Arterial blood gas (ABG) determinations are made not only to determine levels of actual blood gases (i.e., oxygen and carbon dioxide) but also to assess the client's overall acid-base balance. Thus, ABG levels may indicate hypoxia, hypercapnia or hypocapnia, acidosis, alkalosis, and physiological compensation for acid-base imbalance. The components of an ABG determination are as follows:

1. pH reflects the number of hydrogen ions in the body and is influenced primarily by the ratio of bicarbonate ions (HCO₃^-) to carbonic acid (H₂CO₃), which is essentially carbon dioxide (CO₂), in the blood. The normal HCO₃^--to-CO₂ ratio is 20:1. When the hydrogen ion concentration increases (acidosis), the pH falls; when the hydrogen ion concentration decreases (alkalosis), the pH rises. Bicarbonate levels are regulated by the kidneys, whereas carbon dioxide levels are controlled by the lungs. Both the lungs and the kidneys respond to alterations in pH levels by either retaining or excreting carbon dioxide and bicarbonate, respectively.
2. pO₂ indicates the partial pressure of oxygen in the blood. When oxygen levels are lower than normal, the client is hypoxic. Hypoxemia may be caused by either low cardiac output or impaired lung function.
3. pCO₂ indicates the partial pressure of carbon dioxide in the blood, which is regulated by the lungs. Except in cases of compensation for metabolic acid-base imbalances, elevated levels (hypercapnia, hypercarbia) indicate impaired gas exchange in the lungs so that excess CO₂ is not eliminated. Decreased levels (hypocapnia, hypocarbia) indicate increased loss of CO₂ through the lungs (hyperventilation).
4. HCO₃^- indicates the bicarbonate ion concentration in the blood, which is regulated by the kidneys. Altered levels are associated with metabolic acid-base imbalances or reflect response to respiratory alterations in CO₂ levels.
5. O₂ saturation (O₂ Sat, SaO₂) indicates the oxygen content of the blood expressed as percent of oxygen capacity (the amount of oxygen the blood could carry if all of the hemoglobin were fully saturated with oxygen). If the blood is 50 percent saturated, for example, the oxygen content is one-half of the oxygen capacity.
6. Base excess (BE) usually indicates the difference between the normal serum bicarbonate (HCO₃^-) level and the client's bicarbonate level. Positive values indicate excess bicarbonate relative to normal values, whereas negative values indicate decreased HCO₃^- levels.

• Fever may falsely elevate pO₂ and pCO₂; hypothermia may lower them.
• Suctioning of respiratory passages within 20 to 30 minutes of the test may alter results. Excessive heparin in the sample will lower pH and pCO₂.
• Exposure of the sample to atmospheric air (e.g., air bubbles in the sample) may alter results. Exposure of the sample to room temperature for more than 2 minutes may alter test results.

• Evaluation of the effectiveness of pulmonary ventilation in maintaining adequate oxygenation and in removing carbon dioxide, especially in disorders such as chronic pulmonary disease, neurological insults, and drug intoxication
• Evaluation of the effectiveness of cardiac output in maintaining adequate oxygenation, especially in shock and acute myocardial infarction
• Determination of the need for oxygen therapy (Oxygen is generally indicated if the pO₂ is 70 mm Hg or less, except in pulmonary disorders characterized by chronic hypoxemia in which lower oxygen levels may be tolerated by the client without supplemental oxygen.)
• Determination of respiratory failure, which is defined as a pO₂ of 50 mm Hg or less with a pCO₂ of 50 mm Hg or more
• Determination of acid-base balance, type of imbalance, and degree of compensation (see Table 5-26)
• Determination of need for mechanical ventilation (For example, elevated or rising pCO₂ levels may indicate the need for mechanical ventilation, especially when pO₂ is decreased.)
• Evaluation of effectiveness of mechanical ventilation and indication for modification of ventilator settings
• Evaluation of response to weaning from mechanical ventilation

Nursing Care Before the Procedure

Explain to the client:

• That repeat determinations may be necessary until cardiopulmonary function or acid-base balance, or both, are stabilized
• The method and site for obtaining the sample (e.g., arterial puncture or arterial line sample)
• Any anticipated discomforts (Arterial punctures cause a brief, sharp pain unless a local anesthetic is used.)
• That if an arterial puncture is performed, it will be necessary to maintain digital pressure on the puncture site for 5 minutes or more, after which a pressure dressing will be applied

Prepare the client for the procedure:

