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Course Sample:
Introduction and Purpose
The purpose of this course is to provide the nurse with basic information on arterial blood gases, or ABGs. This course will review the purpose of the test as well as how to obtain a sample. This course will spend the majority of content on the interpretation of the results of a patient's arterial blood gas sample and will review how to interpret the results focusing on the four main alterations: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. For each of these main alterations, causes as well as treatments are touched upon; however, the main purpose of the course focuses on correct interpretation, not treatment. Additionally, samples will be provided that demonstrate the concepts discussed. Finally, the post-test will contain samples for interpretation.
Arterial blood gas sampling is a common diagnostic test the purpose of which is to assess respiratory functioning. As the name implies, arterial blood gas (ABG) sampling uses arterial blood to establish both the pH of the blood and the pressures exerted by oxygen and carbon dioxide in the blood ( Taylor, Lillis, & LeMone, 2001). Examination of an ABG will allow the nurse to assess the sufficiency of not only oxygenation, but also of perfusion and ventilation. Pulmonary indications are the main reasons for the ABG. However, acid-base imbalances and kidney diseases can also be identified and monitored via the ABG.
Obtaining an ABG
Arterial blood gas samples can be drawn directly from an artery with the radial, brachial, or femoral arteries being the most common sites. Samples can also be drawn through an existing arterial line per institution specific policy. Samples should only be drawn into prepackaged, specific ABG syringes that are used only for ABG sampling. These syringes are either packaged with a small amount of heparin to prevent coagulation, or will need to be heparinized before use (Wikipedia).
While both the brachial and femoral arteries may be used, the radial artery is typically the site of choice. The reason for this is two-fold. First of all, the radial artery is generally closer to the surface and more easily palpated. Furthermore the radial artery has collateral supply from the ulnar artery. During arterial blood gas sampling, there is a risk of bleeding, hematoma formation, obstruction, or clotting off of the artery. Any of these complications would compromise or completely limit blood flow to distal tissues. Therefore, having an adequate collateral supply of blood, such as is found in the radial/ulnar pairing, is ideal (Williams, 1998).
Collateral circulation is assessed by performing a modified Allen test. To perform this test, the radial and ulnar arteries of one hand are located via palpation. The patient is asked to make a fist then both arteries are occluded briefly by holding firm pressure. The patient is then asked to open the hand, and the arteries are released one at a time to assess for their ability to return blood to the hand (Williams, 1998).
Once the sample is obtained, air bubbles should be removed since they can dissolve and interfere with the accuracy of the results. Additionally, if the ABG sample can not be immediately analyzed, it should be placed on ice. This will prevent any metabolic processes from occurring which could also cause an inaccuracy in the results. Typically, results are available within minutes (Wikipedia).
If the ABG was drawn from a direct arterial stick as opposed to a line draw, the nurse must ensure that firm pressure is held on the site for at least five minutes. In the case of a patient on anticoagulation, pressure may need to be held for a longer period of time. The nurse must also assess frequently for bleeding and/or hematoma formation (Taylor, Lillis, & LeMone, 2001).
Usefulness of the ABG
As will be discussed later, there are many different components to an ABG. Therefore, there are a multitude of reasons that arterial blood gases may be drawn. The following are a few instances where the nurse may anticipate the need for an ABG.
- In surgery, drawing an ABG will aid the anesthesiologist in properly ventilating a patient.
- In an intensive care unit, an ABG will be drawn to properly manage a ventilated patient, or to provide assessment data when weaning a patient toward extubation, or removal of the ventilator.
- Based on the results of an ABG, the respiratory therapist or physician may change the ventilator settings to optimize respiratory functioning, such as increasing the oxygen content when oxygen levels are found to be low, or increasing the rate of ventilations per minute to blow off carbon dioxide when levels are found to be high.
- In the emergency department, an ABG can be used as part of a complete work up on a patient presenting with a variety of respiratory complaints from shortness of breath to chest pain to flu- or pneumonia-like symptoms.
- On a medical-surgical floor, an ABG can be used to assess for decompensation or improvement in patients admitted with exacerbations of asthma, COPD, or pneumonia.
- An ABG can be used to monitor for base deficit or base excess in patients presenting with either vomiting or diarrhea.
- An ABG can be used also in the management of patients with acid-base imbalances or kidney disorders to monitor the pH and bicarbonate levels.
Factors assessed with ABGs
A typical arterial blood gas consists of five values with reference ranges as follows: (Please note that reference ranges vary slightly depending on the source or your place of work, and you should be familiar with the normal reference ranges at your institution).
Table 1. ABG Tests and Reference Ranges
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Test: |
Reference Range: |
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pH |
7.35-7.45 (no units) |
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pCO2 |
35-45 mm Hg |
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pO2 |
80-100 mm Hg |
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Bicarbonate |
24-30 mEq/L or 22-26 mEq/L |
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