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Introduction ⬇

Partial Pressure of Oxygen (PO2)

Oxygen is carried in the blood in two forms: dissolved in plasma (<2%) and combined with hemoglobin (98%). The partial pressure of a gas determines the force it exerts in attempting to diffuse through the pulmonary membrane. The arterial partial pressure of oxygen (PaO2) is the pressure exerted by O2 on the arterial walls and reflects the amount of O2 passing from the pulmonary alveoli into the blood; it is directly influenced by the fraction of inspired oxygen (FIO2). The partial pressure of oxygen in mixed venous blood (PvO2) reflects the balance between oxygen consumption and oxygen delivery, and the blood sample should be obtained from a pulmonary artery catheter.

Partial pressure of oxygen in the brain (PbtO2) reflects cerebral tissue perfusion and is measured in patients with traumatic or neurologic brain injury.

Procedure ⬆ ⬇

  1. Obtain an arterial (or venous, if ordered) blood sample (see Chapter 2 for venous and arterial blood collection).

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

  3. Place the sample on ice and transfer it to the blood gas laboratory.

Clinical Implications ⬆ ⬇

  1. Increased PaO2 is associated with:

    1. Polycythemia

    2. Increased FIO2

    3. Hyperventilation

  2. Decreased PaO2 is associated with:

    1. Anemias

    2. Cardiac decompensation

    3. Insufficient atmospheric O2

    4. Intracardiac shunts

    5. COPD

    6. Restrictive pulmonary disease

    7. Hypoventilation caused by neuromuscular disease

  3. Increased PvO2 is associated with:

    1. Decreased O2 consumption (such as with hypothermia)

    2. Impaired tissue uptake

    3. Sepsis

    4. Left-to-right shunt

  4. Decreased PvO2 is associated with:

    1. Increased O2 consumption (such as with exercising)

    2. Hypoxia

    3. Low cardiac output

  5. Decreased PaO2 with normal or decreased PaCO2 is associated with:

    1. Diffuse interstitial pulmonary infiltration

    2. Pulmonary edema

    3. Pulmonary embolism

    4. Postoperative extracorporeal circulation

Clinical Alert

In some persons with COPD, ventilatory efforts are stimulated by the hypoxic state, whereas for a healthy person, the respiratory stimulus is the buildup of CO2. Administering a high concentration of supplemental oxygen may knock out the hypoxic drive, resulting in increased retention of CO2. In persons with community-acquired pneumonia, arterial hypoxemia can signal impending respiratory failure

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the purpose, benefits, and risks of arterial blood sampling. Assess the patient’s level of cooperation and understanding.

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

Posttest Patient Care

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

  2. Assess, monitor, and intervene appropriately for bleeding at the puncture site and for respiratory or ventilatory disturbances.

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

Reference Values ⬆

Normal

Clinical Alert

Pbto2 levels below 15 mm Hg may indicate cerebral ischemia; levels below 5 mm Hg indicate brain cell death