Pulse Oximetry Functions and Applications

Pulse oximetry is one of the most commonly used bits of monitoring equipment for anaesthesia in veterinary clinics today. Utilizing a pulse oximeter we can monitor the percentage of haemoglobin (Hb) which is saturated with oxygen in a non-invasive way, allowing us to discover hypoxia prior to the patient is visibly cyanotic. The pulse oximeter contains a probe mounted on the patient (usually tongue, ear, or prepuce/vulva) which is linked to a computerised device. The unit displays the percentage of Hb saturated with air and a determined heartrate, often with an audible transmission for every pulse beat. Some units also have a graphical screen of the blood flow at night probe called a plethysmograph.

The pulsative pattern is caused by the arterial move as the heart beats, which is the same process that causes us to feel a 'pulse' when we palpate arteries. During systole, a new wave of arterial bloodstream enters the vascular foundation, and blood amount and light absorption boosts. During diastole, blood amount and light absorption declines to its minimum level.

The pulse oximeter can determine the ratio of haemoglobin saturated with air, commonly known as SpO2, by emitting red and infrared light from the light-emitting diodes (LEDs) on one side of the probe, which trips through the tissue (or reflects off with respect to the probe type) to the photodiode on the other side of the probe. The device analyses the light that gets to the photodiode and can detect subtle dissimilarities in the absorption of light by oxyhaemoglobin and deoxyhaemoglobin. As these differ in absorption levels, the amount of red and infrared light assimilated by blood is related to haemoglobin oxygen saturation.

The pulse oximeter can compute the heart rate as it picks up the pulsations as the volume of arterial blood vessels in the structure changes during the pulsative cycle, impacting on light absorption.

Adequate oxygenation is essential at all times for the body to perform its metabolic techniques. The heart and soul and brain will be the body's biggest consumers of air, in case oxygenation levels lower to critical levels, injury occurs extremely quickly. Oxygen journeys in the blood in two forms - as unbound oxygen dissolved in plasma and as oxygen that will the haemoglobin. In healthy patients inhaling and exhaling room air (which contains approximately 21% air), air dissolved in plasma compatible a very small percentage of the full total oxygen in the blood (most text messages list this as less than 1. 5%), and the majority of blood oxygen is bound to haemoglobin (the remaining 98. 5%). Measuring and monitoring oxygenation via pulse oximetry is very useful as it is monitoring the air that is bound to haemoglobin, which is what is utilised by the body for normal cell function.

Monitoring SpO2 however will not offer you a good indication of how well the patient is ventilating (or deep breathing) for itself, especially during anaesthesia. A typical fault veterinary nurses make is to presume that if an individual has a SpO2 of 95% or higher under anaesthesia, then it is inhaling and exhaling adequately. We can get lulled into a incorrect sense of security by having a good saturation amount when the patient's respiration is totally inadequate.

There are two main functions of respiration, one gets oxygen out of the air and into the body, and the other gets carbon dioxide from the body and in to the air. It possible for the individual to be getting enough air into their body but not being able to eliminate enough skin tightening and, so the SpO2 will show a good reading, but the patient may be hypercapnic (elevated levels of carbon dioxide). A capnograph should be utilized to measure end tidal carbon dioxide (ETCO2) levels and examine patient respiration.

Partial pressure of air in arterial bloodstream (PaO2) is a measurement of the levels of unbound oxygen in the plasma, so that as discussed above, accocunts for a small ratio of the full total air in the blood. However PaO2 is important as it affects the saturation of haemoglobin because there has to be an adequate degree of dissolved oxygen in the blood to be accessible to bind to the haemoglobin.

It is also important to comprehend that oxygen saturation and PaO2 are associated (when one rises the other goes up and vice versa), nonetheless it does not have a direct linear correlation. As PaO2 reduces, the saturation level diminishes slowly at first, but then lessens rapidly (see desk **).

In a patient which is breathing room air, the PaO2 is about 100mmHg, whereas for an individual breathing 100% air (for anaesthesia), their Pa02 is just about 500mmHg and SpO2 is 100%. If this patient has a PaO2 drop to 100mmHg (a drop of 400mmHg) their SpO2 will drop to around 98%. If a further drop to 80mmHg occurs, their SpO2 will drop to around 95%. After this point, the SpO2 will start a more dramatic drop; if PaO2 drops to 60mmHg (another 20mmHg drop) means their SpO2 will be about 90%. A further drop of 20mmHg to a PaO2 of 40mmHg, the saturation goes from 90% to 75%.

In request, when monitoring SpO2 in a standard healthy dog or cat, it should be 95-100%. Levels between 90-95% must be investigated, and critical prices for oxygen saturation are below 90%.

