Acid base balance, respiratory and metabolic acidosis, causes, pathophysiology, symptoms, diagnosis and treatment.
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Acidosis refers to a process that causes increased acidity in the blood and body’s tissues.
The body’s blood pH is maintained by the balance between acids and bases. Acidosis occurs when acids accumulate or bases are lost. The major players are carbon dioxide and bicarbonate. Carbon dioxide acts as an acid as it combines with water to make carbonic acid, releasing hydrogen ions; while bicarbonate binds to and neutralizes hydrogen ions, acting as a base.
Normal cellular metabolism constantly produces and excretes carbon dioxide into the blood, and therefore constantly making the blood more acidic. The body keeps the blood pH within the normal range by 2 mechanisms: removal of carbon dioxide through exhalation by the lungs, and excretion of acids and RE-absorption of bicarbonate through the kidneys. Pulmonary regulation is fast, acting within minutes to hours. Renal regulation is slower, taking days to respond to pH changes.
There are 2 major types of acidosis: respiratory and metabolic.
Respiratory acidosis results from inadequate function of the lungs. As carbon dioxide is not exhaled fast enough, it accumulates in the blood, raising acidity. Respiratory acidosis is characterized by primary increase in arterial partial pressure of carbon dioxide. Compensatory increase in bicarbonate may or may not present; pH is usually low but may also be near normal. Common causes include chronic lung diseases, neuromuscular disorders that affect respiratory muscles, chest deformities or injuries, conditions that impair the central respiratory center, and overuse of sedative drugs. Some of these causes may lead to a fast-developing, or acute form, of the disease; while others result in a chronic, stable respiratory acidosis. The acute form can cause headache, confusion, drowsiness, stupor, or coma, due to its effects on the brain; while the chronic form may or may not have symptoms of its own. This is because the kidneys, over the course of several days, are generally able to compensate by increasing bicarbonate reabsorption.
Metabolic acidosis may result from excessive production of metabolic acids, ingestion of acids, decreased ability of the kidneys to excrete acids, or loss of alkali. Metabolic acidosis is characterized by primary decrease in plasma bicarbonate, secondary decrease in carbon dioxide partial pressure, and a low blood pH. Acute forms most frequently result from overproduction of metabolic acids such as ketoacids or lactic acid. Chronic forms are often caused by impaired renal function or loss of bicarbonate, such as in severe diarrhea. Apart from symptoms of the underlying condition, metabolic acidosis itself causes rapid breathing, because the body tries to correct acidity by expelling more carbon dioxide. Severe metabolic acidosis may lead to respiratory or cardiovascular failure.
Diagnosis of acidosis requires measurement of blood pH and carbon dioxide in an arterial blood (arterial blood gas, ABG, plus serum electrolyte measurements) sample, usually taken from the radial artery in the wrist. Arterial blood is taken because it is generally more reliable than venous blood for carbon dioxide measurement. Serum bicarbonate, ketoacids and lactic acid are tested to help identify the cause. Other tests include chest X-ray, abdominal CT scan, urinalysis, urine ketones, and urine pH.
Treatment aims at the cause. Improving ventilation is the mainstay for managing respiratory acidosis.