Respiratory Acidosis (Carbon Dioxide Blood Acidity)
Respiratory Acidosis Definition
Respiratory acidosis refers to the condition in which body fluids, especially the blood, become too acidic due to higher than normal levels of carbon dioxide. It is an indication that breathing (ventilation) is not adequately expelling the carbon dioxide from the body. There are a number of different causes of respiratory acidosis. It is a state that arises with certain diseases, and is not a disease on its own. Without proper intervention respiratory acidosis can lead to a host of severe complications and even progress to death.
Respiratory vs Metabolic Acidosis
The term acidosis broadly refers to any decrease in pH (rise in acidity) within the body fluids. It is more correctly known as acidemia when referring to the pH within the blood in an artery. There are broadly two types – metabolic acidosis and respiratory acidosis. In metabolic acidosis, the rise is acidity is result of the acidic byproducts of metabolism which are normally excreted by the kidneys. With respiratory acidosis, the increase in acidity is associated with a build up of carbon dioxide (hypercapnia) related to inadequate expulsion via the lungs (hypoventilation).
Respiratory Acidosis Reasons
The body uses oxygen for energy products and in the process makes carbon dioxide as a byproduct. The lungs remove this carbon dioxide through exhalation. However, if lungs fail to remove all of the carbon dioxide, then its levels in the bloodstream rises rapidly. Carbon dioxide then mixes with water present in the body and forms carbonic acid. Carbonic acid makes the body fluids very acidic.
Although the kidney is capable of removing these acidic compounds and therefore carbon dioxide, it is a very slow process when compared to passing out carbon dioxide via the lungs. The body has buffers which are chemicals that act to limit any drastic change in pH. However, the action of these buffers are limited and should the relevant elimination processes not restore a normal state,
The increase in carbon dioxide is measured in terms of pressure carbon dioxide (PaCo2). It is also called partial pressure because it is not the only gas in the blood or lungs, as oxygen and nitrogen are also abundant gases in air. Simply, an increased PaCO2 means more acidity, which is also measured in terms of p – a low pH signifies more acidity.
Respiratory Acidosis Types
Respiratory acidosis can be sub-divided into acute and chronic.
Acute respiratory acidosis
In acute respiratory acidosis, both PaCO2 and acidity are both higher. Acute respiratory acidosis occurs when ventilation through the lungs fails suddenly. Use of some drugs or various medical conditions (like cerebral or neuromuscular diseases) can cause acute respiratory acidosis. Airway blockage due to asthma can also result in such situation.
Chronic respiratory acidosis
In chronic respiratory acidosis, only the PaCO2 is higher. Chronic respiratory acidosis can result from many disorders, like obesity hypoventilation syndrome, neuromuscular disorders, and severe defects in ventilation.
Respiratory Acidosis Pathophysiology
Energy production and utilization is the cornerstone of the life process in the body. Nutrients are broken down into the simplest components, primarily into glucose, or converted into glucose, and then utilized by the cells along with oxygen to produce energy. Waste products and carbon dioxide is then formed as byproducts of these activities. Not only does it “power” the life processes, but it also contributes to growth and development. Broadly this is referred to as the metabolism.
The body’s metabolism generates a large quantity of carbon dioxide rapidly, which may combine with water to form carbonic acid (H2CO3). The lungs excrete carbon dioxide through ventilation, and prevents the accumulation of acidic compounds. If there is a problem in ventilation, the removal of carbon dioxide is affected. This can cause the carbon dioxide partial pressure to rise above the normal range of 35 – 45 mmHg. Ventilation is controlled by various factors like PaCO2, partial arterial pressure of oxygen or PaO2, and pH in the brain areas that regulate ventilation.
Should there be a rise in the carbon dioxide or drop in the oxygen levels, ventilation increases. This means that a person breathes faster and deeper to attempt to restore the normal levels of the gases, and ultimately restore the pH of the body fluids. While buffering does offer some relief in dealing with changes in pH and both the kidneys and lungs play central roles in relieving excess acidic compounds, it may at times not be enough to prevent drastic changes in the pH. Acidosis is one of the consequences of these disruptions.
Changes in the pH of the blood and body fluids can then affect the chemical structure of other crucial substances in the body. This in turn can compromise various chemical processes leading to a host of signs and symptoms and leading to complications if treatment is not instituted rapidly. Ultimately death is possible.
Respiratory Acidosis Symptoms
Respiratory acidosis often presents with the symptoms of the underlying disorders that cause respiratory acidosis. Symptoms may vary depending on the severity of the diseases and on the rate of increase in PaCO2. Mild to moderate and slow increase in PaCO2 shows minimal symptoms.
Following non-specific symptoms may be seen in patients suffering from respiratory acidosis:
- Anxiety
- Breathlessness
- Disturbed sleep
- Excessive daytime sleepiness
- Hallucinations
- Confusion
- Dullness
- Wheezing
- Increased chest expansion
- Decreased breath sounds
- Bluish skin
- Clubbing of fingers
- Depression
- Hand tremors
- Twitching
- Seizures
If left untreated, other symptoms may also become evident as the complications of respiratory acidosis arises.
