Blood Gas Analysis Health Article

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 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 [P_O2] and carbon dioxide P_O2 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.