Blood Gas Analysis

Definition

Blood gas analysis, also called arterial blood gas (ABG) analysis, is a procedure to measure the partial pressure of oxygen (O₂) and carbon dioxide (CO₂) gases and the pH (hydrogen ion concentration) in arterial blood.

Purpose

Blood gas analysis is used to diagnose and evaluate respiratory diseases and conditions that influence how effectively the lungs deliver oxygen to and eliminate carbon dioxide from the blood. The acid-base component of the test is used to diagnose and evaluate metabolic conditions that cause abnormal blood pH.

Because high concentrations of inhaled oxygen can be toxic and can damage lungs and eyes, repeated blood gas analysis is especially useful for monitoring patients on oxygen, for example, premature infants with lung disease, so that the lowest possible inhaled oxygen concentration can be used to maintain the blood oxygen pressure at a level that supports the patient. In intubated patients under artificial ventilation, monitoring the levels of arterial carbon dioxide and oxygen allow assessment of respiratory adequacy so that the rate or depth of ventilation, the ventilator dead space, or airway pressure can be changed to preserve the patient's optimal physiologic balance.

The measurement of arterial blood pH and carbon dioxide pressure with subsequent calculation of the concentration of bicarbonate (HCO₃⁻), especially in combination with analysis of serum electrolytes, aids in the diagnosis of many diseases. For example, diabetes mellitus is often associated with a condition known as diabetic acidosis. Insulin deficiency often results in the excessive production of ketoacids and lactic acid that lower extracellular fluid and blood pH. Unabated acid-base disorders are life threatening. Acidosis is associated with severe consequences, including shock and cardiac arrest, and alkalosis with mental confusion and coma.

Precautions

The syringe used to collect the sample for a blood gas analysis must contain a small amount of heparin to prevent clotting of the blood. It is very important that air be excluded from the syringe both before and after the sample is collected. The syringe must be filled completely and never exposed to air. For transportation, the syringe should be capped with a blind hub, placed on ice, and immediately sent to the laboratory for analysis to guarantee the accuracy of the results.

A blood gas analysis requires a sample of arterial blood in order to evaluate gas exchange by the lungs. Arterial puncture is associated with a greater risk of bleeding than venipuncture. The test may be contraindicated in persons with a bleeding disorder such as hemophilia or low platelet count. During the arterial puncture, the patient may feel a brief throbbing or cramping at the puncture site. In cases where the primary concern is ascertaining that the blood is adequately oxygenated, a pulse oximeter may be used in lieu of arterial blood gas analysis. Medical personnel must follow standard precautions for prevention of exposure to bloodborne pathogens when performing arterial blood collection.

Description

The sample of choice for blood gas analysis is arterial blood. This is usually collected from the radial artery in the wrist, but in cases where no radial pulse is obtained, the femoral or brachial artery may be used. The sample may also be collected from an arterial line after flushing the line to remove excess anticoagulant and fluid. In neonates and in adults when arterial puncture is contraindicated or unsuccessful, a capillary blood sample may be used.

The sample is inserted into an analytical instrument that uses electrodes to measure the concentration of hydrogen ions (H⁺), which is reported as pH, and the partial pressures of oxygen [Po₂)] and carbon dioxide Po₂) gases. The pH-measuring electrode consists of a special glass membrane that is selectively permeable to hydrogen ions. An electical potential develops across the inner and outer surfaces of this membrane that is related to the log of hydrogen ion activity in the sample. A Severinghaus electrode is used to measure Pco₂. The measuring principle is the same as for hydrogen ions, except that the electrode tip is covered with a gas permeable membrane, so that the pH change is proportional to carbon dioxide diffusing from the sample to the electrode surface. The Po₂ is measured using a polarographic (Clark) electrode. Oxygen diffuses from the sample to the cathode, where it is reduced to peroxide ions. The electrons come from a silver anode that is oxidized, generating current in proportion to oxygen concentration at the cathode. Electrode signals are dependent upon temperature as well as concentration, and all measurements are performed at 98.6°F (37°C). Since the in vivo pH and levels of oxygen and carbon dioxide are temperature dependent, results may need to be adjusted for the patient's actual temperature. Portable blood gas analyzers are available that can be used at the bedside.

