Blood oxygen saturation at your fingertips

There are many things humans can do without and still survive, but oxygen is not one of them. We all breathe it from the same atmosphere and have similar baseline requirements. Any significant deviation in oxygen saturation is therefore a cause for concern.

The carrier of oxygen in the human body is haemoglobin and the percentage of (haemoglobin) binding sites in the bloodstream occupied by oxygen is of great clinical significance, particularly if it is unusually low [1]. There are two distinct methods of measuring oxygen saturation, differing in complexity and invasiveness, but first we must properly define what oxygen saturation is and how it is measured.

Oxygen saturation is simply the fraction of oxygen-saturated haemoglobin relative to total haemoglobin in the patient’s blood, represented in percentages. However, that percentage value varies according to the location where the measurement was taken. Arterial oxygen saturation (SaO₂) measurement entails the drawing of blood and analysis using a blood gas analyser [2]. Normal values are between 100% and 94%, while a value of 90% or lower indicates hypoxaemia [3, 4]. It is the most accurate method of measuring oxygen saturation, although it has one significant drawback—the need to draw blood each time a measurement is taken.

This puts the SaO₂ measurement in the same league as mixed venous oxygen saturation (SvO₂) measurement, although the latter differs importantly in the location of puncture and the measuring technology used. More specifically, the procedure does not require the actual drawing of blood but is comparably invasive and even more complex. First, a flow-directed pulmonary artery catheter (PAC) is introduced into the pulmonary artery, followed by a fibre-optic probe connected to an oximeter monitor.

Utilising the principles of reflection spectrophotometry (determining the amount of saturated haemoglobin through light absorption in examined blood), the diagnostic system then measures the result of oxygen (O₂) consumption and delivery (i.e. how well the body delivers oxygen to tissues). Normal SvO₂ values are between 70% and 80% and any significant deviation is indicative of possible pathological issues: lower values are common in patients with anaemia and higher in those with distributive shock [5, 6].

At the other end of the spectrum are tissue oxygen saturation (StO₂) and peripheral oxygen saturation (SpO₂), non-invasive and generally more convenient methods, but not equal to both SaO₂ and SvO₂. StO₂ measurement is based on near-infrared spectroscopy (NIRS), an optical method of using light to illuminate chemical compounds. The optical sensor is simply pressed to the skin without any special procedures.

As a relatively new technology there are ambiguities regarding normal values for healthy individuals and it is less widely utilised than other methods of measuring oxygen saturation. It is suitable for monitoring foot oxygenation in patients undergoing endovascular revascularisation as a part of critical limb ischaemia, the most serious complication of peripheral artery disease (PAD).

Peripheral oxygen saturation (SpO₂), on the other hand, is a more versatile and far more widely utilised method and has several important advantages in comparison with other methods. Pulse oximetry, as the method of SpO₂ measuring is called, is fairly similar to the StO₂ method: pulse oximetry utilises two light sources of red and near infrared light and measures their absorption in the tissue. The device takes several measurements per second and calculates oxygen saturation from the detected difference between greater oxygen saturation in the arterial blood from the less oxygenated tissue of the venous system.

Pulse oximetry is used in a wide variety of applications, ranging from diagnosing sleep apnoea and detecting hypoxaemia in perioperative patients to assessing the severity of asthma and/or COPD and titrating supplemental oxygen therapy in ventilator-dependent patients [12, 13, 14, 15]. However, no two pulse oximeters are alike and there can be notable differences in measurement results, particularly between cheap and high-end devices, which are more accurate.

The MESI mTABLET SPO2 diagnostic tool

The two main factors affecting the quality of pulse oximetry readings are movements (motion artifact) of the finger or earlobe where the sensor is attached during measurement and low perfusion [17, 18, 19]. Advancements in the field of software and algorithmic filtering have generally made them less of an issue, but to varying extents. They are, for example, no match for the complex algorithms as used by the MESI mTABLET SPO2 to give clinicians accurate results. That is, of course, not all that it offers. The SPO₂-related features include pulse frequency and advanced measuring procedures such as the 6-minute walk test, etc.

Additionally, the MESI mTABLET has integrated support for MESI mRECORDS, an electronic health record (HER) management and sharing system. This enables automatic saving of SPO₂ results in the patient’s EHR and easy sharing of data with appropriate specialists and other healthcare professionals, even if they aren’t users of the MESI mTABLET.

Pulse oximetry is a valuable non-invasive diagnostic method for assessing oxygen saturation and suitable for use in both general practice and inpatient and outpatient facilities. Users in the latter group usually require only a simple pulse oximeter, while the former have far more rigorous requirements that can only be fulfilled by devices such as the MESI mTABLET SPO2, which offers features with benefits that go beyond pulse oximetry.