Respiratory Alkalosis

Respiratory alkalosis is a disturbance in acid and base balance due to alveolar hyperventilation. Alveolar hyperventilation leads to a decreased partial pressure of arterial carbon dioxide (PaCO₂). In turn, the decrease in PaCO₂ increases the ratio of bicarbonate (HCO₃^-) concentration to PaCO₂, thereby increasing the pH level; thus the descriptive term respiratory alkalosis. The decrease in PaCO₂ (hypocapnia) develops when a strong respiratory stimulus causes the respiratory system to remove more carbon dioxide than is produced metabolically in the tissues.

Respiratory alkalosis can be acute or chronic. In acute respiratory alkalosis, the PaCO₂ level is below the lower limit of normal and the serum pH is alkalemic. In chronic respiratory alkalosis, the PaCO₂ level is below the lower limit of normal, but the pH level is relatively normal or near normal due to compensatory mechanisms.

Respiratory alkalosis is the most common acid-base abnormality observed in patients who are critically ill. It is associated with numerous illnesses and is a common finding in patients on mechanical ventilation. Many cardiac and pulmonary disorders can manifest with respiratory alkalosis as an early or intermediate finding. When respiratory alkalosis is present, the cause may be a minor, non–life-threatening disorder. However, more serious disease processes should also be considered in the differential diagnosis.

Breathing or alveolar ventilation is the body’s method of providing adequate amounts of oxygen for metabolism while removing carbon dioxide produced in the tissues. By sensing the body’s partial pressure of arterial oxygen (PaO₂) and partial pressure of arterial carbon dioxide (PaCO₂), the respiratory system adjusts pulmonary ventilation so that oxygen uptake and carbon dioxide elimination at the lungs are balanced with the amount used and produced by the tissues.

The PaCO₂ must be maintained at a level that ensures that hydrogen ion concentrations remain in the narrow limits required for optimal protein and enzymatic function. PaO₂ is not as closely regulated as the PaCO2. Adequate hemoglobin saturation can be achieved over a wide range of PaO₂ levels. The movement of oxygen from the alveoli to the vascular system is dependent on pressure gradients. On the other hand, carbon dioxide diffuses much more easily through an aqueous environment.

Metabolism generates a large quantity of volatile acid (carbonic acid excreted as carbon dioxide by the lungs) and nonvolatile acid. The metabolism of fats and carbohydrates leads to the formation of a large amount of carbon dioxide, ^[] which combines with water to form carbonic acid. The lungs excrete the volatile fraction through ventilation so that acid accumulation does not occur. Significant alterations in ventilation can affect the elimination of carbon dioxide and lead to a respiratory acid-base disorder.

PaCO₂ is normally maintained in the range of 35-45 mm Hg. Chemoreceptors in the brain (central chemoreceptors) and in the carotid bodies (peripheral chemoreceptors) sense hydrogen concentrations and influence ventilation to adjust the partial pressure of carbon dioxide (PCO₂) and pH. This feedback regulator is how the PaCO₂ is maintained within its narrow normal range. When these receptors sense an increase in hydrogen ions, breathing is increased to "blow off" carbon dioxide and subsequently reduce the amount of hydrogen ions. Various disease processes may cause stimulation of ventilation with subsequent hyperventilation. If hyperventilation is persistent, it leads to hypocapnia.

Hyperventilation refers to an increase in alveolar ventilation that is disproportionate to the rate of metabolic carbon dioxide production, leading to a PaCO₂ level below the normal range, or hypocapnia. Hyperventilation is often associated with dyspnea, but not all patients who are hyperventilating complain of shortness of breath. Conversely, patients with dyspnea need not be hyperventilating.

Acute hypocapnia causes a reduction of serum levels of potassium and phosphate secondary to increased intracellular shifts of these ions. A reduction in free serum calcium also occurs. Calcium reduction is secondary to increased binding of calcium to serum albumin due to the change in pH. Many of the symptoms present in persons with respiratory alkalosis are related to hypocalcemia. ^[] Hyponatremia and hypochloremia may also be present.

