Oxygen saturation is a measure of how much oxygen the blood is carrying as a percentage of the maximum it could carry
Oxygen saturation is also refered to as SpO 2 .
.One haemoglobin molecule can carry a maximum of four molecules of oxygen,.One hundred haemoglobin molecules could together carry a maximum of 400 (100 x 4) oxygen molecules, if these 100 haemoglobin molecules were carrying 380 oxygen molecules they would be carrying (380 / 400) x 100 = 95% of the maximum number of oxygen molecules that could carry and so together would be 95% saturated.
This will vary with age, degree of fitness, current altitude, oxygen therapy etc etc.
The edges of the visible light spectrum blend into the ultraviolet and infrared levels of radiation. Red light (630-740 nm) Infrared light (750nm- 1mm)
The ratio of absorbencies at these two wavelengths is calibrated empirically against direct measurements of arterial blood oxygen saturation (SaO2) in volunteers, and the resulting calibration algorithm is stored in a digital microprocessor within the pulse oximeter
Pulse oximeters can either be used to take a 'one-off' reading from someone or can be left on for period of time. A single one-off reading often isn't much use, trends over a period of time give more information.It is important to remember that pulse oximetry is only one way of monitoring breathing. It is also necessary, as a minimum, to record respiratory rate and if pulse oximetry is used the amount of oxygen they are receiving must be recorded. As with all clinical assessments the 'whole picture' must be looked at.
Calibration is performed by company on normal patients breathing various gas mixtures, so cal is accurate only down to 80%. Carboxyhemoglobin leads to overestimation of sats because it absorbs at 660 nm like oxyHb does. . So if someone has 25% of their haemoglobin saturated with carbon monoxide and a true oxygen saturation of 70% a pulse oximeter will display an oxygen saturation of about 95%. This is obviously extremely dangerous and for this reason pulse oximeters should not be used with people who may have inhaled smoke, ie anyone who has been involved with any sort of fire
To compensate for LED differences To anticipate arterial pulses by simultaneously recording the electrocardiograph. To help with reducing artifact and low pulsatile flow situations. Researchers at MIT have developed a ring sized pulse oximeter.
Hemoglobin is a protein molecule that binds to oxygen. In its oxygen-loaded form, it is called oxyhemoglobin and is bright red. In the oxygen-unloaded form it is called deoxyhemoglobin and is purple-blue.
Based on two physical principles: <ul><li>The presence of a pulsate signal generated by arterial blood. </li></ul><ul><li>Oxyhemoglobin and reduced hemoglobin have different absorption spectra. </li></ul>
Pulse: The rhythmic contraction and expansion of an artery due to the surge of blood from the beat of the heart.
The oximeter is dependant on a pulsate flow and produces a graph of the quality of flow.
Where flow is sluggish, the pulse oximeter may be unable to function. The computer within the oximeter is capable of distinguishing pulsatile flow from other more static signals (such as tissue or venous signals) to display only the arterial flow.
What about oxyhemoglobin and reduced hemoglobin having different absorption spectra? Pulse oximetry uses spectrophotometry based on Beer-Lambert law
Spectrophotometry is the measurement of the amount of light that is absorbed as it passes through a substance.
The "Beer-Lambert Law" states that there is a linear relationship between the concentration of a solution and the absorbance.
Oxygen saturation is based on the ratio of light absorption during pulsate and baseline phases.
Absorption Graph Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated (or reduced) hemoglobin absorbs more red light and allows more infrared light to pass through.
It is used in evaluation of various medical conditions that affect the heart and lungs.
It is used to detect hypoxia. Hypoxia is a pathological condition in which the body as a whole ( generalized hypoxia ) or a region of the body ( tissue hyoxia ) is deprived of adequate oxygen supply.
Even under ideal conditions, skilled observers cannot detect hypoxemia until oxygen saturation is below 80%.
Areas of Use <ul><li>Anesthesia standards require pulse oximetry on all anesthetized patients. </li></ul><ul><li>Used with ventilator dependant patients. </li></ul><ul><li>Frequently incorporated into vital signs monitors measuring heart rate, blood pressure, and temperature. </li></ul><ul><li>It is commonly used in the hospital in the continuous mode for critical applications and intermittently for less critical patients. </li></ul>
Benefits of Use <ul><li>Low cost (Finger units <$50). </li></ul><ul><li>Ease of Use (Clip and Press). </li></ul><ul><li>Degree of Accuracy. </li></ul>
For a resting patient under normal conditions, the accuracy of pulse oximeters is about +/- 2% in the typical range of clinical interest - a SaO2 value of 70% - 100%.
Limitations of Use <ul><li>Intravenous Dyes. </li></ul><ul><li>Motion. </li></ul><ul><li>Low Perfusion states. </li></ul><ul><li>Black or blue nail polish. </li></ul>
Areas of concern <ul><li>Sensitive to motion. </li></ul><ul><li>Readings below 85% have increased error. </li></ul><ul><li>Low perfusion state increases error. </li></ul><ul><li>Ambient light interferes with reading. </li></ul><ul><li>Delay in reading of about 12 seconds. </li></ul><ul><li>Dysfunctional hemoglobin. </li></ul>
Since first generation devices, technical advances which have been made to improve pulse oximetry include:
<ul><li>Calibration resistors and chips embedded into the sensor. </li></ul><ul><li>The use of ECG synchronization techniques. </li></ul><ul><li>Various motion sensing improvements. </li></ul><ul><li>Specialty sensors for high altitude climbers. </li></ul><ul><li>Smart alarm systems for pulse oximeters. </li></ul><ul><li>A reduction in size, cost and power use. </li></ul><ul><li>Wireless connection via Bluetooth technology. </li></ul>
Pulse oximeters are relatively safe devices with a few safety issues: <ul><li>Infection especially with reusable sensors. </li></ul><ul><li>Possible heating and minor burns to sensitive skin due to the red/infra-red LEDs. </li></ul><ul><li>Routine electrical safety concerns. </li></ul>