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 dis- ease, 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 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 x 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.
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.
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.
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+].
BOOKS
Burtis, C.A., and E.R. Ashwood, eds. Fundamentals of Clinical Chemistry. 5th ed. Philadelphia: Saunders, 2001.
Marshall, William J. Clinical Chemistry 4th ed. 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>.
