Respiratory Acidosis is a state in which there is usually a failure of ventilation and an accumulation of carbon dioxide. The primary disturbance of elevated arterial PCO2 is the decreased ratio of arterial bicarbonate to arterial PCO2, which leads to a lowering of the pH. In the presence of alveolar hypoventilation, 2 features commonly are seen are respiratory acidosis and hypercapnia. To compensate for the disturbance in the balance between carbon dioxide and bicarbonate (HCO3-), the kidneys begin to excrete more acid in the forms of hydrogen and ammonium and reabsorb more base in the form of bicarbonate. This compensation helps to normalize the pH.[rx]
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 PaCO2 (a condition called hypercapnia). The increase in PaCO2 in turn decreases the HCO3−/PaCO2 ratio and decreases pH.
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 45 mm Hg) with an accompanying acidemia (pH <7.36). Acute respiratory acidosis is a condition in which carbon dioxide builds up very quickly before the kidneys can return the body to a state of balance.
- 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 compensation and elevated serum bicarbonate (HCO3− >30 mEq/L). Chronic respiratory acidosis occurs over a long time. This leads to a stable situation because the kidneys increase body chemicals, such as bicarbonate, that help restore the body’s acid-base balance.
Some people with chronic respiratory acidosis get acute respiratory acidosis because an acute illness makes their condition worse and disrupts their body’s acid-base balance.
Pathophysiology
Carbon dioxide plays a remarkable role in the human body mainly through pH regulation of the blood. The pH is the primary stimulus to initiate ventilation. In its normal state, the body maintains CO2 in a well-controlled range from 38 to 42 mm Hg by balancing its production and elimination. In a state of hypoventilation, the body produces more CO2 than it can eliminate, causing a net retention of CO2. The increased CO2 is what leads to an increase in hydrogen ions and a slight increase in bicarbonate, as seen by a right shift in the following equilibrium reaction of carbon dioxide:
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CO2 + H2O -> H2CO3- -> HCO3- + H+
The buffer system created by carbon dioxide consists of the following three molecules in equilibrium: CO2, H2CO3-, and HCO3-. When H+ is high, HCO3- buffers the low pH. When OH- is high, H2CO3 buffers the high pH. In respiratory acidosis, the slight increase in bicarbonate serves as a buffer for the increase in H+ ions, which helps minimize the drop in pH. The increase in hydrogen ions inevitably causes a decrease in pH, which is the mechanism behind respiratory acidosis.[rx][rx]
Causes of Respiratory Acidosis
The respiratory centers in the pons and medulla control alveolar ventilation. Chemoreceptors for PCO2, PO2, and pH regulate ventilation. Central chemoreceptors in the medulla are sensitive to changes in the pH level. A decreased pH level influences the mechanics of ventilation and maintains proper levels of carbon dioxide and oxygen. When ventilation is disrupted, arterial PCO2 increases, and an acid-base disorder develop. Another pathophysiological mechanism may be due to ventilation/perfusion mismatch of dead space.
- asthma
- chronic obstructive pulmonary disease (COPD)
- acute pulmonary edema
- severe obesity (which can interfere with the expansion of the lungs)
- neuromuscular disorders (such as multiple sclerosis or muscular dystrophy)
- scoliosis
- lung disorders (COPD, emphysema, asthma, pneumonia)
- conditions that affect the rate of breathing
- muscle weakness that affects breathing or taking a deep breath
- obstructed airways (due to choking or other causes)
- sedative overdose
- cardiac arrest
Causes of respiratory acidosis include:
- Diseases of the airways, such as asthma and COPD
- Diseases of the lung tissue, such as pulmonary fibrosis, which causes scarring and thickening of the lungs
- Diseases that can affect the chest, such as scoliosis
- Diseases affecting the nerves and muscles that signal the lungs to inflate or deflate
- Medicines that suppress breathing, including powerful pain medicines, such as narcotics, and “downers,” such as benzodiazepines, often when combined with alcohol
- Severe obesity, which restricts how much the lungs can expand
- Obstructive sleep apnea
- This may be due to cerebrovascular accidents, use of central nervous systems (CNS) depressants such as opioids, or inability to use muscles of respiration because of disorders like myasthenia gravis, muscular dystrophy, or Guillain-Barre Syndrome.
