Respiratory Alkalosis

Type

pCO₂ (mmHg)

Expected renal response (compensation)

Expected serum [HCO₃ ⁻]

pH (calculated from Henderson equation)

Normal

–

7.40

Acute

20^a

For each mmHg decrease in pCO₂, HCO₃ ⁻ decreases by 0.2 mEq/L, ΔpCO₂ = 20 (40 − 20 = 20 × 0.2 = 4)

20 (24 − 4 = 20)

7.56

Chronic

20^a

For each mmHg decrease in pCO₂, HCO₃ ⁻ decreases by 0.4 mEq/L, ΔpCO₂ = 20 (40 − 20 = 20 × 0.4 = 8)

16 (24 − 8 = 16)

7.50

^aArbitrary value

Causes of Acute and Chronic Respiratory Alkalosis

With few exceptions, all causes are associated with both acute and chronic respiratory alkalosis. Acute respiratory alkalosis is generally caused by anxiety-hyperventilation syndrome, pain, acute illnesses such as pneumonia, acute asthmatic attack, septicemia, pulmonary embolism, pulmonary edema, salicylate ingestion, or increased minute ventilation in mechanically ventilated patients. Therefore, acute respiratory alkalosis is rather common in general wards and intensive care units. Table 33.2 shows the most important causes of respiratory alkalosis.

Table 33.2

Causes of respiratory alkalosis

Direct stimulation of medullary respiratory center

Voluntary or psychogenic hyperventilation

Stroke

CNS infection, tumor, or trauma

Gram negative sepsis

Liver failure

Pregnancy

Hypermetabolic state (fever, thyrotoxicosis)

Drugs (salicylates, progesterone, nicotine, xanthine derivatives)

Pain

Hypoxemic stimulation of medullary respiratory center

Pulmonary diseases (pneumonia, asthma, pulmonary edema, pulmonary embolus, interstitial lung disease, high altitude, hypotension, severe anemia)

Mechanical ventilation

High minute ventilation

Clinical Manifestations

Acute Respiratory Alkalosis

When pCO₂ is lowered acutely, paresthesias of lower extremities, circumoral numbness, tingling of hands, mouth, and a feeling of chest tightness occur. Headache, light headedness, confusion, and disorientation are also common. Severe alkalosis may precipitate seizures and cardiac arrythmias. These manifestations are related to central nervous system (CNS), cardiovascular, and metabolic effects.

CNS Effects

Acute hypocapnia causes sudden decrease in brain blood flow due to cerebral vasoconstriction. This results in a decrease in intracranial and intraocular pressures.

Light headedness, confusion, and other manifestations are related to decreased cerebral perfusion.
Electroencephalogram (EEG) changes such as slowing and high-voltage wave forms also occur.
Acid–base changes are similar in both cerebrospinal fluid (CSF) and systemic blood during acute hypocapnia.
Acral parasthesias are due to decreased blood flow to the skin.

Cardiovascular Effects

Acute hypocapnia decreases myocardial blood flow and O₂ supply.

Cardiac output is decreased.
Coronary vasoconstriction and chest pain occur.
Cardiac arrythmias are common with severe hypocapnia.

Metabolic Effects

Lactate and ketoacid production is increased.

A decrease in plasma volume is observed.
Acute hypocalcemia due to increased binding of Ca²⁺ can occur with precipitation of tetany and perioral tingling.
Mild hyponatremia, hypokalemia, and hypophosphatemia may be present due to cellular shift.

Chronic Respiratory Alkalosis

Individuals with chronic respiratory alkalosis are generally less symptomatic than subjects with acute respiratory alkalosis. Hyperventilation is subtle in many patients, and it is not even recognized by patients and clinicians. However, certain CNS, cardiovascular, and metabolic effects do occur in patients with chronic respiratory alkalosis.

CNS Effects

CSF pH rises when pCO₂ is sufficiently low.
CSF lactate levels may be high (alkalosis-induced).
Light headedness may be common.

Cardiovascular Effects

Common in individuals who climb mountains and live at high altitudes. These effects are:

Initially, cardiac output increases due to a decrease in peripheral vascular resistance.
Heart rate goes up.
Blood pressure remains normal secondary to decreased peripheral vascular resistance.
Cardiac output returns to baseline 1–6 weeks after exposure to high altitude, but heart rate remains elevated.
Blood flow to the kidneys and skin is reduced.
Both blood and plasma volumes are reduced due to renal Na⁺ loss; however, after several days of high altitude exposure, only blood volume returns to normal due to an increase in hemoglobin and hematocrit increase.
All of the above manifestations are related to hypoxemia.

Metabolic Effects

In addition to low plasma volume, hyponatremia, hypokalemia, and hypophosphatemia due to cellular shifts are common. Decreased ionized Ca²⁺ is also common, causing tetany, Chvostek’s and Trousseau’s signs.

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