Blood gas analyzers calculate blood bicarbonate concentration using the formula: pH = 6.1 + Log bicarbonate/.0306 × Pco₂. They also calculate oxygen content, total carbon dioxide, base excess, and percent oxygen saturation of hemoglobin. These values are used by physicians to assess the extent of hypoxia and acid-base imbalance.

Preparation

Patients do not need to restrict food or drink before the test. For patients receiving oxygen therapy, the oxygen concentration must remain constant for 20 minutes before sample collection; if the test is specifically ordered to be without oxygen, the gas must be turned off for 20 minutes before the blood sample is taken to guarantee accurate test results. The patient should breathe normally during sample collection.

Infants and children may require physical and psychological preparation appropriate to the child's age. A parent or other trusted adult may be enlisted to restrain the child during sample collection.

Aftercare

After the blood sample has been taken, the health care practitioner or patient applies pressure to the puncture site for about 10 minutes or until bleeding has stopped, after which a dressing is applied. The patient should rest quietly while applying pressure to the puncture site and be observed for signs of bleeding or impaired circulation at the puncture site.

Complications

Complications posed by the arterial puncture are minimal when the procedure is performed correctly, but may include bleeding or delayed bleeding or bruising at the puncture site, or, rarely, impaired circulation around the puncture site.

Results

Normal values

The following results are for arterial blood at sea level (at altitudes of 3,000 feet and above, the values for oxygen are lower).

• Partial pressure of oxygen (Po₂): 75-100 millimeters of mercury (mm Hg). Note that Po₂ values normally decline with age.
• Partial pressure of carbon dioxide Pco₂: 35-45 mm Hg.
• pH: 7.35-7.45.
• Oxygen content (O₂CT): 15-23 volume%.
• Oxygen saturation (SaO₂): 94-100%.
• Concentration of bicarbonate(HCO₃⁻): 22-26 millimols per liter (mEq/liter).

Total CO₂ is often reported with blood gas analysis results and is defined as the sum of carbonic acid and bicarbonate concentrations. Normally, the ratio of bicarbonate to carbonic acid at physiological pH is about 20:1, thus, the total CO₂ is normally about 5% higher than the bicarbonate value.

The A-a gradient (alveolar-arterial Po₂ difference) is calculated from the partial pressures of oxygen and carbon dioxide as returned from the blood gas analysis, and the partial pressure of oxygen in the air and a factor called the respiratory quotient that are specific to the site of the test. A normal value for A-a gradient may be estimated as one-fourth the patient's age plus 2.5.

Abnormal results

Values that differ from the normal values may indicate the presence of respiratory, metabolic, or renal diseases.

For most clinical decisions, the bicarbonate value, Pco₂, and pH are used to evaluate acid-base status. The pH value defines the magnitude of the disturbance and the bicarbonate and Pco₂ determine the cause. The bicarbonate level is under the control of the kidneys, which may increase or decrease bicarbonate blood levels in response to pH changes. Bicarbonate is also the principal blood buffer anion, and it functions as the conjugate base to increase pH. Pco₂ is the respiratory component because it is regulated by the lungs. It is determined by the concentration of dissolved carbon dioxide (anhydrous carbonic acid) and is the principal acid component of the blood. Abnormal results are classified on the basis of pH and whether the abnormal pH is caused by the metabolic or respiratory component. pH < 7.35 indicates acidosis, either metabolic (non-respiratory) or respiratory, and pH > 7.45 indicates alkalosis.

Metabolic or non-respiratory acidosis is characterized by pH < 7.35 (i.e. increased [H⁺]) and decreased [HCO₃⁻]. In most cases, the decrease in pH stimulates the respiratory center causing hyperventilation. The loss of carbon dioxide that results serves to decrease the severity of the acidosis and is referred to as compensation. Metabolic acidosis is caused by bicarbonate deficit which may result from increased H⁺ formation or ingestion, from decreased H⁺ excretion, or failure to produce or retain bicarbonate. Common causes of metabolic acidosis are diabetes mellitus, alcoholism, lactic acidosis (associated with hypoxia), acid poisoning, renal failure, renal tubular acidosis (an inherited defect of the renal tubules), and diarrhea.

