Respiratory Alkalosis

Background

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 concentration to PaCO₂ and, thereby, increases 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.^{[1, 2]}

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.

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Pathophysiology

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 easier 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,^[3] 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 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.^[4] 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.^[5] After 2-6 hours, renal compensation begins via a decrease in bicarbonate reabsorption. The kidneys respond more to the decreased PaCO₂ rather than the increased pH. Complete kidney compensation may take several days and requires normal kidney function and intravascular volume status.^[5] 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).^[4]

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.^[6]

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Epidemiology

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.^[5]

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.^[7]

Sex

Respiratory alkalosis is equally prevalent in males and females.

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Prognosis

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.^[8]

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.^[9]

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 ratio for those with respiratory alkalosis or metabolic acidosis, compared with controls, was 1.21 or 1.17, respectively.^[10]

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Patient Education

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.

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History

Clinical manifestations of respiratory alkalosis depend on its duration, its severity, and the underlying disease process. Note the following:

• The hyperventilation syndrome can mimic many conditions that are more serious. Symptoms may include paresthesia, circumoral numbness, chest pain or tightness, dyspnea, and tetany.^[11]
• Acute onset of hypocapnia can cause cerebral vasoconstriction. An acute decrease in PaCO₂ reduces cerebral blood flow and can cause neurologic symptoms, including dizziness, mental confusion, syncope, and seizures. Hypoxemia need not be present for the patient to experience these symptoms.^[5]
• The first cases of spontaneous hyperventilation with dizziness and tingling leading to tetany were described in 1922 in patients with cholecystitis, abdominal distention, and hysteria.^[12]
• Haldane and Poulton described painful tingling in the hands and feet, numbness and sweating of the hands, and cerebral symptoms following voluntary hyperventilation.^[13]
• Respiratory alkalosis may impair vitamin D metabolism, which may lead to vitamin D deficiency and cause symptoms such as fibromyalgia.^[14]

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Physical

Physical examination findings in patients with respiratory alkalosis are nonspecific and are typically related to the underlying illness or cause of the respiratory alkalosis. Note the following:

• Many patients with hyperventilation syndrome appear anxious and are frequently tachycardic. Understandably, tachypnea is a frequent finding.
• In acute hyperventilation, chest wall movement and breathing rate increase. In patients with chronic hyperventilation, these physical findings may not be as obvious.
• Positive Chvostek and Trousseau signs may be elicited.^[4]
• Patients with underlying pulmonary disease may have signs suggestive of pulmonary disease, such as crackles, wheezes, or rhonchi. Cyanosis may be present if the patient is hypoxic.
• If the underlying pathology is neurologic, the patient may have focal neurologic signs or a depressed level of consciousness.^[15]
• Cardiovascular effects of hypocapnia in healthy and alert patients are minimal, but in patients who are anesthetized, critically ill, or receiving mechanical ventilation, the effects can be more significant. Cardiac output and systemic blood pressure may fall as a result of the effects of sedation and positive-pressure ventilation on venous return, systemic vascular resistance, and heart rate.^[5]
• Cardiac rhythm disturbances may occur because of increased tissue hypoxia related to the leftward shift of the hemoglobin-oxygen dissociation curve.^[5]

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Causes

The differential diagnosis of respiratory alkalosis is broad; therefore, a thorough history, physical examination, and laboratory evaluation are helpful in arriving at the true diagnosis.

Central nervous system causes are as follows:

• Pain
• Hyperventilation syndrome
• Anxiety
• Panic disorders
• Psychosis
• Fever
• Cerebrovascular accident
• Meningitis
• Encephalitis
• Tumor
• Trauma

Hypoxia-related causes are as follows:

• High altitude
• Right-to-left shunts

Drug-related causes are as follows:

• Progesterone
• Methylxanthine toxicity
• Salicylate toxicity
• Catecholamines
• Nicotine

Endocrine-related causes are as follows:

• Pregnancy
• Hyperthyroidism

Pulmonary causes are as follows:

• Pneumothorax/hemothorax
• Pneumonia
• Pulmonary edema
• Pulmonary embolism
• Aspiration
• Interstitial lung disease
• Asthma
• Emphysema
• Chronic bronchitis

Miscellaneous causes are as follows:

• Sepsis
• Severe anemia
• Hepatic failure
• Mechanical ventilation
• Heat exhaustion
• Recovery phase of metabolic acidosis
• Congestive heart failure

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Laboratory Studies

An essential laboratory analysis is as follows:

• Arterial blood gas determination: Alkalemia is documented by the presence of an increased pH level (>7.45) on arterial blood gas determinations. The presence of a decreased PaCO₂ level (< 35 mm Hg) indicates a respiratory etiology of the alkalemia.

