What is the difference between sa02 and sp02

We can measure how many of these binding sites are combined, or saturated, with oxygen. Pa02, put simply, is a measurement of the actual oxygen content in arterial blood. Partial pressure refers to the pressure exerted on the container walls by a specific gas in a mixture of other gases. When dealing with gases dissolved in liquids like oxygen in blood, partial pressure is the pressure that the dissolved gas would have if the blood were allowed to equilibrate with a volume of gas in a container.

In other words, if a gas like oxygen is present in an air space like the lungs and also dissolved in a liquid like blood, and the air space and liquid are in contact with each other, the two partial pressures will equalize. As the partial pressure of oxygen rises, there are more and more oxygen molecules available to bind with Hgb. As each of the four binding sites on an Hgb molecule binds to an oxygen molecule, its attraction to the next oxygen molecule increases and continues to increase as successive molecules of oxygen bind.

The more oxygen is bound, the easier it is for the next oxygen molecule to bind, so the speed of binding increases and the oxygen saturation percentage rises rapidly on the curve. As all of the binding sites fill up, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen.

This tendency makes it easy for Hgb to rapidly pick up oxygen in the lungs as it passes through. As PaO2 falls, the Hgb saturation also falls as Hgb releases oxygen to the tissues in the areas of lower oxygen supply. This is because Hgb binding sites become less attracted to oxygen as it is bound to fewer oxygen molecules. This property allows Hgb to rapidly release oxygen to the tissues.

Since a normal PaO2 is between mmHg, some people may think that an O2 saturation of 90 is normal as well — after all 90 was a pretty good grade to get in school. However, this interpretation is very wrong. This is the minimum oxygen concentration providing enough oxygen to prevent ischemia in tissues.

A simple formula to estimate what the arterial oxygen concentration should be is to multiply the inspired oxygen concentration by 5. As good as they are they can have problems. Movement can cause inaccurate readings. This is especially common in small children. Another problem is that poor perfusion from extreme vasoconstriction, hypotension, hypovolemia, and septic shock can all decrease peripheral blood flow. This sometimes makes it impossible for the sensor to measure the concentration correctly, or at all.

When peripheral flow is disturbed, the body tends to try to protect central blood flow to the head. You can often put the sensor on the ear lobe and get a more accurate reading. Disposable sensors can also be placed on the forehead, bridge of the nose, and can also be pinched around the corner of the mouth making sure that the light and the detector are directly opposite each other.

Feiner et al also describes discrepancies based on gender. With all these limitations, the use of an SpO 2 pulse oximeter for the estimation of SaO 2 for a diagnosis of hypoxaemia should be used with caution. It can be seen that excessive hypoxia, or over-oxygenation in patients, can cause significant harm, both in the short and long term Martin and Grocott, SpO 2 peripheral oxygen saturation can provide an estimate of SaO 2 arterial oxygen saturation with a fair accuracy.

Changes in SaO 2 might not be reflected in real-time by SpO 2 and pulse oximetry. History Arterial oxygen saturation or the discovered oxygen level in arterial blood SaO 2 is invasive, and it is difficult to monitor trends in a practice setting. The pulse oximeter The principle behind the current approach towards pulse oximetry is that oxygenated blood absorbs light at a certain wavelength a shade of red , and deoxygenated blood, another infrared Nitzan, Application As with all patient assessment, excellent hand and equipment hygiene Bradley and Fraise, and an informed patient consent are of the upmost importance, with an explanation of the procedure and reasoning behind it.

Select a site that is well perfused with a proximal palpable pulse; warm; with a brisk capillary refill; immobile; comfortable; and easily accessible Welch et al, This is commonly either the fingers or earlobes, but other sites such as the tongue, cheeks, toes, or nose may be used in a patient with low peripheral perfusion Barnett et al, Select an appropriately sized pulse oximeter probe so that it comfortably fits the patient Figure 1.

This is an especially important consideration when using the adhesive probes—this type of probe may be more stable if not too tight In paediatric patients weighing less than 3 kg, the ball of the foot can be considered; above this weight, the nail bed of the big toe is recommended Figure 2. When using the toe, orientate the cable medially, and replace the sock to stabilise and ensure a strong signal Remember that the probe measures the absorbency of pulsatile blood, so although a loose probe may have an artificially low SpO 2 , venous pulsations from a tight probe or tape may also cause an artificially low SpO 2 by incorrectly measuring the venous blood Chan et al, Consider environmental factors to reduce interference and improve signal strength.

Electromagnetic radiation such as that produced by mobile phones can cause artefacts Ortega et al, Patient or probe movements including muscular spasms, shivering, seizures or an inconsolable infant can also cause interference Clarke et al, Figure 1. An appropriately sized pulse oximeter probe must be selected for comfort and access Figure 2. The nail bed of the big toe is recommended for paediatric patients weighing more than 3 kg Limitations Despite the usefulness of this tool, there are limitations as a result of physiology, pathology or physical changes.

Figure 3. Treat your patient With all these limitations, the use of an SpO 2 pulse oximeter for the estimation of SaO 2 for a diagnosis of hypoxaemia should be used with caution. Keep up to date with Journal of Paramedic Practice! An explanation for our observations remains unclear. Secondly, arteriolar dilatation secondary to tissue hypoxia may lead to venous pulsations, which in turn contributes to falsely low S p O 2 readings because venous oxyhaemoglobin saturation is also measured in the pulsatile vein [ 3 , 4 ].

Finally, a possible formation of a complex between the virus and haemoglobin may result in increased red light absorbance relative to infrared absorbance, thereby resulting in a lower S p O 2. On our unit, oxygen titration is mostly guided by S p O 2 , and therefore patients may have been administered a higher inspired oxygen fraction than was necessary.

The authors thank M. Columb for reviewing our statistics. No competing interests declared. National Center for Biotechnology Information , U. McDonnell , 1 and A. Bentley 1. The following formula helps to calculate the SpO2 value.

Moreover, the SpO 2 value serves as an unreliable surrogate marker for SaO 2 in critically ill patients. H owever, the validity of the relationship between SpO 2 and SaO 2 is dependent on a number of factors, including the adequacy of peripheral perfusion. Also, besides its non-invasive nature, pulse oximetry provides attempts to the continuous measurement of oxygen saturation, which is useful in several clinical situations.

These include surgery and post-anesthetic care units, neonatal care and NICU, emergency care, noninvasive transcutaneous pacing, etc. Moreover, a significant feature of SpO 2 measurement is that it only measures the oxygen saturation of the functional hemoglobin, which is more accurate. Here, the functional hemoglobin refers to the hemoglobin capable of carrying oxygen.

On the other hand, non-functional hemoglobins include carboxyhemoglobin HbCO and methemoglobin METHb , which are incapable of carrying oxygen. Thus, the formula for the ideal blood saturation measurement is:. SaO 2 refers to the measurement of oxygen saturation in arterial blood, but SpO 2 refers to the oxygen saturation in the arterial blood as measured by a pulse oximeter.

SaO 2 can be measured by blood gas analysis, while SpO 2 refers to the SaO 2 measured by the pulse oximetry.