moorVMS-OXY

FAQ

This section lists the more common questions our customers have. If you have a question you would like answered that does not appear below then please email us. We are happy to help!

Q. What parameters does the moorVMS-OXY monitor measure?

A. The moorVMS-OXY monitor measures tissue haemoglobin concentration (oxyHb, deoxyHb and totalHb), oxygen saturation (SO₂) and temperature.

Q. Why is it important to measure tissue oxygen saturation and total haemoglobin?

A. Tissue haemoglobin and oxygen saturation are two important physiological parameters that indicate tissue health at a microcirculatory level. Oxygen is essential for almost all cells of biological tissue. As well as the continued health of all the cells involved, oxygen is also essential to many aspects of wound healing, including immune function, accumulation of new tissue and growth of new blood vessels. Reduction in oxygen delivery is, therefore, a potentially serious impediment to successful and prompt healing as evidenced by the development of ulcers in patients with peripheral vascular disease. The importance of tissue oxygen measurement in many clinical applications, such as prediction of amputation levels, assessment of reconstructive and plastic surgery, and monitoring of peripheral circulation (e.g. in diabetes patients) have also been recognised for many years.

Q. What is the measurement principle used by moorVMS-OXY?

A. The moorVMS-OXY is based on the theory of white-light reflectance spectroscopy. Its measurement relies on spectrophotometric principles that relate light absorption to chromophore concentrations. The colour spectra of the back-scattered light is analysed to give the overall oxygen saturation of the tissue reflecting and absorbing the light.

Q. Why is haemoglobin concentration unit Arbitrary Unit (AU)?

A. Light scattering confounds the light path in tissue and is generally difficult to quantify. Therefore an absolute quantification of haemoglobin concentrations is difficult. However, the relative concentrations from the oxygenated and deoxygenated haemoglobin and trend measurements are reliable parameters.

Q. How is moorVMS-OXY tissue oxygen saturation different from other measures of oxygenation (SpO₂, NIRS, tcpO₂)?

A. The moorVMS-OXY SO₂ is a measure of the percent haemoglobin oxygen saturation in the capillaries and venous area of the tissue microcirculation and therefore reflects changes in the local conditions of supply and consumption in the tissue. Tissue oxygen saturation is generally lower than arterial oxygen saturation (SaO₂) and pulse oximetry saturation (SpO₂), and is more close to venous oxygen saturation (SvO₂).

SpO₂:

SpO₂, measured with a pulse oximeter, is the most widely used measure of haemoglobin oxygen saturation. The primary difference is that SpO₂ relies on the difference in path length during the pulse cycle to calculate oxygen saturation so that it only measures arterial oxygen saturation. This gives a good indication of lung function, but it gives no information about tissue oxygenation and oxygen uptake by organs as SO₂, measured with moorVMS-OXY. In addition, because a pulse oximeter is dependent on detecting the pulse of hemoglobin in the artery, SpO₂ requires a pulsatile flow while SO₂ readings do not.

NIRS:

Tissue oxygen measurement using near-infrared spectroscopy (NIRS) is also a measure of the local haemoglobin saturation at the measurement site. The difference lies in the light source (white light or laser light), wavelength range (visible region or near infrared region) and algorithms for the derivation of the tissue oxygen saturation. NIRS has been primarily developed to monitor deeper tissues such as muscle and brain while moorVMS-OXY based on the visible light spectroscopy (VLS) measures oxygen saturation in the superficial layer of the tissue. Furthermore, the hemoglobin absorption signal is much stronger in the visible range compared to the NIR, resulting in the higher signal-to-noise ratio and better accuracy with the VLS method.

tcpO₂:

tcpO₂ is transcutaneous measurement of the oxygen partial pressure. It uses an electrode to heat the underlying tissue to create a local hyperaemia. This means that the transcutaneous measurement values represent the maximum capacity of the vascular bed and tissue to deliver oxygen and transport carbon dioxide away. Therefore, tcpO₂ value is close to arterial oxygen saturation and is not a measure of tissue oxygen saturation at normal condition as measured by moorVMS-OXY. The tcpO₂ method requires heating the skin tissue to 40°C or higher in order to make a measurement, so it takes several minutes to have the 1st reading and is not suitable for long-term monitoring. In comparison, moorVMS-OXY is a non-invasive, much quick and convenient method for tissue oxygenation measurement. Also, due to the relative large electrode size, tcpO₂ cannot be used for measuring oxygen of the internal tissue and in combination with laser Doppler monitor for blood flow measurement at the same site.

