Type of test Photodiagnostic
Normal findings
≥ 95%
Possible critical values
≤ 75%
Test explanation and related physiology
Oximetry is a noninvasive method of monitoring arterial blood oxygen saturation (Sao2 ). The Sao2 is the ratio of oxygenated hemoglobin to the total amount of hemoglobin. The Sao2 is expressed as a percentage; for example, a saturation of 95% indicates that 95% of the total hemoglobin attachments for oxygen have oxygen attached to them. The Sao2 is an accurate approximation of oxygen saturation obtained from an arterial blood gas study. By correlating the Sao2 and the patient’s physiologic status, a close estimate of the partial oxygen pressure (Po2 ) can be obtained.
Oximetry is typically used for monitoring the patient’s oxygenation status during the perioperative period (or any time of heavy sedation) and for patients receiving mechanical ventilation. This test is used in many clinical situations, such as pulmonary rehabilitation programs, stress testing, and sleep laboratories. Oximetry can be used to assess the body’s response to various drugs, such as theophylline (which causes bronchodilation) and methacholine (which evokes bronchospasm in people with asthma). This test is commonly used to titrate levels of oxygen on hospitalized patients.
O2 levels can also be measured in various body tissues. For example, monitors that continuously measure tissue O2 partial pressures can be attached to a small catheter placed in the brain, heart, or peripheral muscle. Brain tissue oxygen testing and monitoring is the most common use of this technology. It is used to monitor the condition of the brain after severe head trauma by measuring cerebral blood flow and pulmonary oxygenation. It is more accurate than intracranial pressure for indicating brain injury.
Procedure and patient care
Before
* Explain the procedure to the patient.
* Tell the patient that no fasting is required.
During
• Rub the patient’s earlobe, pinna (upper part of the ear), or fingertip to increase blood flow.
• Clip the monitoring probe or sensor to the ear or finger. A beam of light passes through the tissue, and the sensor measures the amount of light the tissue absorbs (Figure 1).
• Note that this study is usually performed by a respiratory therapist or nurse at the patient’s bedside in a few seconds.
* Tell the patient that no discomfort is associated with this study.
Fig1. Oximetry. The pulse oximeter passes a beam of light through the tissue. The amount of light absorbed by the oxygen saturated hemoglobin is measured by the sensor.
After
• Note that no special aftercare is needed.
Abnormal findings
Increased levels
- Increased inspired O2
- Hyperventilation
Decreased levels
- Inadequate O2 in inspired air
- Hypoxic lung diseases
- Hypoxic cardiac diseases
- Severe hypoventilation states