Respiratory acidosis is caused by deficient ventilation that results in retention of carbon dioxide. The pH is <7.35, and Pco₂ is increased. If time has permitted renal compensation, the [HCO₃⁻] is somewhat increased. Respiratory acidosis is associated with airway obstruction such as occurs with asthmatic bronchial spasm, bronchitis, and emphysema; pulmonary diseases such as severe pneumonia and pulmonary fibrosis; thoracic conditions such as multiple broken ribs and kyphoscoliosis. Respiratory acidosis is also caused by neuromuscular disease, and by depression of the respiratory center in the brain due to drugs, head trauma, or cranial tumor. The blood gas analysis results may deviate only slightly from normal values, and pH may even fall within the normal range (compensated respiratory acidosis) in cases of chronic compared to acute acidosis.

Metabolic alkalosis is caused by excess blood bicarbonate and usually involves a renal factor. Metabolic alkalosis is characterized by pH > 7.45 and elevated [HCO₃⁻]. The Pco₂ is usually elevated due to respiratory compensation. Metabolic alkalosis can be caused by mineralcorticoid excess (e.g. Cushing's or Conn's syndromes), which promotes increased acid excretion and bicarbonate retention by the kidney. Other causes are diuretic therapy, vomiting, severe dehydration, hypokalemia (low blood potassium), and hypoparathyroidism.

Respiratory alkalosis is caused by hyperventilation. The pH is >7.45 and the Pco₂ is low. If the kidneys are functioning normally and given sufficient time, the HCO₃⁻ will be decreased in compensation. Respiratory alkalosis may be caused by hyperventilation psychologically induced (anxiety), by drugs that stimulate the respiratory center, excessive ventilation therapy, and mild hypoxia.

A decrease in Po₂ is a sensitive measure of respiratory function and hypoxia. In addition to ventilation defects that also result in increased Pco₂, Po₂ will be low in persons with poor ratios of ventilation to perfusion; mild emphysema and other gas diffusion defects; pulmonary arterial-venous shunts; and those breathing air with a low oxygen content. Elevated Po₂ is caused by excessive administration of oxygen which can lead to optic nerve damage and acidosis by displacing hydrogen ions from hemoglobin.

It is important to note that in cases of carbon monoxide poisoning the Po₂ will be normal, but life-threatening hypoxia may be present. Blood gas analyzers calculate the oxygen saturation of hemoglobin from Po₂, temperature, and pH. In cases of CO poisoning, the calculation will be falsely elevated. Accurate assessment of hypoxia in CO poisoning requires direct measurements of carboxyhemoglobin and oxygen saturation of hemoglobin by oximetry or colorimetry methods.

KEY TERMS

Acid— A chemical compound that reacts with a base to form a salt, that can give off hydrogen ions in water solution, or that contains an atom that can accept a pair of electrons from a base.

Acidosis— A blood condition in which the pH is < 7.35 and the bicarbonate concentration is below normal.

Alkalosis— A blood condition in which the pH is > 7.45 and the bicarbonate concentration is above normal.

Base— A chemical compound that reacts with an acid to form a salt, that takes up or accepts protons, or that contains an atom with a free pair of electrons to be donated to an acid.

Buffer— A chemical substance that resists changes in pH in response to changes in acid and base concentration; a buffer system consists of a weak acid or weak base in combination with its salt.

Hemoglobin— The red-colored, iron-containing protein in red blood cells that carries oxygen to the tissues.

Heparin— A biochemical that may be isolated from various animal tissues that has anticoagulant properties.

Ketoacidosis— An excessive level of acid accompanied by an increase in the level of ketones in blood that occurs as a complication of diabetes mellitus; ketones are substances normally processed by the liver from fats.