Q. What is the measurement depth of moorVMS-OXY?

A. The actual sampling depth depends on probe design and tissue characteristics. In general, the probe with wider separation between delivery and collecting fibres has deeper measurement depth. Physical models predict that the measurements are sensitive to the upper 1 – 2 mm of tissue.

Q. What is the fastest acquisition rate for moorVMS-OXY?

A. The moorVMS-OXY is capable of taking measurements with a rate up to 40 Hz, where heart rate can be clearly visualised in recorded temporal traces of totalHb.

Q. How does skin pigmentation (melanin) affect moorVMS-OXY measurement?

A. As for any device based on reflectance spectroscopy, very strong skin pigmentation might compromise its performance. In such extreme cases, a warning message will be displayed and it is suggested to try alternative measurement sites or set a longer integration time using moorVMS-PC software.

Q. Are there normal / standard values for tissue oxygen saturation?

A. Tissue oxygen saturation is generally lower than arterial oxygen saturation (SaO₂) and is more close to venous oxygen saturation (SvO₂). Measurement values depend on target tissue site, skin temperature, and patient conditions. Tissue oxygen saturation values for Finger are typically 50% - 80%, and for Leg/Arm are 20% - 50%.

Q. How do I choose which probe to use?

A. The moorVMS-OXY is supplied with a broad probe selection. The standard probes will satisfy most research and clinical purposes. These probes are categorised either by probe type or fibre spacing; Fibre spacing affects penetration depth. Therefore, it is suggested to use skin probe for skin tissues and needle probes for internal tissues. Please consult our sale engineers for more details. We are also happy to discuss any special probe designs or modifications to standard.

Q. Do I need to calibrate the moorVMS-OXY probe before use?

A. No. There is no need to calibrate the probe every time before measurement. The moorVMS-OXY is factory calibrated for each type of probe during manufacture.

Q. Can I regularly check the moorVMS-OXY's performance?

A. Yes. A “Check Probe” is supplied so the user can regularly check the monitor’s integrity.

Q. Does ambient light affect the readings?

A. No. Normally ambient light does not affect the readings because the tissue site is shielded by the probe. To see if the ambient lighting has adverse effect on the measurement, turn the lighting off and on, or cover / uncover the tissue site while reading the SO₂ measurements. External lighting that causes a noticeable change in the SO₂ and totalHb should be avoided or the tissue shielded from it.

Q. Does probe pressure affect the readings?

A. Yes. High probe pressure will press blood out of the tissue and therefore reduce SO₂ and totalHb on skin tissues if not well controlled. All the moorVMS-OXY skin probes are supplied with probe holders and adhesive discs, which will hold the probe on skin firmly with neutral contact pressure.

Q. Is it possible to save the absorption spectra and measurements for post-analysis?

A. Yes. The absorption spectra (raw and reference spectra) and measurements can be continuously saved for post measurement analysis in a text file format using moorVMS-PC software.

Q. Is it possible to measure tissue oxygenation and blood flow simultaneously?

A. Yes. The moorVMS-OXY can be used with the moorVMS-LDF laser Doppler monitor to simultaneously measure SO₂, totalHb and blood flow. Combined probe heads are available. The combination of laser Doppler Flux and SO₂ measurements would allow clinicians to investigate oxygen metabolism in tissue.

Q. Can I use moorVMS-OXY with other protocol modules?

A. Yes. The moorVMS-OXY is intended to form a modular system when used in combination with the LDF monitor, Skin heater, Iontophoresis controller and Pressure controller. When used in this way the instruments will be controlled and monitored via USB using the moorVMS-PC software.The moorVMS–DAQ is also available to integrate 8 channels of other physiological / analogue data from other systems found in the laboratory or clinic.

Q. Does the technique cause any local heating effects or tissue damage?

A. No. The moorVMS-OXY is a non-invasive device hence causing no damage to target tissues. The light induced heating from the probe tip is negligible. Therefore, moorVMS-OXY is suitable for long term monitoring.

Q. Can the moorVMS-OXY be used for tissues other than skin?

A. Yes. The moorVMS-OXY can also be used for internal tissues, such as brain, with needle probes.