More and more general practitioners are including pulse oximeters in their medical kit. A pulse oximeter is used routinely in anesthesiology, critical care, and the emergency department; and they are also frequently utilized in ambulances.
Roles of pulse oximetry in primary care:
• Evaluating severity and deciding management in babies with bronchiolitis
• Determining severity and oxygen requirements for patients with community-acquired pneumonia
• Diagnosing and handling caustic increase of chronic obstructive pulmonary disease (COPD) in the community
• Classifying the severity of an asthma attack
Pulse oximetry is a clear, comparatively inexpensive, and non-invasive method in monitoring oxygenation. It checks the percentage of hemoglobin that is oxygen-saturated. Oxygen saturation must constantly be above 95%, though in patients with cyanotic heart defect or COPD, it may be lower, consistent to its severity. The oxyhemoglobin dissociation curve becomes severely vertical below 90%, demonstrating the fast desaturation that happens with falling oxygen partial pressure. The default low oxygen saturation alarm setting in most pulse oximeters is 90%.
Still, pulse oximetry does not deliver information on the oxygen content of the blood nor ventilation; therefore, in the presence of anemia and in patients with respiratory failure because of carbon dioxide retention, proper care is required.
Things to remember when using a pulse oximeter:
• To rule out significant motion artefact, the shown heart rate must equate to a manually checked heart rate (within 5 beats per minute).
• Make sure that the finger is fully inserted into the probe, and that flexible probes are correctly attached.
• Suitably-sized probes must be used for children and infants.
• So as to minimize motion artefact when using a finger probe, the hand must be placed on the chest at the level of the heart.
• Poor perfusion (as a result of hypotension) is the chief cause of insufficient pulse wave. A sharp waveform with a dicrotic notch shows good perfusion, whereas a sine wave-like waveform conveys poor perfusion.
• Resting readings must be taken for at least 5 minutes.
A pulse oximeter works by the principles of spectrophotometry. Measurements of relative light absorption are completed numerous times every second, and these are handled by the device to provide a new reading every 0.5 to 1 second, which computes for the average of the readings over the last 3 seconds.
Two light-emitting diodes, infrared and red, are placed opposite their corresponding detectors through 5 to 10 mm of tissue. Probes are typically set on the fingertip; but the earlobes and the forehead are occasionally used as another course of action. Probes usually have clip or wrap sensors.
Sources of error:
• Environmental intervention: extreme movement, high level of ambient light (infrared heat lamps), and vibration at 0.5 to 3.5 Hz.
• Intravascular dyes, like methylthioninium chloride, may incorrectly decrease saturation readings temporarily.
• Nail polish must be removed because it may cause false readings.
• Pulse oximetry cannot distinguish different forms of hemoglobin. Carboxyhemoglobin is recorded as 90% oxygenated hemoglobin and 10% desaturated hemoglobin, thus causing an overestimation of correct saturation levels.
How to improve oximeter signal:
• Apply a topical vasodilator, such as glyceryl trinitrate (GTN) cream.
• Use a different probe.
• Use an alternative probe site.
• Rub and warm skin.