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Introduction ⬇

ABG analysis is essential to assess adequacy of oxygenation and ventilation, to evaluate acid–base status by measuring the respiratory and nonrespiratory components, and to monitor effectiveness of therapy (e.g., supplemental oxygen). ABGs are also used to monitor critically ill patients, to establish baseline values in the perioperative and postoperative period, to detect and treat electrolyte imbalances, to titrate appropriate oxygen flow rates, to qualify a patient for use of oxygen at home, and in conjunction with pulmonary function testing. ABG analysis usually include the following: pH, PaCO2, SaO2, CO2 content, O2 content, PaO2, base excess or deficit, HCO3−, hemoglobin, hematocrit, CO, Na+, and K+ (see Table 14.3).

Reasons for using arterial rather than venous blood to measure blood gases include the following:

  1. Arterial blood provides a better way to sample a mixture of blood from various parts of the body.

    1. Venous blood from an extremity gives information mostly about that extremity. The metabolism in the extremity can differ from the metabolism in the body as a whole. This difference is accentuated in the following instances:

      1. In shock states, when the extremity is cold or poorly perfused

      2. During local exercise of the extremity, as in opening and closing a fist

      3. If the extremity is infected

    2. Blood from a central venous catheter usually is an incomplete mix of venous blood from various parts of the body. For a sample to be completely mixed, the blood would have to be obtained from the right ventricle or pulmonary artery.

  2. Arterial blood measurements indicate how well the lungs are oxygenating blood.

    1. If it is known that the arterial O2 concentration is normal (indicating that the lungs are functioning normally) but the mixed venous O2 concentration is low, it can be inferred that the heart and circulation are failing.

    2. Oxygen measurements of central venous catheter blood reveal tissue oxygenation but do not separate contributions of the heart from those of the lungs. If central venous catheter blood has a low O2 concentration, it means either that the lungs have not oxygenated the arterial blood well or that the heart is not circulating the blood effectively. In the latter case, the body tissues must take on more than the normal amount of O2 from each cardiac cycle because the blood is flowing slowly and permits this to occur; this produces a low venous O2 concentration.

    3. ABG analysis does not indicate the degree of an abnormality. For this reason, the vital signs and mental function of the patient must be used as guides to determine adequacy of tissue oxygenation.

  3. Arterial samples provide information about the ability of the lungs to regulate acid–base balance through retention or release of CO2. Effectiveness of the kidneys in maintaining appropriate bicarbonate levels also can be gauged.

The radial artery is usually the ABG site of choice, but brachial and femoral arteries can also be used. Samples can be drawn from direct arterial sticks or from an indwelling arterial line.

Procedure ⬆ ⬇

  1. Observe standard precautions and follow agency protocols.

  2. Have the patient assume a sitting or supine position.

  3. Perform a modified Allen test to assess collateral circulation before performing a radial puncture as follows:

    1. Apply pressure to obliterate both radial and ulnar pulses of the selected upper extremity.

    2. Release pressure over the ulnar artery once blanching of the hand occurs. In a positive test, the hand will then flush immediately; the radial artery may then be used for puncture.

    3. Choose another site if flushing of the hand does not occur.

  4. Elevate the patient’s wrist with a small pillow and ask the patient to extend the fingers downward (this flexes the wrist and positions the radial artery closer to the surface).

  5. Palpate the artery and maneuver the patient’s hand back and forth until a satisfactory pulse is felt.

  6. Swab the area liberally with an antiseptic agent (e.g., an agent with an iodine base).

  7. Optional: After assessing for allergy, inject the area with a small amount (<0.25 mL) of 1% lidocaine if necessary to anesthetize the site. This allows for a second attempt without undue pain.

  8. Prepare a 20- or 21-gauge needle on a preheparinized self-filling syringe, puncture the artery, and collect a 3- to 5-mL sample. During the procedure, if the patient feels a dull or sharp pain radiating up the arm, withdraw the needle slightly and reposition it. If repositioning does not alleviate the pain, the needle should be withdrawn completely.

  9. Withdraw the needle and place a 4 × 4 inch absorbent bandage over the puncture site. Maintain pressure over the site with two fingers for a minimum of 2 minutes or until no bleeding is evident; it may be necessary to use a pressure dressing, secured to the site with elastic tape, for several hours.

  10. Meanwhile, ensure that all air bubbles in the blood sample are expelled as quickly as possible. Air in the sample changes ABG values. Cap the syringe and gently rotate to mix heparin with the blood.

  11. Label the sample with patient’s name, identification number, date, time, mode of O2 therapy, and flow rate.

  12. Place the sample on ice and transfer it to the blood gas laboratory. This prevents alterations in gas tensions resulting from metabolic processes that continue after blood is drawn.

  13. See Chapter 1 guidelines for intratest care.

Procedural Alert

  1. Some patients experience lightheadedness, nausea, or vasovagal syncope during arterial puncture. Respond according to established protocols.

  2. Pressure must be applied to the arterial puncture site, and the site must be watched carefully for several hours for bleeding. Instruct the patient to report any bleeding or swelling at the site.

  3. Information on the specimen should include the patient’s name, date, and test(s) ordered as well as the fraction of inspired oxygen (FIO2), and the patient’s temperature. Do not use blood for ABG measurements if the sample is >3 hours old.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the purpose and procedure for obtaining an arterial blood sample.

  2. If the patient is apprehensive, explain that a local anesthetic can be used.

  3. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Clinical Alert

  1. Before obtaining an arterial blood sample, assess for the following contraindications to an arterial stick or indwelling line:

    1. Absent palpable radial artery pulse

    2. Negative modified Allen test, indicating obstruction in the ulnar artery (i.e., compromised collateral circulation)—do not attempt to use radial artery for blood sample

    3. Cellulitis or infection in the area

    4. Arteriovenous fistula or shunt

    5. Severe thrombocytopenia

    6. Prolonged prothrombin or partial thromboplastin time (relative contraindication)

  2. A Doppler probe or finger-pulse transducer may be used to assess circulation. This may be especially helpful with dark-skinned or uncooperative patients.

  3. Before obtaining an arterial blood sample, record the most recent Hb concentration, the mode and flow of oxygen therapy, and the temperature. If the patient has recently undergone suctioning or been placed on mechanical ventilation, or if the inspired oxygen concentration has been changed, wait at least 15 minutes before drawing the sample. This waiting period allows circulating blood levels to return to baseline. Hyperthermia and hypothermia also influence oxygen release from hemoglobin at the tissue level.

Posttest Patient Care

  1. Evaluate color, motion, sensation, degree of warmth, capillary refill time, and quality of pulse in the affected extremity or at the puncture site.

  2. Monitor puncture site and dressing for arterial bleeding for several hours. No vigorous activity of the extremity should be undertaken for 24 hours.

  3. Review test results; report and record findings. Modify the nursing care plan as needed.

  4. Explain the possible need for follow-up testing and treatment.

  5. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Reference Values ⬆

Normal