The following laboratory studies may be helpful:

• Serum chemistries: Acute respiratory alkalosis causes small changes in electrolyte balances. Minor intracellular shifts of sodium, potassium, and phosphate levels occur. A minor reduction in free calcium occurs due to an increased protein-bound fraction. Compensation for respiratory alkalosis is by increased renal excretion of bicarbonate. In acute respiratory alkalosis, the bicarbonate concentration level decreases by 2 mEq/L for each decrease of 10 mm Hg in the PaCO₂ level. In chronic respiratory acidosis, the bicarbonate concentration level decreases by 5 mEq/L for each decrease of 10 mm Hg in the PaCO₂ level. Plasma bicarbonate levels rarely drop below 12 mm Hg secondary to compensation for primary respiratory alkalosis.
• Complete blood cell count: An elevation of the WBC count may indicate early sepsis as a possible etiology of respiratory alkalosis. A reduced hematocrit value may indicate severe anemia as the potential cause of respiratory alkalosis.
• Liver function test: Findings may be abnormal if hepatic failure is the etiology of the respiratory alkalosis.
• Cultures of blood, sputum, urine, and other sites: These should be considered, depending on information obtained from the history and physical examination and if sepsis or bacteremia are thought to be the cause of the respiratory alkalosis.
• Thyroid testing: Thyroid-stimulating hormone and thyroxine levels may be indicated to rule out hyperthyroidism.
• Beta-human chorionic hormone levels may be helpful in ruling out pregnancy.
• Drug screens and theophylline and salicylate levels may be useful to determine whether drugs or medications are the cause.

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Imaging Studies

Consider the following imaging studies:

• Chest radiography: Chest radiography should be performed to help rule out pulmonary disease as a cause of hypocapnia and respiratory alkalosis. Potential etiologies that may be confirmed based on chest radiography findings include pneumonia, pulmonary edema, aspiration pneumonitis, pneumothorax, and interstitial lung disease.
• Computerized tomography (CT) scanning: CT scanning of the chest may be performed if chest radiography findings are inconclusive or a pulmonary disorder is strongly considered as a differential diagnosis. CT scanning is more sensitive for helping detect disease, and findings may reveal abnormalities not seen on the chest radiograph. Consider spiral CT angiography of the chest if pulmonary embolism is suggested. Consider CT scanning of the brain if a central cause of hyperventilation and respiratory alkalosis is suspected. Specific etiologies that may be diagnosed based on brain CT scan findings include cerebrovascular accident, central nervous system tumor, and central nervous system trauma.
• Ventilation perfusion scanning: Consider this scan in patients who are unable to undergo an intravenous contrast injection associated with CT scanning to assess the patient for pulmonary embolism.
• Brain magnetic resonance imaging (MRI): If a central cause of hyperventilation and respiratory alkalosis is suggested and the initial brain CT scan findings are negative or inconclusive, an MRI of the brain can be considered. MRIs may reveal abnormalities not seen on CT scans, particularly lesions of the brain stem. Possible etiologies based on MRIs include cerebrovascular accident, central nervous system tumor, and central nervous system trauma.

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Other Tests

Echocardiography can be performed to assess myocardial and valvular function. A "bubble" study is helpful when assessing patients for unexplained hypoxemia and right-to-left shunting of blood.

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Procedures

Perform a lumbar puncture if the history and physical examination findings are suggestive of a CNS infectious process.

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Medical Care

The treatment of respiratory alkalosis is primarily directed at correcting the underlying disorder. Respiratory alkalosis itself is rarely life threatening. Therefore, emergent treatment is usually not indicated unless the pH level is greater than 7.5. Because respiratory alkalosis usually occurs in response to some stimulus, treatment is usually unsuccessful unless the stimulus is controlled. If the PaCO₂ is corrected rapidly in patients with chronic respiratory alkalosis, metabolic acidosis may develop due to the renal compensatory drop in serum bicarbonate.

In mechanically ventilated patients who have respiratory alkalosis, the tidal volume and/or respiratory rate may need to be decreased. Inadequate sedation and pain control may contribute to respiratory alkalosis in patients breathing over the set ventilator rate.

In hyperventilation syndrome, patients benefit from reassurance, rebreathing into a paper bag during acute episodes, and treatment for underlying psychological stress. Sedatives and/or antidepressants should be reserved for patients who have not responded to conservative treatment. Beta-adrenergic blockers may help control the manifestations of the hyperadrenergic state that can lead to hyperventilation syndrome in some patients.^[4]

In patients presenting with hyperventilation, a systematic approach should be used to rule out potentially life-threatening, organic causes first before considering less serious disorders.

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Consultations

Based on the findings from the history, physical examination, laboratory studies, and imaging modalities, the necessity for assistance from consultants such as pulmonologists, neurologists, or nephrologists can be determined.

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