22/10/2017
Acid-Base Disorders
What are acidosis and alkalosis?
Acidosis and alkalosis describe the abnormal conditions that result from an imbalance in the pH of the blood caused by an excess of acid or alkali (base). This imbalance is typically caused by some underlying condition or disease.
Normal blood pH must be maintained within a narrow range of 7.35-7.45 to ensure the proper functioning of metabolic processes and the delivery of the right amount of oxygen to tissues. Acidosis refers to an excess of acid in the blood that causes the pH to fall below 7.35, and alkalosis refers to an excess of base in the blood that causes the pH to rise above 7.45. Many conditions and diseases can interfere with pH control in the body and cause a person's blood pH to fall outside of healthy limits.
Normal body functions and metabolism generate large quantities of acids that must be neutralized and/or eliminated to maintain blood pH balance. Most of the acid is carbonic acid, which is created from carbon dioxide (CO2) and water. Lesser quantities of lactic acid, ketoacids, and other organic acids are also produced.
The lungs and kidneys are the major organs involved in regulating blood pH.
The lungs flush acid out of the body by exhaling CO2. Raising and lowering the respiratory rate alters the amount of CO2 that is breathed out, and this can affect blood pH within minutes.
The kidneys excrete acids in the urine, and they regulate the concentration of bicarbonate (HCO3-, a base) in blood. Acid-base changes due to increases or decreases in HCO3- concentration occur more slowly than changes in CO2, taking hours or days.
Both of these processes are always at work, and they keep the blood pH in healthy people tightly controlled.
Buffering systems that resist changes in pH also contribute to the regulation of acid and base concentrations. The main buffers in blood are hemoglobin (in red blood cells), plasma proteins, bicarbonate, and phosphates.
The absolute quantities of acids or bases are less important than the balance between the two and its effect on blood pH (see Figure 1, below).
Acidosis occurs when blood pH falls below 7.35. It can be due to:
Increased acid production within the body
Consumption of substances that are metabolized to acids
Decreased acid excretion
Increased excretion of base
Alkalosis occurs when blood pH rises above 7.45. It can be due to:
Electrolyte disturbances caused by, for example, prolonged vomiting or severe dehydration
Administration or consumption of base
Hyperventilation (with increased excretion of acid in the form of CO2)
Any disease or condition that affects the lungs, kidneys, metabolism or breathing has the potential to cause acidosis or alkalosis. The normal balance between acid and base can be visualized in Figure 1.
Illustration of Acid-Base Balance
Figure 1: Faucets and Drains
Important points:
The blood's pH is normally between 7.35 and 7.45.
The body's goal is a constant balance between incoming/produced acids and bases (faucet on) and eliminated acids and bases (drain open).
Imbalances lead to acidosis (acid sink overflow) or alkalosis (base sink overflow).
Balance can be restored by increasing elimination (faster draining) and/or by decreasing flow (slowing down drippy faucet).
Acid-base disorders are divided into two broad categories:
Those that affect respiration and cause changes in CO2 concentration are called respiratory acidosis (low pH) and respiratory alkalosis (high pH). Respiratory acid-base disorders are commonly due to lung diseases or conditions that affect normal breathing.
Disorders that affect HCO3- concentration are called metabolic acidosis (low pH) and metabolic alkalosis (high pH). Metabolic acid-base disorders may be due to kidney disease, electrolyte disturbances, severe vomiting or diarrhea, ingestion of certain drugs and toxins, and diseases that affect normal metabolism (e.g., diabetes).
Symptoms
Acidosis may not cause any symptoms or it may be associated with nonspecific symptoms such as fatigue, nausea, and vomiting. Acute acidosis may also cause an increased rate and depth of breathing, confusion, and headaches, and it can lead to seizures, coma, and in some cases death.
Symptoms of alkalosis are often due to associated potassium (K+) loss and may include irritability, weakness, and cramping.
