Introduction
A pulse oximeter is a medical device that is used daily in our department; it measures oxygen saturation and pulse rate which are both useful in respiratory and sleep monitoring.
Pulse oximetry is a non-invasive measurement of the oxygen saturation (SpO2). Oxygen saturation is defined as the measurement of the amount of oxygen dissolved in blood, based on the detection of Haemoglobin (Hb) and Deoxyhaemoglobin.
How the pulse oximeter records:

Pulse oximeters measure the absorption of specific wavelengths of light. When blood gets in contact with a cell, the red cell’s haemoglobin releases oxygen and becomes Deoxyhaemoglobin (Hb) (deoxygenated haemoglobin). At this point, blood without oxygen returns back to the heart’s right atrium to repeat the process again in oxygenated and deoxygenated haemoglobin. The colour of blood varies depending on how much oxygen it contains. A pulse oximeter shines two beams of light through
a finger, earlobe or toe probe. One beam is a red light, which you can see when a pulse oximeter is used, and the other one is infrared light, which you don’t see. These two beams of light can let the pulse oximeter detect what colour the arterial blood is, and it can then work out the oxygen saturation. However, there are lots of other bits of a finger which will absorb light, such as venous blood, bone, skin, muscle etc., so to work out the colour of the arterial blood a pulse oximeter looks for the slight change in the overall colour caused by a beat of the heart pushing arterial blood into the finger. This change in colour is very small so pulse oximeters work best when there is a good strong pulse in the finger the probe is on. If the signal is too low the measured oxygen saturation may not be reliable, the pulse oximeter will not be able to work.
There are two different light wavelengths that are used to measure the actual difference in the absorption spectra of HbO2 and Hb. The bloodstream is affected by the concentration of HbO2 and Hb, and their absorption coefficients are measured using theses wavelengths 660 nanometre (nm) for a red-light spectra and 940 nm for an infrared light spectrum. Deoxygenated and oxygenated haemoglobin absorb different wavelengths. Deoxygenated haemoglobin has a higher absorption at 660 nm and oxygenated haemoglobin (HbO2) has a higher absorption at 940nm. Red blood cells contain a protein called haemoglobin. When oxygen reacts with this protein, it gets attached to it and generates Ox haemoglobin (HbO2) 0xygen saturation; ox haemoglobin and deoxyhaemoglobin absorb light differently forming the basis for pulse oximetry, and red cells with oxygenated haemoglobin (Hgb) circulate in the blood through the whole body, irrigating tissues. Biomedical Instrumentation Accessed on (23/04/2018)
Pulse oximetry instrumentation
Overall requirements
Shine light through the finger (constant current light).
Control the pulsing of that light
Biomedical instrumentation Accessed on (23/04/18).

Pulse oximetry assumes that the attenuation of light by the finger can be split into 3 independent components: arterial blood (A), venous blood (V) and tissues (T) the amplitudes of the pulsatile component of light attenuation at the two wavelengths are used to derive arterial oxygen saturation

There is window of opportunity:
The wavelength range between 600 and 1,000 nm is also the range for which there is the least attenuation of light by body tissues. By measuring the light transmission through a body segment at two wavelengths within that range, the arterial SaO2 can be determined. Biomedical Instrumentation Accessed on (23/04/2018).
Attenuation of light through an artery
Principles of Beer-Lambert Law:

Where:
Co is the concentration of oxyhaemoglobin (HbO2)
Cr is the concentration of reduced haemoglobin (Hb)
?on is the absorption coefficient of HbO2 at wavelength ?n
?rn is the absorption coefficient of Hb at wavelength ?n

Calculating oxygen saturation
simple manipulation gives:

Where:

The above equation simplifies further if ?2 is chosen to be the isosbestic wavelength,
i.e. ?r2 = ?o2,
When downloading the pulse oximeter and displaying all the information on the visi- download computer system this is how the information looks at different sampling times. Biomedical Instrumentation Accessed on (23/04/2018).