Simply, which means that the total air available to the body decreases very little when partial pressures are above 80mmHg (Spo2 of 95%), nonetheless they decrease quickly below this level, such as patients with lung disease, insufficient oxygen, limited ventilation etc. Virtually put, if you patient has a Sp02 of 90-95% this may reveal hypoxaemia and must be looked into as your patient's haemoglobin is not totally saturated. In case your patient has a Sp02 of significantly less than 90%, then immediate remedy must be initiated - oxygen if not acquiring already, ventilation assistance etc. Sp02 of 85% or below for more than 30 seconds is considered a crisis.

Placing the SpO2 Probe

There are two main types of probes available on the market - transmission or reflective. Transmission probes will be the most usual, and are usually attached in a clip. These are typically applied to the tongue, pinna, toe webbing, vulva or prepuce, or any other area that is thin and relatively hairless.

Reflective probes have the source of light and sensor side by side and tend to be taped to the bottom of the tail after it has been clipped, or covered and inserted into the oesophagus or rectum. When placing rectally, it's important to ensure that there are no faeces between your sensor and the rectum wall.

Tongue, Cheeks, Prepuce, Vulva

With tongues, start at the tip and work the right path toward the bottom. Always direct the light downward, toward the ground; whatever the animal's position to lessen the consequences of ambient light (ambient light will influence accuracy and reliability). For patient comfort, keep carefully the tongue moist during longer procedures through the use of a dampened gauze swab between the tongue and the probe. Do not have the gauze too thick as it could alter the reading by impeding the light transmission.

To get an improved reading on smaller tongues, bring the sides of the tongue up and complete the light through both layers. Do not fold the tip of the tongue, as you will limit blood circulation to the tongue.

The same principals apply to placing the probe on the cheek, prepuce or vulva.


Moisten the hock area with isopropyl alcohol and/or drinking water, and clip hair if needed.

Pinna (Ear canal)

The probe can be positioned on the ear canal using the same strategy as the tongue. Long haired animals may desire a patch shaved first for the sensor to work accurately.


Probes can be located on the metatarsals or metacarpals or in the webbing between them.


Place the reflective probe on the ventral foot of the tail. The LEDs should be positioned dorsally. You may want to clip a small patch of scalp, only large enough for the LEDs to place on your skin. Be sure the skin is clean. Contain the sensor snugly up against the tail and wrap with non-adhesive wrap.

Poor SpO2 Readings

When you identify a poor or low saturation reading, it is vital that you check the patient before you check the device. Make sure your patient is secure by evaluating all vital indications. Pulse oximeters need a strong regular pulse where the probe is situated. If there is merely a fragile pulse, the pulse oximeter may display a reading but it might not be exact. Most pulse oximeters have a pulse durability indicator as a pub graph which should be utilized to see whether you have accurate placement.

If the clip of the probe is too strong, this can also have an impact on your reading by constricting the blood circulation before the sensor. If this is actually the case, swapping the clip for a more gentler is your best option, in any other case reposition the sensor to somewhere that may take the pressure (this will most likely be thicker).

An irregular transmission triggered by an unusual heartbeat or by the individual moving, shivering or fitting can cause problems for a pulse oximeter. If a patient movements too much, try relocating the probe to another location.

Ambient light may be too bright for the sensor to operate correctly. Theatre lighting can especially cause issues. Any sensor that is found in bright light should have a drape located over it to reduce light contamination to get more accurate readings.

Do not place the sensor is on a single limb as a blood pressure cuff, the blood flow restriction from the cuff during way of measuring will hinder the pulse oximeter sensor operating correctly.

Other factors that make a difference SpO2 readings include pigmented epidermis - either normal pigment or jaundiced patients; peripheral vasoconstriction - eg hypothermia, impact, drug-induced; or excess hair - can cause interference and should be clipped away to allow the probe to take a seat directly against the skin. Wetting down with alcohol can also assistance with excess hair if you are prohibited to clip.

Pulse Oximeter Maintenance

Read your manufacturer's instructions about the good care of your pulse oximeter and probe. For infections control, you should always wipe the probe sensor and clip between patients. Most detectors can be surface-cleaned by wiping with 70% isopropyl alcohol. Usually do not immerse the sensor in liquid unless the manufacturer instructions talk about immersion is possible.

After each cleaning and prior to each use, examine the sensor and cable television for fraying, cracking, breakage, or other damage. Inspect the clip for cracking or breakage, or loss of spring tension that would allow slippage or movements of the sensor from its proper position. If defects are observed, do not use the sensor or clip as it may offer an inaccurate reading.

When used properly, pulse oximeters are an easy to utilize and readily available little bit of monitoring equipment specifically for anaesthesia, nonetheless it is important to note that they do not replace hands-on monitoring, and are not a valid approach to assessing whether a patients respiration is satisfactory, as they offer a late indicator of respiratory issues.

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