respiratory acidosis

Respiratory acidosis is a state in which decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH (a condition generally called acidosis).

Carbon dioxide is produced continuously as the body's cells respire, and this CO₂ will accumulate rapidly if the lungs do not adequately expel it through alveolar ventilation. Alveolar hypoventilation thus leads to an increased ''Pa''CO₂ (a condition called hypercapnia). The increase in ''Pa''CO₂ in turn decreases the HCO₃⁻/''Pa''CO₂ ratio and decreases pH.

Types

Respiratory acidosis can be acute or chronic.
* In ''acute respiratory acidosis'', the ''Pa''CO₂ is elevated above the upper limit of the reference range (over 6.3 kPa or 45 mm Hg) with an accompanying acidemia (pH <7.36).
* In ''chronic respiratory acidosis'', the ''Pa''CO₂ is elevated above the upper limit of the reference range, with a normal blood pH (7.35 to 7.45) or near-normal pH secondary to renal compensation and an elevated serum bicarbonate (HCO₃⁻ >30 mEq/L).

Causes

Acute

Acute respiratory acidosis occurs when an abrupt failure of ventilation occurs. This failure in ventilation may be caused by depression of the central respiratory center by cerebral disease or drugs, inability to ventilate adequately due to neuromuscular disease (e.g., myasthenia gravis, amyotrophic lateral sclerosis, Guillain–Barré syndrome, muscular dystrophy), or airway obstruction related to asthma or chronic obstructive pulmonary disease (COPD) exacerbation.

Chronic

Chronic respiratory acidosis may be secondary to many disorders, including COPD. Hypoventilation in COPD involves multiple mechanisms, including decreased responsiveness to hypoxia and hypercapnia, increased ventilation-perfusion mismatch leading to increased dead space ventilation, and decreased diaphragm function secondary to fatigue and hyperinflation.

Chronic respiratory acidosis also may be secondary to obesity hypoventilation syndrome (i.e., Pickwickian syndrome), neuromuscular disorders such as amyotrophic lateral sclerosis, and severe restrictive ventilatory defects as observed in interstitial lung disease and thoracic deformities.

Lung diseases that primarily cause abnormality in alveolar gas exchange usually do not cause hypoventilation but tend to cause stimulation of ventilation and hypocapnia secondary to hypoxia. Hypercapnia only occurs if severe disease or respiratory muscle fatigue occurs.

Physiological response

Mechanism

Metabolism rapidly generates a large quantity of volatile acid (H₂CO₃) and nonvolatile acid. The metabolism of fats and carbohydrates leads to the formation of a large amount of CO₂. The CO₂ combines with H₂O to form carbonic acid (H₂CO₃). The lungs normally excrete the volatile fraction through ventilation, and acid accumulation does not occur. A significant alteration in ventilation that affects elimination of CO₂ can cause a respiratory acid-base disorder. The ''Pa''CO₂ is maintained within a range of 35–45 mm Hg in normal states.

Alveolar ventilation is under the control of the respiratory center, which is located in the pons and the medulla. Ventilation is influenced and regulated by chemoreceptors for ''Pa''CO₂, PaO₂, and pH located in the brainstem, and in the aortic and carotid bodies as well as by neural impulses from lung stretch receptors and impulses from the cerebral cortex. Failure of ventilation quickly increases the ''Pa''CO₂.

In acute respiratory acidosis, compensation occurs in 2 steps.
* The initial response is cellular buffering (plasma protein buffers) that occurs over minutes to hours. Cellular buffering elevates plasma bicarbonate (HCO₃⁻) only slightly, approximately 1 mEq/L for each 10-mm Hg increase in ''Pa''CO₂.
* The second step is renal compensation that occurs over 3–5 days. With renal compensation, renal excretion of carbonic acid is increased and bicarbonate reabsorption is increased. For instance, PEPCK is upregulated in renal proximal tubule brush border cells, in order to secrete more NH₃ and thus to produce more HCO₃⁻.

Estimated changes

In renal compensation, plasma bicarbonate rises 3.5 mEq/L for each increase of 10 mm Hg in ''Pa''CO₂. The expected change in serum bicarbonate concentration in respiratory acidosis can be estimated as follows:

* Acute respiratory acidosis: HCO₃⁻ increases 1 mEq/L for each 10 mm Hg rise in ''Pa''CO₂.
* Chronic respiratory acidosis: HCO₃⁻ rises 3.5 mEq/L for each 10 mm Hg rise in ''Pa''CO₂.

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

* Acute respiratory acidosis: Change in pH = 0.08 X ((40 − ''Pa''CO₂)/10)
* Chronic respiratory acidosis: Change in pH = 0.03 X ((40 − ''Pa''CO₂)/10)

Respiratory acidosis does not have a great effect on electrolyte levels. Some small effects occur on calcium and potassium levels. Acidosis decreases binding of calcium to albumin and tends to increase serum ionized calcium levels. In addition, acidemia causes an extracellular shift of potassium, but respiratory acidosis rarely causes clinically significant hyperkalemia.

Diagnosis

Diagnoses can be done by doing an ABG (Arterial Blood Gas) laboratory study, with a pH <7.35 and a PaCO2 >45 mmHg in an acute setting. Patients with COPD and other Chronic respiratory diseases will sometimes display higher numbers of PaCO2 with HCO3- >30 and normal pH.

Terminology

*Acidosis refers to disorders that lower cell/tissue pH to < 7.35.
*Acidemia refers to an arterial pH < 7.36.

See also

* Acidosis
* Alkalosis
* Arterial blood gas
* Hypercapnia
* Chemical equilibrium
* pCO2
* pH
* pKa
* Metabolic acidosis
* Metabolic alkalosis
* Respiratory alkalosis

References

External links

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{{Fluid, electrolyte, acid base metabolic pathology

Acid–base disturbances