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Introduction ⬇

Oxygen Saturation (SO2)

Oxygen saturation (SO2) is a ratio between the actual O2 content of the hemoglobin and the potential maximum O2-carrying capacity of the hemoglobin. The SO2 is a percentage indicating the relationship between O2 and hemoglobin; it does not indicate the O2 content. The maximum amount of O2 that can be combined with hemoglobin is called the oxygen capacity. The combined measurements of SO2, PO2, and Hb indicate the amount of O2 available to tissues (tissue oxygenation). Pulse oximetry (SpO2) (Figure 14.2) is a noninvasive technique that permits continuous real-time monitoring and trending of arterial oxygen saturation (SaO2). However, it cannot differentiate COHb. As a result, the SpO2 is generally higher than the actual SaO2 by the amount of COHb, and a more direct measurement involves taking an arterial blood sample and measuring with a blood gas analyzer. Mixed venous oxygen saturation (SvO2) is the percentage of oxygen bound to hemoglobin in blood returning to the right side of the heart (venous blood) and reflects the amount of remaining oxygen after tissues have utilized oxygen extracted from arterial blood.

  1. SO2 measures only the percentage of oxygen being carried by hemoglobin; it does not reveal the actual amount of oxygen available to the tissues (oxygen content).

  2. Pulse oximetry equipment evaluates pulsatile blood flow. Many factors can interfere with the ability to measure flow:

    1. Digit motion

    2. A decrease in blood flow to the digit (e.g., cool extremity, decreased peripheral pulses, vasoconstriction, nail bed thickening, ambient light, digit malformation, vasoconstrictive drugs, localized obstruction)

    3. Decreased hemoglobin (anemia) or abnormal hemoglobin (COHb)

    4. Pulse rate and rhythm

Procedure ⬆ ⬇

  1. Obtain an arterial blood sample (see Blood Specimen Collection Procedures and Venipuncture and Arterial Puncture in Chapter 2). Two methods are used for determining SO2:

    1. Direct method: Blood sample is entered into a hemoximeter, a spectrophotometric device for direct determination of SO2.

    2. Calculated method: Calculated SO2 from oxygen content (the volume of O2 actually combined with hemoglobin) and oxygen capacity (the volume of O2 to which hemoglobin could combine). Both of these values are expressed as volume percentages (vol%) or milliliters per deciliter of blood. Use the following formula:

  2. Pulse oximetry: A small, clip-type sensor is placed on a digit over the fingernail (or toenail or earlobe, if necessary). The instrument, using transmitted light waves (in the infrared spectrum) and sensors, determines SO2 noninvasively and is referred to as the SpO2.

Clinical Implications ⬆ ⬇

  1. Abnormal results occur in pulmonary diseases involving cyanosis and erythrocytosis.

  2. Abnormal results occur with venous-to-arterial shunts.

  3. Values are abnormal in Rh incompatibility caused by blocking antibodies.

  4. Values usually are normal in polycythemia vera.

  5. Values are decreased in ventilation–perfusion mismatching.

  6. Values decrease with age.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the purpose, benefits, and risks of invasive arterial blood sampling. Assess the patient’s ability to comply with the procedure.

  2. Explain the use of pulse oximetry and that it does not cause pain or discomfort.

  3. 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 puncture site and for hypoxemia or other respiratory dysfunctions.

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

Interfering Factors ⬆ ⬇

Recent smoking or exposure to close secondhand smoke or to CO can increase the level of COHb, as can use of certain paint and varnish-type stripping agents, especially when they are applied in closed or poorly ventilated areas. The effect is to decrease the SaO2 with little or no effect on the PaO2. Interfering factors for pulse oximetry measurement include presence of fingernail polish; use of intravascular dyes, such as methylene blue; and exposure to ambient light.

Reference Values ⬆

Normal