Common Causes of Acid-Base Disorders
Respiratory acidosis
Reduced CO2 elimination
Decreased breathing rate (respiratory drive) due to drugs or central nervous system disorders
Impaired breathing and lung movement (respiratory mechanics) due, for example, to trauma or abnormal presence of air between the lung and the wall of the chest (pneumothorax)
Respiratory muscle/nerve disease (myasthenia gravis, botulism, amyotrophic lateral sclerosis (ALS), Guillain-Barre syndrome)
Airway obstruction (food or foreign object)
Lung disease
Respiratory alkalosis
Increased CO2 elimination
Hyperventilation due to anxiety, pain, shock
Drugs (aspirin, salicylates)
Pneumonia, pulmonary (lung) congestion, or embolism
Exercise
Fever
Central nervous system tumor, trauma, infection (meningitis, encephalitis)
Liver failure
Metabolic acidosis
Decreased HCO3-, due to increased acid or loss of bicarbonate
Alcoholic ketoacidosis
Diabetic ketoacidosis
Kidney failure
Lactic acidosis
Toxins – overdose of salicylates (aspirin), methanol, ethylene glycol
Gastrointestinal bicarbonate loss, such as from prolonged diarrhea
Renal bicarbonate loss
Metabolic alkalosis
Increased HCO3-, due to loss of acid or gain of bicarbonate
Diuretics
Prolonged vomiting
Severe dehydration
Diseases that cause loss of potassium
Administration of bicarbonate, ingestion of alkali
Tests
The goals of testing are to identify whether an individual has an acid-base imbalance, to determine how severe the imbalance is, and to help diagnose underlying diseases or conditions (such as diabetic ketoacidosis or the ingestion of a toxin) that may have caused the acid-base disturbance. Testing is also done to monitor critically ill people as well as those with conditions known to affect acid-base balance, such as chronic lung disease and kidney disease.
The primary tests used to identify, evaluate, and monitor acid-base imbalances are:
Blood gases
Electrolytes
Blood gases are a group of tests performed together, usually on an arterial blood sample (blood obtained from an artery instead of a vein). They are a snapshot of the blood's pH, pO2 (the amount of oxygen in the blood), and pCO2 (the amount of carbon dioxide the blood). From these results, bicarbonate (HCO3-) can be calculated.
In most acid-base disorders, both HCO3- and pCO2 are outside the reference range. That is because the body tries to keep the pH close to normal. Therefore, if one of these substances is abnormally high or low, the other one will change in order to bring the pH back toward its healthy range. This process is called compensation. The table below shows the expected values for each of the four main acid-base disorders.
Results seen in the four primary acid-base disorders:
ACID-BASE DISORDER pH HCO3- PCO2 BODY COMPENSATION
Metabolic acidosis Less than 7.35 Low Low Increased breathing rate (hyperventilation) to increase CO2 elimination
Metabolic alkalosis Greater than 7.45 High High Slowed breathing (hypoventilation) to decrease CO2 elimination
Respiratory acidosis Less than 7.35 High High Kidney increases retention of HCO3- and excretion of acid
Respiratory alkalosis Greater than 7.45 Low Low Kidney decreases retention of HCO3- and excretion of acid
Electrolytes refers to a group of four tests: Na+ (sodium), K+ (potassium), Cl- (chloride), and bicarbonate (usually measured as total CO2 content). Body fluid levels, electrolyte concentrations, and acid-base balance are interconnected, and one or more of the electrolytes is usually increased or decreased in metabolic acid-base disorders.
In a person with a metabolic acidosis, the anion gap is calculated using the results of an electrolyte panel to help determine which disorders might be responsible for the acid-base abnormality. For example, an increase in the anion gap can indicate diabetic ketoacidosis. (For more on anion gap, see Common Questions #1 here.)
People with a metabolic alkalosis usually have low chloride (Cl-) and potassium (K+) values, which again provides clues as to the cause of the acid-base disturbance.
Based on these results, other tests may be ordered to diagnose the disease or condition that is producing the acidosis or alkalosis. Some examples include:
Glucose—to detect and/or monitor diabetes
Lactate—a high level indicates lactic acidosis
Ketones—a high level indicates ketoacidosis
Osmolality—evaluates water balance and may be used to detect a low sodium level or toxins such as methanol and ethylene glycol
Emergency overdose and drug testing may detect one of several drugs or toxins
Complete blood count (CBC)—this is a group of general tests, but a high white blood cell (WBC) count may indicate sepsis
Urinalysis—another general test; a low or high urine pH may provide clues to the cause of acidosis or alkalosis
It is possible for a person to have more than one acid-base disturbance at the same time. Examples include ingestion of aspirin (which can produce both a respiratory alkalosis and metabolic acidosis) and those with lung disease who are taking diuretics (respiratory acidosis plus metabolic alkalosis). By using formulas to calculate the expected degree of compensation, it is possible to determine if a mixed acid-base disorder is present.