Oxygen saturation of hemoglobin— The percentage of hemoglobin that is bound to oxygen.

pH— An exponential measurement scale for expressing the concentration of acid in solution pH = −log [H⁺].

Health care team roles

A physician, nurse, respiratory care technician, or laboratory technician collects the blood sample by arterial puncture and sees to the timely and appropriate transport to the laboratory for analysis. A member of the health care team should observe the patient for 10-15 minutes to ensure that bleeding from the puncture site has stopped. Blood gas measurements are performed by a registered respiratory therapist, RRT; certified respiratory technician, CRTT; clinical laboratory scientist CLS(NCA) or medical technologist MT(ASCP); clinical laboratory technician CLT(NCA) or medical laboratory technician MLT(ASCP). A physician interprets the blood gas analysis results with a thorough understanding of the acid-base chemistry and physiology of blood and in view of the clinical situation, and applies the results to the diagnosis, treatment, and management of the patient.

Resources

BOOKS

Burtis, C.A., and E.R. Ashwood, eds. Fundamentals of Clinical Chemistry. 5th ed. Philadelphia: Saunders, 2001.

Marshall, William J. Clinical Chemistry 4thed. Edinburgh, London, New York, Philadelphia, St. Louis, and Toronto: Mosby, 2000.

OTHER

Argyle, B. Mad Scientist Software, Blood Gases Computer Program Manual. 1996 Mad Scientist Software, Alpine UT. 〈http://www.madsci.com/manu/gas_acid.htm〉.

Clinical Web Server 2. University of Kansas Medical Center. 〈http://clinweb2.kumc.edu/〉.

HealthCentral website. 1998 A.D.A.M. Software, Inc. «http://www.healthcentral.com/mhc/top/003855.cfm».

Blood Gas Analysis

Definition

Blood gas analysis, also called arterial blood gas (ABG) analysis, is a test which measures the amounts of oxygen and carbon dioxide in the blood, as well as the acidity (pH) of the blood.

Purpose

An ABG analysis evaluates how effectively the lungs are delivering oxygen to the blood and how efficiently they are eliminating carbon dioxide from it. The test also indicates how well the lungs and kidneys are interacting to maintain normal blood pH (acid-base balance). Blood gas studies are usually done to assess respiratory disease and other conditions that may affect the lungs, and to manage patients receiving oxygen therapy (respiratory therapy). In addition, the acid-base component of the test provides information on kidney function.

Description

Blood gas analysis is performed on blood from an artery. It measures the partial pressures of oxygen and carbon dioxide in the blood, as well as oxygen content, oxygen saturation, bicarbonate content, and blood pH.

Oxygen in the lungs is carried to the tissues through the bloodstream, but only a small amount of this oxygen can actually dissolve in arterial blood. How much dissolves depends on the partial pressure of the oxygen (the pressure that the gas exerts on the walls of the arteries). Therefore, testing the partial pressure of oxygen is actually measuring how much oxygen the lungs are delivering to the blood. Carbon dioxide is released into the blood as a by-product of cell metabolism. The partial carbon dioxide pressure indicates how well the lungs are eliminating this carbon dioxide.

The remainder of oxygen that is not dissolved in the blood combines with hemoglobin, a protein—iron compound found in the red blood cells. The oxygen content measurement in an ABG analysis indicates how much oxygen is combined with the hemoglobin. A related value is the oxygen saturation, which compares the amount of oxygen actually combined with hemoglobin to the total amount of oxygen that the hemoglobin is capable of combining with.

KEY TERMS

Acid-base balance— The condition that exists when the body's carbonic acid-bicarbonate buffer system is in equilibrium, helping to maintain the blood pH at a normal level of 7.35-7.45.

Hemoglobin— A protein—iron compound in red blood cells that functions primarily in carrying oxygen from the lungs to the tissues of the body.

pH— A measure of the acidity of a solution. Normal blood pH ranges from 7.35-7.45.

