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Arterial Blood Gas Analysis (ABG)

Arterial blood gas (ABG) analysis is used to measure the partial pressures of oxygen (PaO2) and carbon dioxide (pacO2)' and the pH of an arterial sample. Oxygen content (O2CT), oxygen saturation (SaO2) and bicarbonate (RCO3 -) values are also measured. A blood sample for ABG analysis may be drawn by percutaneous arterial puncture or from an arterial line.

Purpose

• To evaluate gas exchange in the lungs.
• To assess integrity of the ventilatory control system.
• To determine the acid-base level of the blood.
• To monitor respiratory therapy.

Patient preparation

• Explain to the patient that this test is used to evaluate how well the lungs are delivering oxygen to blood and eliminating carbon dioxide.
• Tell him that the test requires a blood sample. Explain who will perform the arterial puncture and when and which site - radial, brachial, or femoral artery - has been selected for the puncture.
• Inform him that he needn't restrict food or fluids.
• Instruct the patient to breathe normally during the test, and warn him that he may experience a brief cramping or throbbing pain at the puncture site.

Procedure and posttest care

• Perform an arterial puncture.
• After applying pressure to the puncture site for 3 to 5 minutes, tape a gauze pad firmly over it. (If the puncture site is on the arm, don't tape the entire circumference; this may restrict circulation.)
• If the patient is receiving anticoagulants or has a coagulopathy, hold the puncture site longer than 5 minutes if necessary.
• Monitor vital signs, and observe for signs of circulatory impairment, such as swelling, discoloration, pain, numbness, and tingling in the bandaged arm or leg.
• Watch for bleeding from the puncture site.

Precautions

• Wait at least 15 minutes before drawing arterial blood when starting, changing, or discontinuing oxygen therapy.
• Before sending the sample to the laboratory, note on the laboratory slip whether the patient was breathing room air or receiving oxygen therapy when the sample was collected.
• If the patient was receiving oxygen therapy, note the flow rate. If he is on a ventilator, note the fraction of inspired oxygen and tidal volume.
• Note the patient's rectal temperature and respiratory rate.

Reference values

Normal ABG values fall within the following ranges:

• PaO2: 75 to 100 mm Hg
• PacO2: 35 to 45 mm Hg
• pH: 7.35 to 7.45
• O2CT: 15% to 22%
• SaO2: 95% to 100%
• HCO3 -: 24 to 28 mEq/L.

Abnormal findings

Low PaO2, O2CT, and SaO2 levels and a high PacO2 may result from conditions that impair respiratory function, such as respiratory muscle weakness or paralysis, respiratory center inhibition (from head injury, brain tumor, or drug abuse, for example), and airway obstruction (possibly from mucus plugs or a tumor). Similarly, low readings may result from bronchiole obstruction caused by asthma or emphysema, from an abnormal ventilation-perfusion ratio due to partially blocked alveoli or pulmonary capillaries, or from alveoli that are damaged or filled with fluid because of disease, hemorrhage, or near-drowning.

When inspired air contains insufficient oxygen, PaO2, O2CT, and SaO2 decrease, but PacO2 may be normal. Such findings are common in pneumothorax, impaired diffusion between alveoli and blood (due to interstitial fibrosis, for example), or an arteriovenous shunt that permits blood to by­pass the lungs.

Low O2CT - with normal PaO2, Sa02 and, possibly, PacO2 values ­may result from severe anemia, de­creased blood volume, and reduced hemoglobin oxygen-carrying capacity.

Interfering factors

• Failure to heparinize syringe, place Rumple in an iced bag, or send the sample to the laboratory immediately (possible altered PaO2 and PacO2 because metabolic processes continue after sample is drawn)
• Exposing the sample to air (increase or decrease in PaO2 and PacO2)
• Venous blood in the sample (possible decrease in Pa02 and increase in PII(02)
• Bicarbonate, ethacrynic acid hydrocortisone, metolazone, prednisone, and Ihlllzides (possible increase in PacO2) . Acetazolamide, methicillin, nitrofurantoin, and tetracycline (possible decrease in PacO2)
• Fever (possible false-high PaO2 and PacO2).