Arterial Blood Gas (ABG) Analysis
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.
Blood gases test The blood gases test is performed by collecting a sample of blood through a needle from an artery. The test is used to evaluate respiratory diseases and conditions that affect the lungs, and it is used to determine the effectiveness of oxygen therapy. The acid-base component of the test also gives information on how well the kidneys are functioning.
The Six Steps to ABG Analysis In order for our analysis to be effective, notes will have to be written next to the results on our lab slip. Alternately, the ABG results can be transcribed onto another paper for analysis (see example one below). 1.
The first step in analyzing ABGs is to look at the pH. Normal blood pH is 7.4, plus or minus 0.05, forming the range 7.35 to 7.45. If blood pH falls below 7.35 it is acidic. If blood pH raises above 7.45, it is alkalotic. If it falls into the normal range, label what side of 7.4 it falls on. Lower than 7.4 is normal/acidic, higher than 7.4 is normal/alkalotic. Label it.
The second step is to examine the pCO2. Normal pCO2 levels are 35-45mmHg. Below 35 is alkalotic, above 45 is acidic. Label it.
The third step is to look at the HCO3 level. A normal HCO3 level is 22-26 mEq/L. If the HCO3 is below 22, the patient is acidotic. If the HCO3 is above 26, the patient is alkalotic. Label it.
Next match either the pCO2 or the HCO3 with the pH to determine the acid-base disorder. For example, if the pH is acidotic, and the CO2 is acidotic, then the acid-base disturbance is being caused by the respiratory system. Therefore, we call it a respiratory acidosis. However, if the pH is alkalotic and the HCO3 is alkalotic, the acid-base disturbance is being caused by the metabolic (or renal) system. Therefore, it will be a metabolic alkalosis.
Fifth, does either the CO2 or HCO3 go in the opposite direction of the pH? If so, there is compensation by that system. For example, the pH is acidotic, the CO2 is acidotic, and the HCO3 is alkalotic. The CO2 matches the pH making the primary acid-base disorder respiratory acidosis. The HCO3 is opposite of the pH and would be evidence of compensation from the metabolic system.
Finally, evaluate the PaO2 and O2 sat. If they are below limits there is evidence of hypoxemia.
Interpreting an arterial blood gas report Arterial blood gases are one of the first tests done on any critically ill patient, because the results give useful information about A--oxygenation, B--ventilation, and C--perfusion. Analysis of arterial blood gas has five steps:
• Look at pH. This tells you the primary acid-base abnormality. The body never overcompensates
• Next, look at the arterial carbon dioxide tension (Paco2) and standard bicarbonate to find out if this is a respiratory or metabolic problem
• Check the appropriateness of compensation. For example, in a severe metabolic acidosis, you would expect the Paco2 to be low as a result. If this is normal, it means there is a hidden respiratory acidosis as well
• Measure the anion gap in any metabolic acidosis
• Finally look at the Pao2 in the context of the inspired oxygen concentration.
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
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.
Arterial Blood Gas (ABG) Analysis.pdf
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