Carbon dioxide dissolves more readily in the blood than oxygen does, primarily forming bicarbonate and smaller amounts of carbonic acid. When present in normal amounts, the ratio of carbonic acid to bicarbonate creates an acid-base balance in the blood, helping to keep the pH at a level where the body's cellular functions are most efficient. The lungs and kidneys both participate in maintaining the carbonic acid-bicarbonate balance. The lungs control the carbonic acid level and the kidneys regulate the bicarbonate. If either organ is not functioning properly, an acid-base imbalance can result. Determination of bicarbonate and pH levels, then, aids in diagnosing the cause of abnormal blood gas values.

The procedure

The blood sample is obtained by arterial puncture (usually in the wrist, although it could be in the groin or arm) or from an arterial line already in place. If a puncture is needed, the skin over the artery is cleaned with an antiseptic. A technician then collects the blood with a small sterile needle attached to a disposable syringe. The patient may feel a brief throbbing or cramping at the site of the puncture. After the blood is drawn, the sample must be transported to the laboratory as soon as possible for analysis.

Preparation

There are no special preparations. Patients have no restrictions on drinking or eating before the test. If the patient is receiving oxygen, the oxygen concentration must remain the same for 20 minutes before the test; if the test is to be taken without oxygen, the gas must be turned off for 20 minutes before the test is taken. The patient should breathe normally during the test.

Aftercare

After the blood has been taken, the technician or the patient applies pressure to the puncture site for 10-15 minutes to stop the bleeding, and then places a dressing over the puncture.The patient should rest quietly while applying the pressure to the puncture site. Health care workers will observe the patient for signs of bleeding or circulation problems

Risks

Risks are very low when the test is done correctly. Risks include bleeding or bruising at the site, or delayed bleeding from the site. Very rarely, there may be a problem with circulation in the puncture area.

Normal results

Normal blood gas values are as follows:

• partial pressure of oxygen (PaO₂): 75-100 mm Hg
• partial pressure of carbon dioxide (PaCO₂): 35-45 mm Hg
• oxygen content (O₂CT): 15-23%
• oxygen saturation (SaO₂): 94-100%
• bicarbonate (HCO₃): 22-26 mEq/liter
• pH: 7.35-7.45

Abnormal results

Values that differ from those listed above may indicate respiratory, metabolic, or kidney disease. These results also may be abnormal if the patient has experienced trauma that may affect breathing (especially head and neck injuries). Disorders, such as anemia, that affect the oxygen-carrying capacity of blood, can produce an abnormally low oxygen content value.

Resources

BOOKS

Thompson, June, et al. Mosby's Clinical Nursing. 4th ed. St. Louis: Mosby, 1997.

Blood gas analysis

Blood gas analysis, also called arterial blood gas (ABG) analysis, is a means of determining the amount of oxygen or carbon dioxide being carried in the blood, and in some cases, of discovering the identity of a toxic gas, such as carbon monoxide, that may be present. In addition, the determination can be made as to whether the blood is too acidic or too alkaline, which may help the physician in his/her diagnosis.

An ABG analysis evaluates how effectively the lungs are delivering oxygen to the blood and how efficiently they are eliminating carbon dioxide from it. The test also indicates how well the lungs and kidneys are interacting to maintain normal blood pH (acid-base balance). Blood gas studies are usually performed to assess respiratory disease and other conditions that may affect the lungs, and to manage patients receiving oxygen therapy. In addition, the acid-base component of the test provides information on kidney function.

Among other functions, blood carries oxygen to the body’s tissues and removes carbon dioxide. The blood laden with carbon dioxide passes through the right side of the heart into the lungs and exchanges the carbon dioxide for fresh oxygen. The oxygenated blood is then pumped by the left side of the heart out into the body to repeat the cycle.

The red blood cells, or erythrocytes, carry the blood gases. Hemoglobin, the substance that gives blood its red color, is the molecular substance in the erythrocyte that attaches to oxygen and exchanges it for carbon dioxide.

Carbon monoxide, a colorless, very toxic gas, can displace oxygen in the bloodstream. Hemoglobin has approximately 12 times the affinity for carbon monoxide as it does for oxygen, so it will pick up carbon monoxide if both gases are present. This means that, in this case, the body does not get the oxygen it needs, and death will eventually occur.