Respiratory Acidosis Complications
Complications related to respiratory acidosis include chronic hypoxemia (severe oxygen deficiency). Due to chronic hypoxemia, more red blood cells are produced (polycythemia). Chronic hypoxemia can also cause narrowing of pulmonary arteries, which may increase blood pressure in the arteries of the lungs (pulmonary hypertension). This may result in heart failure. Sleep quality is often disturbed in patients with respiratory acidosis, which causes morning headaches, sleepiness, and daytime fatigue.
Respiratory Acidosis Causes
There is a wide range of causes of respiratory acidosis related to dysfunction in ventilation.
Lungs and airways
These are considered to be among the main causes of respiratory acidosis. The inability of the airways and lungs to carry out its normal functions hampers normal gas exchange, and therefore the acidity of the blood and body fluids.
- Chronic obstructive pulmonary disease (COPD) including chronic bronchitis and emphysema.
- Narrowing of the airways as is seen with asthma, tracheal and laryngeal stenosis.
- Obstructive sleep apnea
Muscles and Bones
The bones of of the chest cavity, primarily the ribs and sternum (breastbone), not only protect the lungs but also serve as site of attachment for the respiratory muscles. These muscles can then expand and contract the chest cavity to draw in air into the lungs or push it out.
- Neuromuscular diseases including amyotrophic lateral sclerosis, Guillain-Barré syndrome, muscular dystrophy, myasthenia gravis, botulism, diaphragm dysfunction and paralysis.
- Chest wall disorders such as severe kyphoscoliosis, flail chest, ankylosing spondylitis, pectus carinatum or pectus excavatum.
Nervous system
The nervous system plays an integral role in the ventilation. Centers in the brain keep monitoring the gas levels in the blood and ensure that ventilation is in line with the body’s oxygen demands. Nerves carry the signals from the brain to the muscles of respiration thereby controlling the rate of respiration.
- Central nervous system (CNS) depression due to drug abuse (benzodiazepines, narcotics, or barbiturates) and neurologic disorders (encephalitis, trauma).
- Neuropathy (injury or disease) of the phrenic or intercostal nerves.
Respiratory Acidosis Diagnosis
The following tests may be performed to detect respiratory acidosis:
- Drug screening: A drug screen can detect if some drugs (like barbiturates, opiates, and benzodiazepines) are causing respiratory acidosis.
- Imaging studies: X-ray, MRI or CT scan of chest may be helpful to detect obstruction in airways, weakness or paralysis of diaphragm, and skeletal deformities. CT or MRI scan can detect tumors or injuries in brain areas that control respiration.
- Lab tests: Blood samples are tested for bicarbonate level, serum electrolytes level, thyroid functions, and a complete blood cell (CBC) count.
- Pulmonary or lung function tests: These tests can determine the severity of disease.
- Nerve and muscle tests: Electromyography (EMG) and nerve conduction velocity (NCV) tests can detect neuromuscular disorders.
Respiratory Acidosis Treatment
Treatment depends on the underlying disorders. Supportive measures such as oxygen therapy or mechanical ventilation may at times be necessary irrespective of the cause.
Lifestyle measures
Weight loss may help patients with obesity hypoventilation syndrome.
Muscle or nerve pacing
In this technique, electrical currents are administered. Rhythmic electrical stimulation to diaphragm or phrenic nerve results in breathing.
Medications
- Bronchodilating agents like beta-agonists (salmeterol, albuterol), anti-cholinergic agents (tiotropium, ipratropium bromide), and methylxanthines (theophylline) help patients with severe bronchospasm (spasm of lung muscles) and obstructive lung disease.
- Beta-agonists increase ventilation by relaxing muscles of both small and large airways in the lungs.
- Anti-cholinergics also increase ventilation by decreasing constriction of bronchi and bronchioles of lungs.
- Theophylline also relaxes lung muscles but is given with caution due to its toxic side-effects.
- Respiratory stimulants (medroxyprogesterone, acetazolamide) promote ventilation. Some of these are corticosteroids, which act by decreasing inflammation. These drugs help patients who developed respiratory acidosis due to asthma.
- If respiratory acidosis is caused by over-dose of some sedating drugs, drug antagonists are used to reverse the effects. Drugs such as flumazenil reverses the effects of benzodiazepines. Naloxone reverses the effects of narcotics.
Bicarbonate infusion therapy
Sodium bicarbonate is given in rare cases, when patients show extremely low pH and cardiopulmonary arrest.
Oxygen therapy
Oxygen therapy can help patients with oxygen deficiency. It prevents the complications associated with prolonged oxygen deficiency.
Ventilation
Mechanical ventilation is done through nose or through a tube in trachea. Ventilation helps in decreasing the partial arterial pressure of carbon dioxide (PaCO2) and increasing the partial arterial pressure of oxygen (PaO2).
References :
http://www.nlm.nih.gov/medlineplus/ency/article/000092.htm