Function of an oximeter.
This device can be used in many clinical situations:
Theatres Post-operative recovery
Intensive care Rehabilitation programmes
Tolerance of diagnose procedures e.g. Bronchoscopy
Detecting desaturation during stress tests
Monitoring of Chronic Obstructive Pulmonary Disease
Shortness of Breath in acute situations
CPAP ; NIV management
They are also now fitted into mobile phones for fitness monitoring. This equipment is simple to use, non-invasive it comes in all different sizes, portable devices, watches with finger probes, toe probes, earlobe probes, and ward oximeters for patients overnight, it is also fitted into theatre equipment to monitor patients under anaesthetic.
Ward oximeters work of the main electricity supply.
How a power supply works for the ward oximeters.
When power comes into a building, it is in AC, or “alternating current.” AC current switches back and forth from positive to negative 60 times a second. It is carried into the building on the live wire. A second wire, called the return wire, carries the current back out of the house to complete the circuit, Unlike AC, DC, or “direct current,” only flows in one direction. A DC power supply has two wires–one with a negative charge and the other with a positive charge. A device called a rectifier is used to turn AC into DC. The central component of a rectifier is the diode. Diodes are one-way electric valves. When the electricity in the circuit turns negative, a diode lets it flow down the negative wire. When the electricity cycles back to positive, that diode closes automatically, and another diode lets the positive current flow down the positive wire. There are several different types of rectifiers, but they all use diodes in essentially the same way to separate the negative current from the positive.
AC current is carried in at 120 volts, far too high a voltage for most DC appliances. The voltage must be reduced through a step-down transformer. The AC current runs through a coil, which creates a magnetic field. A second coil, with fewer turns of wire, is placed next to it. The magnetic field from the first coil creates an electric current in the second coil. Because there are fewer turns in the second coil, it creates lower-voltage AC electricity. Sciencing, (2017)

Block diagram of a power supply system.

Transformer- steps down high voltage AC mains to low voltage AC
Rectification – converts AC to DC, but the DC output is varying.
Smoothing- smooths the DC from varying greatly to a small ripple.
Regulator- eliminates ripple by setting Dc output to a fixed voltage.

https://www.google.com/search?q=explanation+how+a+dc+power+supply+circuit+works;source=lnms;tbm=isch;sa=X;ved=0ahUKEwjq_Y278tXaAhUKIMAKHbI_D9sQ_AUICygC;biw=1280;bih=627#imgrc=6ldJO6gCMxQ5eM:;spf=1524675761979

Technical limitations
The oximeter reads the oxygen levels through the finger. If the finger is compromised, then it will struggle to direct the light absorption. Below are some examples of how this can be interrupted, and I have also listed the advantages and disadvantages of using such device in my department.
Nail Varnish Motion
Vascular Conditions Deformities
Sickle Cell Anaemia Carbon Monoxide presence

Advantages of a Pulse Oximeter for use in sleep monitoring
1. Simple
2. Non-invasive compared to arterial blood gases.
3. Relatively available and cheap
4. Portable
5. Can be used for multiple nights
6. Easy application
7. Good sensitivity for diagnosing Sleep Apnoea
Disadvantages of a Pulse Oximeter for use in sleep monitoring
1. Cannot confidently differentiate types of sleep disordered breathing.
2. Artefact due to body movement
3. Interpretations is not comparable or consistent
4. Unaware if the patient was asleep.
In sleep monitoring we are specifically looking at a patient’s recorded oxygen level and pulse rate throughout a night of sleep. We also look in more detail at the oxygen desaturation index (ODI); a desaturation occurs following an apnoea (pause in breath) or hypopnoea (reduction in breath), the arterial blood passing through the lungs picks up less oxygen, leading to lower levels of saturation (desaturation).

When the airflow returns to normal the oxygen saturation will normally respond by returning to or close to the originally level. (Reference McGill Criteria)
It is a useful screening tool and can be used to diagnose the severity of sleep apnoea, we can do this by looking at how many times on average in an hour the patient has desaturations. This is categorised by 0-5 normal, 5-15 mild, moderate 15 to 30 and 30 and over severe sleep apnoea.
Overnight pulse oximetry can also be used to monitor patients who are using CPAP and NIV patients on their BIPAP machines. We are assessing that with the overnight support with their positive airway pressure (PAP) therapy, their oxygen levels are maintained and that their sleep disordered breathing is controlled. If the results show lower than expected oxygen levels or a raised ODI then we may have to adjust their PAP. Starship (2009).