Testing blood gases is a means to determine whether an acid-base (biochemical) disturbance is of respiratory or metabolic origin. Respiratory conditions such as pneumonia, bronchitis, emphysema, or severe asthma can cause the blood to become more acid. Respiratory conditions such as aspirin toxicity, strenuous exercise, fever, or overactive thyroid can cause the blood to be more alkaline. Kidney failure, burns, heart attack, or starvation are the metabolic reasons for the blood to become acid, and liver failure, vomiting, ulcer, or cystic fibrosis cause metabolic alkaline blood.

To perform an ABG analysis, blood is obtained by arterial puncture (usually in the wrist, although it could be in the groin or arm) or from an arterial line already in place. A technician then collects the blood with a small sterile needle attached to a disposable syringe. After the blood is drawn, the sample must be transported to the laboratory as soon as possible for analysis.

When the blood has been drawn, the technician or the patient applies pressure to the puncture site for 10 to 15 minutes to stop the bleeding, and then places a dressing over the puncture. Risks are very low when the test is performed correctly, but common concerns include bleeding or bruising at the site.

Normal blood gas values are as follows:

• Partial pressure of oxygen (PaO₂): 10 to 13 kilopascals (75 to 100 millimeters of mercury),
• Partial pressure of carbon dioxide (PaCO₂): 5 to 6 kilopascals (35 to 45 millimeters of mercury),
• Oxygen content (O₂ CT): 15% to 23%,
• Oxygen saturation (SaO₂): 94% to 100%,
• Bicarbonate (HCO₃): 22 to 26 milliequivalents/liter, where one milliequivalent is the mass in grams of a substance that will react with 6.022 × 10²³ electrons and milli denotes that the equivalent is divided by 1,000, and
• pH: 7.35 to 7.45.

Values that differ from those listed above may indicate respiratory, metabolic, or kidney disease. These results also may be abnormal if the patient has experienced trauma that affects breathing (especially head and neck injuries). Disorders such as anemia that affect the oxygen-carrying capacity of blood, can produce an abnormally low oxygen content value.

See also Bronchitis; Circulatory system.

Blood gas analysis

Blood gas analysis is a means of determining the amount of oxygen or carbon dioxide being carried in the blood, and in some cases, of discovering the identity of a toxic gas, such as carbon monoxide , that may be present. Also, the determination can be made as to whether the blood is too acidic or too alkaline, which may help the physician in his diagnosis .

Among other functions, blood carries oxygen to the body's tissues and removes carbon dioxide. The blood laden with carbon dioxide passes through the right side of the heart into the lungs and exchanges the carbon dioxide for fresh oxygen. The oxygenated blood is then pumped by the left side of the heart out into the body to repeat the cycle.

The red blood cells, or erythrocytes, carry the blood gases. Hemoglobin, the substance that gives blood its red color , is the molecular substance in the erythrocyte that attaches to oxygen and exchanges it for carbon dioxide.

Carbon monoxide, a colorless, very toxic gas, can displace oxygen in the bloodstream. Hemoglobin has approximately 12 times the affinity for carbon monoxide as it does for oxygen, so it will pick up carbon monoxide if both gases are present. This means that the body does not get the oxygen it needs, and eventually death will occur.

Testing blood gases is a means to determine whether an acid-base (biochemical) disturbance is of respiratory or metabolic origin. Respiratory conditions such as pneumonia , bronchitis , emphysema , or severe asthma can cause the blood to become more acid. Respiratory conditions such as aspirin toxicity, strenuous exercise , fever, or overactive thyroid can cause the blood to be more alkaline. Kidney failure, burns, heart attack, or starvation are the metabolic reasons for the blood to become acid, and liver failure, vomiting, ulcer, or cystic fibrosis cause metabolic alkaline blood.

To determine blood gases, the blood specimen must be taken from an artery (usual blood specimens are taken from a vein) and the blood specimen is placed in ice to prevent any changes in blood gases, and rushed to the laboratory for analysis.

See also Bronchitis; Circulatory system.