- Chronic respiratory acidosis may be caused by COPD where there is a decreased responsiveness of the reflexes to states of hypoxia and hypercapnia. Other individuals who develop chronic respiratory acidosis may have the fatigue of the diaphragm resulting from a muscular disorder.
- Chronic respiratory acidosis can also be seen in obesity hypoventilation syndrome, also known as Pickwickian syndrome, amyotrophic lateral sclerosis, and in patients with severe thoracic skeletal defects. In patients with chronic compensated respiratory disease and acidosis, an acute insult such as pneumonia or disease exacerbation can lead to ventilation/perfusion mismatch.
Respiratory acidosis may cause slight elevations in ionized calcium and an extracellular shift of potassium. However, hyperkalemia is usually mild. In chronic respiratory acidosis, renal compensation occurs gradually over the course of days.[rx][rx]
Symptoms of Respiratory Acidosis
Symptoms may include of Respiratory Acidosis
- Confusion
- Anxiety
- Easy fatigue
- Lethargy
- Shortness of breath
- Sleepiness
- Tremors (shaking)
- Warm and flushed skin
- Sweating
Initial signs of acute respiratory acidosis include
- headache
- anxiety
- blurred vision
- restlessness
- confusion
- sleepiness or fatigue
- lethargy
- delirium or confusion
- shortness of breath
- coma
- memory loss
- sleep disturbances
- personality changes
Diagnosis of Respiratory Acidosis
History and Physical
The clinical presentation of respiratory acidosis is usually a manifestation of its underlying cause. Signs and symptoms vary based on the length, severity, and progression of the disorder. Patients can present with dyspnea, anxiety, wheezing, and sleep disturbances. In some cases, patients may present with cyanosis due to hypoxemia. If the respiratory acidosis is severe and accompanied by prolonged hypoventilation, the patient may have additional symptoms such as altered mental status, myoclonus, and possibly even seizures. Respiratory acidosis leads to hypercapnia, which induces cerebral vasodilation. If severe enough, increased intracranial pressure and papilledema may ensue, increasing the risk of herniation and possibly even death. Cases of chronic respiratory acidosis may cause memory loss, impaired coordination, polycythemia, pulmonary hypertension, and heart failure. Persistence of apnea during sleep can lead to daytime somnolence and headaches. In patients with an obvious source of respiratory acidosis, the offending agent needs to be removed or reversed.[rx]
Evaluation
An arterial blood gas (ABG) and serum bicarbonate level is necessary to evaluate patients with suspected respiratory acidosis. Other tests can be conducted to evaluate the underlying causes. In respiratory acidosis, the ABG will show an elevated PCO2 (>45 mmHg), elevated HCO3- (>30 mmHg), and decreased pH (<7.35). The respiratory acidosis can be further classified as acute or chronic based on the relative increase in HCO3- with respect to PCO2. In cases of acute respiratory acidosis, HCO3- will have increased by one mEq/L for every ten mmHg increase in PCO2 over a few minutes. In cases of chronic respiratory acidosis, HCO3- will have increased by four mEq/L for every ten mmHg increase in PCO2 over a time course of days. If the compensation does not occur in this pattern, a mixed respiratory-metabolic disorder may be present. In a patient who presents with unexplained respiratory acidosis, a drug screen may also be warranted.[rx][rx]
Tests that may be done include
Treatment of Respiratory Acidosis
Once the diagnosis has been made, the underlying cause of respiratory acidosis has to be treated. The hypercapnia should be corrected gradually because rapid alkalization of the cerebrospinal fluid (CSF) may lead to seizures. Pharmacologic therapy can also be used to help improve ventilation. Bronchodilators like beta-agonists, anticholinergic drugs, and methylxanthines can be used in treating patients with obstructive airway diseases. Naloxone can be used in patients who overdose on opioid use.[rx][rx]
- Bronchodilator medicines and corticosteroids to reverse some types of airway obstruction
- Noninvasive positive-pressure ventilation (sometimes called CPAP or BiPAP) or a breathing machine, if needed
- Oxygen if the blood oxygen level is low
- Treatment to stop smoking
- For severe cases, a breathing machine might be needed
The goal is to improve airway function. Some strategies include
- antibiotics (to treat infection)
- diuretics (to reduce excess fluid affecting the heart and lungs)
- bronchodilators (to expand the airways)
- corticosteroids (to reduce inflammation)
- mechanical ventilation (in severe cases)
References
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