Acute hyperventilation with hypocapnia causes a small, early reduction in serum bicarbonate levels resulting from cellular shift of bicarbonate. Acutely, plasma pH and bicarbonate concentration vary proportionately with the PaCO₂ along a range of 15-40 mm Hg. The relationship of PaCO₂ to arterial hydrogen and bicarbonate is 0.7 mmol/L per mm Hg and 0.2 mmol/L per mm Hg, respectively. ^[] After 2-6 hours, renal compensation begins via a decrease in bicarbonate reabsorption. The kidneys respond more to the decreased PaCO₂ rather than to the increased pH. Complete kidney compensation may take several days and requires normal kidney function and intravascular volume status. ^[] The expected change in serum bicarbonate concentration can be estimated as follows:

• Acute - Bicarbonate (HCO₃^-) falls 2 mEq/L for each decrease of 10 mm Hg in the PCO₂; that is, ΔHCO₃^- = 0.2(ΔPCO₂); maximum compensation: HCO₃^- = 12-20 mEq/L
• Chronic - Bicarbonate (HCO₃^-) falls 5 mEq/L for each decrease of 10 mm Hg in the PCO₂; that is, ΔHCO₃^- = 0.5(ΔPCO₂); maximum compensation: HCO₃^- = 12-20 mEq/L

Note that a plasma bicarbonate concentration of less than 12 mmol/L is unusual in pure respiratory alkalosis alone and should prompt the consideration of a metabolic acidosis as well (ie, the presence of a mixed acid-base disorder). ^[]

The expected change in pH with respiratory alkalosis can be estimated with the following equations:

• Acute respiratory alkalosis - Change in pH = 0.008 X (40 – PCO₂)
• Chronic respiratory alkalosis - Change in pH = 0.017 X (40 – PCO₂)

A study by Morel et al suggested that when respiratory alkalosis is present, caution be used in the employment of venous-arterial difference in CO₂ (ΔCO₂) as an indicator of the adequacy of tissue perfusion (as has been proposed for shock states). Using healthy volunteers in whom either hypocapnia or hypercapnia was induced, the investigators found a significant increase in ΔCO₂ in the hypocapnic subjects, who also had a significant decrease in skin microcirculatory blood flow. ^[]

Frequency

United States

The frequency of respiratory alkalosis varies depending on the etiology. The most common acid-base abnormality observed in critically ill patients is respiratory alkalosis. ^[]

Mortality/morbidity

Morbidity and mortality of patients with respiratory alkalosis depend on the nature of the underlying cause of the respiratory alkalosis and associated conditions.

An Iranian study, by Hamdi et al, found primary respiratory alkalosis to be one of the mortality risk factors during hospitalization for poisoning, with the other predictors consisting of age, intensive care unit admission, consciousness level, period of hospitalization, and severe metabolic acidosis. ^[]

A study by Wu et al suggested that an association exists between respiratory alkalosis and the severity of coronavirus disease 2019 (COVID-19). The investigators reported that after adjusting for age, gender, and the presence of comorbidities (specifically, cardiovascular disease and hypertension), the hazard ratio for the development of severe COVID-19 in patients with respiratory alkalosis was 2.445. ^[]

Sex

Respiratory alkalosis is equally prevalent in males and females.

The prognosis of respiratory alkalosis is variable and depends on the underlying cause and the severity of the underlying illness.

Lewis et al hypothesized that respiratory alkalosis may interfere with vitamin D production, contributing to the development of fibromyalgia. The investigators suggested that, possibly by suppressing the kidneys’ ability to release phosphate into the urine, alkalotic pH disrupts endogenous 1,25-dihydroxyvitamin D formation. ^[]

A study by Park et al indicated that in patients with high-risk acute heart failure, respiratory alkalosis is the most frequent acid-base imbalance. However, while acidosis was found to be a significant risk factor for mortality in acute heart failure patients, this was not true for alkalosis. ^[]

A retrospective study by Miró et al reported similar results, finding evidence that alkalosis does not increase the mortality risk in patients with heart failure. The cohort included 676 patients with probable metabolic alkalosis and 937 with probable respiratory alkalosis, with the investigators reporting that the adjusted odds ratio for inhospital mortality was 0.919. ^[]

A study by Raphael et al indicated that in healthy older adults, low serum bicarbonate levels can be linked to a higher mortality rate no matter whether respiratory alkalosis or metabolic acidosis is responsible for the bicarbonate reduction. Among the study’s patients (mean age 76 y), the mortality hazard ratios for those with respiratory alkalosis or metabolic acidosis, compared with controls, were 1.21 and 1.17, respectively. ^[]

Patients with hyperventilation syndrome as the etiology of their respiratory alkalosis may particularly benefit from patient education. The underlying pathophysiology should be explained in simple terms, and patients should be instructed in breathing techniques that may be used to relieve the hyperventilation. Reassurance is key for these patients.