Pulse oximetry is a method of monitoring the percentage of hemoglobin which is saturated with oxygen.
The pulse oximeter consists of a probe attached to patient’s finger or ear lobe which is connected to a computerized unit.
Red and infrared light is passed from one side to the other. Based on the absorption of the red and infrared light caused by the difference in color between oxygen-bound (red) and unbound (blue) hemoglobin in the capillary bed, an approximation of oxygenation can be made.
Pulse oximeter displays the percentage of hemoglobin saturated with oxygen together with an audible signal for each pulse beat. Programmable audible alarms are provided.
Pulse oximeter working:
A light source originates from the probe at two wavelengths (650nm and 805nm). The light is partly absorbed by hemoglobin by amounts which differ depending on whether it is saturated or de-saturated with oxygen.
The processor can compute the proportion of hemoglobin which is oxygenated by calculating the absorption at the two wavelengths.
Pulse oximeter depends on pulsatile flow and produces a graph of the quality of flow. The pulse oximeter may be unable to function where flow is slow (for ex. Hypovolaemia).
The computer within oximeter is capable of differentiating pulsatile flow from other more static signals to display only the arterial flow.
Pulse oximeter and sleep apnea:
When the device is attached to the patient’s finger, it will sense hemoglobin concentration below 33%, which correlates with hypoxia below 75% oxygen saturation on red blood cells.
When oxygen saturation drops below 75%, increased amount of infrared light is sensed by the photo detector. The data is downloaded into the computer software that analyzes reductions in your blood oxygen levels and hypopneas so as to determine de-saturation event index (DEI).
Your doctor considers this information with other symptoms and decides to prescribe the treatment or refer to sleep disorder clinic.
Home oximeters are available to detect sleep apnea but the accuracy is unclear. Home oximeter is helpful in identifying patients with unsuspected and seriously low oxygen levels. Pulse oximeter is the best device as it is comfortable and don’t dry the skin out and widely used in hospitals.
Pulse oximeter and COPD:
This device is used for the assessment of nocturnal hypoxemia in patients with COPD. SpO2 monitoring is performed during stable state of the disease and patients who are agitated or in a state of shock are excluded.
SpO2 is a measurement of the amount of oxygen attached to the hemoglobin cell in the circulatory system. Continuous nocturnal SpO2 monitoring will be usually accurate.
It is better to check the amount of carbon monoxide with a whole-blood oximeter just before starting the sleep study.
Oximeter gives no information about CO2 level and therefore it has limitations in assessment of patients developing respiratory failure due to CO2 retention.
On rare occasions, pulse oximeter may develop faults. It is the greatest advance in patient monitoring and it is hoped that their use will eventually become routine.