Respiratory Alkalosis

Respiratory alkalosis is a primary decrease in Pco₂ (hypocapnia) due to an increase in respiratory rate and/or volume (hyperventilation). Ventilation increase occurs most often as a physiologic response to hypoxia (eg, at high altitude Altitude Illness Altitude illness is caused by the decreased availability of oxygen at high altitudes. Acute mountain sickness (AMS), the mildest form, is characterized by headache plus one or more systemic... read more ), metabolic acidosis Metabolic Acidosis Metabolic acidosis is primary reduction in bicarbonate (HCO3⁻), typically with compensatory reduction in carbon dioxide partial pressure (Pco2); pH may be markedly low or slightly... read more , and increased metabolic demands (eg, fever) and, as such, is present in many serious conditions. In addition, pain and anxiety and some central nervous system (CNS) disorders (eg, stroke, seizure [post-ictal]) can increase respirations without a physiologic need.

Respiratory alkalosis can be

The distinction is based on the degree of metabolic compensation. Excess HCO₃⁻ is buffered by extracellular hydrogen ion (H⁺) within minutes (acute), but more significant compensation occurs over 2 to 3 days (chronic) as the kidneys decrease H⁺ excretion.

Symptoms and signs depend on the rate and degree of fall in Pco₂. Acute respiratory alkalosis causes light-headedness, confusion, peripheral and circumoral paresthesias, cramps, and syncope. Mechanism is thought to be change in cerebral blood flow and pH. Tachypnea or hyperpnea is often the only sign; carpopedal spasm may occur in severe cases due to decreased levels of ionized calcium in the blood (driven inside cells in exchange for hydrogen ion [H⁺]).

Chronic respiratory alkalosis is usually asymptomatic and has no distinctive signs.

Recognition of respiratory alkalosis and appropriate renal compensation (see Diagnosis of Acid-Base Disorders Diagnosis Acid-base disorders are pathologic changes in carbon dioxide partial pressure (Pco2) or serum bicarbonate (HCO3⁻) that typically produce abnormal arterial pH values. Acidemia is serum... read more ) requires ABG and serum electrolyte measurements. Minor hypophosphatemia Hypophosphatemia Hypophosphatemia is a serum phosphate concentration < 2.5 mg/dL (0.81 mmol/L). Causes include alcohol use disorder, burns, starvation, and diuretic use. Clinical features include muscle weakness... read more and hypokalemia Hypokalemia Hypokalemia is serum potassium concentration < 3.5 mEq/L (< 3.5 mmol/L) caused by a deficit in total body potassium stores or abnormal movement of potassium into cells. The most common... read more due to intracellular shifts and decreased ionized calcium (Ca⁺⁺) due to an increase in protein binding may be present.

Presence of hypoxia or an increased alveolar-arterial (A-a) O₂ gradient (inspired Po₂− [arterial Po₂+ ⁵/₄ arterial Pco₂]) requires search for a cause.

Causes are often apparent based on history and examination findings. However, because pulmonary embolism Pulmonary Embolism (PE) Pulmonary embolism (PE) is the occlusion of pulmonary arteries by thrombi that originate elsewhere, typically in the large veins of the legs or pelvis. Risk factors for pulmonary embolism are... read more often manifests without hypoxia, embolism must be strongly considered in a patient who is hyperventilating before ascribing the cause to anxiety alone.

Treatment is directed at finding and treating the underlying disorder. Respiratory alkalosis itself is not life threatening, so no interventions to lower pH are necessary. Increasing inspired carbon dioxide through rebreathing (such as from a paper bag) is common practice but may be dangerous in at least some patients with CNS disorders in whom the pH of cerebrospinal fluid may already be below normal.