• Take the client's temperature. Fever may falsely elevate pO₂ and pCO₂; hypothermia may lower them.
• The client should not have had a respiratory therapy treatment, been suctioned, or had ventilator settings changed less than 20 to 30 minutes before the sample is obtained.
• If the test is being conducted to determine the need for oxygen therapy or response to weaning from mechanical ventilation, the client should be off oxygen, off mechanical ventilation, or on a weaning mode for a preset time, which is specified by the person ordering the test.
• If the sample is to be obtained by radial artery puncture, the Allen test should be performed to assess patency of the ulnar artery; in the event that thrombosis involving the radial artery occurs after the puncture:
  • Extend the client's wrist over a rolled towel or similar support.
  • Ask the client to clench the fist; if the client cannot clench the fist, elevate the hand above heart level.
  • Apply digital pressure over both radial and ulnar arteries.
  • Ask the client to unclench the fist while pressure is maintained on the arteries.
  • Observe the palm for blanching, which is the expected response.
  • Release pressure on the ulnar artery while continuing to maintain pressure on the radial artery.
  • Observe the palm for returning pinkness, which is a positive result.
  • If the palm remains blanched or if return of pinkness takes longer than approximately 5 seconds (a negative result), do not use the wrist for arterial punctures.
  • Inform the client's physician of a negative response to the Allen test.

The procedure varies slightly with the method for obtaining the sample.

Arterial Puncture. A blood gas collection kit is obtained. If prepackaged kits are not available, obtain a 3-mL syringe, heparin (usually in the concentration of 100 U/mL), 20-gauge or 21-gauge needles, povidone-iodine or alcohol swabs or sponges, gauze pads, and tape. Fill a plastic or paper cup or a small plastic bag about halfway with ice.

If the syringe is not preheparinized, draw approximately 1 mL of heparin into the syringe, pull the plunger back to about the 3-mL line, and rotate the barrel. Then expel all except approximately 0.1 mL of heparin and change the needle. Excess heparin in the syringe will lower the pH and pCO₂ of the sample.

Palpate the artery to be used. The radial artery is usually the most accessible, but the brachial or the femoral artery also may be used. If the radial artery is to be used, extend the client's wrist over a rolled towel or similar support.

Cleanse the site with povidone-iodine and allow to dry. It is recommended by some that the iodine solution be removed with an alcohol swab before arterial puncture. If the client is allergic to iodine, use only alcohol to prepare the site. Some authorities also advocate anesthetizing the puncture site with a small amount of 1 percent lidocaine (Xylo-caine).

Using the heparinized syringe with needle attached, puncture the artery. A 45-degree angle is used for radial artery punctures; a 60- to 90-degree angle is used for brachial arteries. A 90-degree angle is generally used for femoral artery punctures. Advance the needle until blood begins to enter the syringe; it should not be necessary to pull back on the plunger. After 2 to 3 mL of blood have been obtained, withdraw the needle and immediately apply firm pressure to the puncture site with a sterile gauze pad. Inform the client that the discomfort felt after the puncture will disappear in a few minutes.

Meanwhile, expel any air or air bubbles from the syringe, because mixing with atmospheric air may alter test results. The needle may be plugged by inserting it into a rubber cap, or it may be removed and the rubber cap supplied in the blood gas collection kit placed on the hub of the syringe. The sample is then placed in ice to inhibit metabolic blood activity; failure to do this within 2 minutes of collecting the sample will alter test results.

The sample is sent immediately for analysis. On the ABG request form or sample label, note the time the sample was collected, the client's temperature, and whether the client was breathing room air, receiving oxygen, or using mechanical ventilation.

Arterial Line Sample. See Appendix I, the section titled "Indwelling Devices and Atrial Venous Catheters."

Nursing Care After the Procedure

• For arterial punctures, maintain digital pressure on the site for 5 minutes and then apply a sterile pressure dressing. If the client is receiving anticoagulants or has bleeding tendencies, apply digital pressure for 10 to 15 minutes.
• Observe the arterial puncture site for bleeding or hematoma formation every 5 to 10 minutes for one-half hour after the pressure dressing is applied.
• Check for presence of pulses distal to the site when performing site observations, if the brachial or the femoral artery was used.
• Check for signs of nerve impairment distal to the site.
• Provide support when test findings are revealed and if repeated or serial testing is necessary in acute conditions.
• Evaluate the results of pH with electrolytes (particularly hypokalemia or hyperkalemia) and oxygen, carbon dioxide, and bicarbonate associated with respiratory or metabolic acidosis or alkalosis.
• Assess respiratory pattern, level of consciousness, neuromuscular irritability, fluid and electrolyte imbalances, and symptoms of impaired tissue perfusion as a result of hypoxia, any of which can be associated with abnormal ABGs.
• Notify physician at once of critical lab values.