pulmonary vascular resistance and maintain cardiac output 1
• High altitude exposure triggers hypobaric hypoxia, leading to increased respiratory rate and tidal volume that promotes respiratory alkalosis 1

Psychogenic and Anxiety-Related Causes

Hyperventilation syndrome and anxiety disorders represent the most frequent causes in emergency department and outpatient settings 3:

• Panic disorder and anxiety reactions commonly present with clustering of suffocating, smothering, and air hunger sensations, occurring even without reduced ventilatory capacity due to excessive ventilatory drive or increased CO₂ sensitivity 1
• Characteristic breathing pattern shows abrupt "turned-on" rapid, shallow respiration disproportionate to metabolic stress, with irregular breathing, intermittent breath-holding, and sighing 3
• Cardiopulmonary exercise testing reveals markedly increased minute ventilation, elevated Ve/VCO₂ ratio, higher respiratory frequency, and pronounced reductions in end-tidal CO₂ and arterial PaCO₂ 3
• Some patients exhibit chronic respiratory alkalosis with a down-regulated PaCO₂ set point even at rest before any exercise challenge 3

Central Nervous System Causes

CNS lesions directly stimulate the respiratory center in the medulla, causing hyperventilation 1:

• Head injury, cerebrovascular accidents, and CNS infections all directly affect respiratory drive 1
• These conditions bypass normal feedback mechanisms and produce sustained hyperventilation 1

Compensatory Mechanisms

Metabolic acidosis compensation occurs when the respiratory system attempts to maintain pH near normal by hyperventilating to blow off CO₂ 1:

• This represents a physiological response that should not be disrupted, as correcting the respiratory alkalosis without addressing the underlying metabolic acidosis will worsen acidemia 1
• Heart failure with Cheyne-Stokes breathing creates respiratory alkalosis that may protect the failing heart from decompensation and should not be aggressively suppressed 1

Iatrogenic Causes

Mechanical ventilation with excessive settings and inappropriate ventilator management in critical care settings can cause respiratory alkalosis 1:

• Improperly configured breathing circuits or increased external dead space can paradoxically lead to respiratory alkalosis when compensatory hyperventilation occurs 3
• Respiratory dyskinesia from antipsychotic medications can lead to respiratory alkalosis, often undiagnosed, including orofacial dyskinesia, dysphonia, and dyspnea 1

Physiological States

Pregnancy produces mild respiratory alkalosis with increased ventilation beginning in the first trimester, reaching 20-40% above baseline by term, mediated by elevated serum progesterone levels, resulting in decreased carbon dioxide levels and plasma bicarbonate of 18-21 mEq/L 1

Critical Diagnostic Pitfalls

Do not disrupt compensatory respiratory alkalosis in patients with underlying metabolic acidosis, and target appropriate oxygen saturation (88-92% in most cases) to avoid worsening the condition 1:

• Avoid excessive oxygen therapy that could disrupt compensatory mechanisms in patients with chronic respiratory alkalosis 1
• Arterial blood gas showing pH >7.45 and PaCO₂ <34 mmHg with HCO₃⁻ below normal but not fully normalized indicates partial renal compensation 1
• Failure to recognize hyperventilation syndrome and pursuing extensive work-up for organic disease delays appropriate therapy and heightens patient anxiety 3, 4

Cardiovascular Manifestations

Respiratory alkalosis produces multiple cardiac effects 5:

• Hyperventilation can produce electrocardiographic changes that mimic myocardial ischemia in individuals with normal coronary arteries 3
• May precipitate tachycardia, ventricular and atrial arrhythmias, and both ischemic and non-ischemic chest pain 3, 5

Metabolic Consequences

Respiratory alkalosis produces multiple metabolic abnormalities including changes in potassium (typically causing hyperkalemia acutely via alpha-adrenergic stimulation), phosphate, and calcium, as well as development of mild lactic acidosis 5, 6