Respiratory acidosis
Encyclopedia
Respiratory acidosis is a medical condition in which decreased ventilation (hypoventilation
) causes increased blood carbon dioxide
concentration and decreased pH
(a condition generally called acidosis
).
Carbon dioxide
is produced constantly 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 PaCO2 (called hypercapnia
). The increase in PaCO2 in turn decreases the HCO3−/PaCO2 ratio and decreases pH.
(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.
. 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 fibrosis 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.
. 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 PaCO2 is maintained within a range of 39-41 mm Hg in normal states.
Alveolar ventilation is under the control of the central respiratory centers, which are located in the pons
and the medulla
. Ventilation is influenced and regulated by chemoreceptors for PaCO2, 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 PaCO2.
In acute respiratory acidosis, compensation occurs in 2 steps.
The expected change in pH with respiratory acidosis can be estimated with the following equations:
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
.
Hypoventilation
In medicine, hypoventilation occurs when ventilation is inadequate to perform needed gas exchange...
) causes increased blood carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
concentration and decreased pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
(a condition generally called acidosis
Acidosis
Acidosis is an increased acidity in the blood and other body tissue . If not further qualified, it usually refers to acidity of the blood plasma....
).
Carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
is produced constantly as the body's cells respire, and this CO2 will accumulate rapidly if the lungs do not adequately expel it through alveolar
Pulmonary alveolus
An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...
ventilation. Alveolar hypoventilation thus leads to an increased PaCO2 (called hypercapnia
Hypercapnia
Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood...
). The increase in PaCO2 in turn decreases the HCO3−/PaCO2 ratio and decreases pH.
Terminology
- Acidosis refers to a low pH in tissue.
- Acidemia refers to a low pH in the blood.
Types of respiratory acidosis
Respiratory acidosis can be acute or chronic.- In acute respiratory acidosis, the PaCO2 is elevated above the upper limit of the reference range (over 6.3 kPa or 47 mm Hg) with an accompanying acidemia (pH <7.35).
- In chronic respiratory acidosis, the PaCO2 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 compensationRenal compensationRenal compensation is a mechanism by which the kidneys can regulate the plasma pH. It is slower than respiratory compensation, but has a greater ability to restore normal values....
and an elevated serum bicarbonateBicarbonateIn inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
(HCO3− >30 mm Hg).
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 diseaseNeuromuscular disease
Neuromuscular disease is a very broad term that encompasses many diseases and ailments that either directly, via intrinsic muscle pathology, or indirectly, via nerve pathology, impair the functioning of the muscles....
(e.g., myasthenia gravis
Myasthenia gravis
Myasthenia gravis is an autoimmune neuromuscular disease leading to fluctuating muscle weakness and fatiguability...
, amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis , also referred to as Lou Gehrig's disease, is a form of motor neuron disease caused by the degeneration of upper and lower neurons, located in the ventral horn of the spinal cord and the cortical neurons that provide their efferent input...
, Guillain-Barré syndrome
Guillain-Barré syndrome
Guillain–Barré syndrome , sometimes called Landry's paralysis, is an acute inflammatory demyelinating polyneuropathy , a disorder affecting the peripheral nervous system. Ascending paralysis, weakness beginning in the feet and hands and migrating towards the trunk, is the most typical symptom...
, muscular dystrophy
Muscular dystrophy
Muscular dystrophy is a group of muscle diseases that weaken the musculoskeletal system and hamper locomotion. Muscular dystrophies are characterized by progressive skeletal muscle weakness, defects in muscle proteins, and the death of muscle cells and tissue.In the 1860s, descriptions of boys who...
), or airway obstruction related to asthma or chronic obstructive pulmonary disease (COPD) exacerbation.
Chronic
Chronic respiratory acidosis may be secondary to many disorders, including COPDChronic obstructive pulmonary disease
Chronic obstructive pulmonary disease , also known as chronic obstructive lung disease , chronic obstructive airway disease , chronic airflow limitation and chronic obstructive respiratory disease , is the co-occurrence of chronic bronchitis and emphysema, a pair of commonly co-existing diseases...
. Hypoventilation in COPD involves multiple mechanisms, including decreased responsiveness to hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
and hypercapnia
Hypercapnia
Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood...
, 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
Pickwickian syndrome
Obesity hypoventilation syndrome is a condition in which severely overweight people fail to breathe rapidly enough or deeply enough, resulting in low blood oxygen levels and high blood carbon dioxide levels...
), neuromuscular disorders such as amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis
Amyotrophic lateral sclerosis , also referred to as Lou Gehrig's disease, is a form of motor neuron disease caused by the degeneration of upper and lower neurons, located in the ventral horn of the spinal cord and the cortical neurons that provide their efferent input...
, and severe restrictive ventilatory defects as observed in interstitial fibrosis 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.
Mechanism
Metabolism rapidly generates a large quantity of volatile acid (H2CO3) and nonvolatile acidNonvolatile acid
A nonvolatile acid is an acid produced from sources other than carbon dioxide, and is not excreted by the lungs. They are produced from e.g. an incomplete metabolism of carbohydrates, fats, and proteins. All acids produced in the body are nonvolatile except carbonic acid, which is the sole...
. 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
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...
(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 PaCO2 is maintained within a range of 39-41 mm Hg in normal states.
Alveolar ventilation is under the control of the central respiratory centers, which are located in the pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
and the medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
. Ventilation is influenced and regulated by chemoreceptors for PaCO2, 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
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
. Failure of ventilation quickly increases the PaCO2.
In acute respiratory acidosis, compensation occurs in 2 steps.
- The initial response is cellular buffering 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 PaCO2.
- 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 NH3AmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and thus to produce more HCO3−BicarbonateIn inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
.
Estimated changes
In renal compensation, plasma bicarbonate rises 3.5 mEq/L for each increase of 10 mm Hg in PaCO2. 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 PaCO2.
- Chronic respiratory acidosis: HCO3− rises 3.5 mEq/L for each 10 mm Hg rise in PaCO2.
The expected change in pH with respiratory acidosis can be estimated with the following equations:
- Acute respiratory acidosis: Change in pH = 0.008 X (40 − PaCO2)
- Chronic respiratory acidosis: Change in pH = 0.003 X (40 − PaCO2)
Respiratory acidosis does not have a great effect on electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
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
Hyperkalemia
Hyperkalemia refers to the condition in which the concentration of the electrolyte potassium in the blood is elevated...
.