Non-Invasive Monitoring of Microvascular Oxygenation and Reactive Hyperemia using Hybrid, Near-Infrared Diffuse Optical Spectroscopy for Critical Care

The goal of this protocol is to evaluate microvascular oxygenation and reactive hyperemia in peripheral muscles, especially in the context of intensive care management of critically ill patients. Together with Diffuse Optical Technologies and Vascular Occlusion Tests, we can assess various parameters to have an insight into microvascular oxygenation and reactive hyperemia. We are using the vascular device that combine various modules, including two near infra diffuse optical techniques, pulse oximetry, and an automatic tourniquet.

It is designed to measure metabolic rate of oxygen and microvascular reactivity using our vascular occlusion test induced by a sustained arterial occlusion of the arm. The information from e-technology is integrated to provide a multi-modal approach to the study of microvascular health. Near infrared spectroscopy utilizes laser passes in the order of picosecond at multiple wavelength and it measure the delay and the broadening of such pulses as they travel through the tissue.

Multiple wavelength are used to calculate different blood and tissue components. In vascular we utilize, we utilize 685 nanometer and 830 nanometer in order to compute the oxygenated hemoglobin and de-oxygenated hemoglobin, and thus percentage of microvascular oxygen saturation. Diffuse correlation spectroscopy uses the variation of Near Infrared light from a continuous wave, coherent laser source.

This technology in order to calculate the blood flow, takes advantage of the decay of an auto correlation function of speckle intensity that is due to the movement of light scattering particles such as red blood cell. In the DCS, in vascular, we utilizes as a wavelength 785 nanometer. Lastly, pulse oximetry measure the heart rate and the percentage of arterial oxygen saturation.

With this protocol, what we are able to measure is tissue oxygenation. Tissue oxygenation is a raw parameter combining tissue perfusion, arterial oxygenation, and metabolic rate of the tissue, so venous oxygenation. When we perform the vascular occlusion test, what we have is the metabolic rate of the tissue, so the deoxygenation, the desaturation of the signal provides information of the metabolic rate, isolated, not perfusion, just metabolic rate.

After that, when we release the cuff after this ischemic challenge, we will have a resaturation, a reoxygenation of the signal and a hyperemic response. So this reoxygenation and this hyperemic response are providing information about the microvascular reactivity of the tissue, which is which talks about the performance of endothelial function. The vascular probe has optical windows for laser sources and detectors for diffuse correlation spectroscopy and time results spectroscopy.

The source detector separation is 25 millimeter for both. The probe is indicated with the capacity touch sensor and accelerometer, a load, and a light sensor. The laser safety system in the device uses the touch sensing to shine only the laser when the probe is placed on the tissue.

As soon as the detachment is sense, the lasers are switched off, such that the patients as well as the operators are safe. Switch on the device. The device starts with in-house developed software.

Turn the safety key to on position. Place the probe completely inside the instrument response function box and press the reset button on the probe, if it is glowing. Wait for the device to be ready.

It performs self-test to ensure stable functioning. When the device is ready, it ask if you want to measure an IRF. Now the device automatically adjusts the laser intensity to reach the desired count rate of 1 million.

Press the stop button when you see a stable count rate and DTOF. This IRF is saved to the device as well as loaded in the software to be utilized for real-time calculations. Now, we can continue to perform a phantom measurement.

Insert the probe in the phantom box properly, such that the probe attached indicator is on. The phantom protocol starts with the quality control test that verify that a sufficient number of photons are received by the DCS and TRS detectors and also checks if the dark counts are within the desired limits. The quality check also confirms that there is no interference between modalities.

Continue the phantom protocol for at least 30 seconds to have a sufficient amount of data saved for further of an analysis. Attach the tourniquet on the upper arm above the elbow as done during a blood pressure measurement. Do not wrap the cuff loosely or very tightly around the arm.

Loosely attaching the tourniquet will need more air to reach desired pressure. Slow inflation can allow body to readjust physiology. Attach the pulse oximeter to the index finger of the same arm.

If it is not possible to attach to the index finger, attach it to any other finger. Locate the muscle to be probed, which is in the lateral forearm just below the elbow. The muscle can be traced by slightly twisting the arm with one hand.

The muscle can be felt between the thumb and fingers with the other hand. Measure the arm circumference around the located muscle using a short measuring tape. Measure the approximate tissue thickness on top of the muscle by using a detailed body fat caliber.

Attach the probe head on the muscle with the optical fibers and cables going towards the hand. Do not attach the probe tightly. It can affect tissue physiology.

Make sure the fibers are not touching any moving objects. It can create artifacts in the data. Cover the probe with a black cloth to block the external light.

If the patient is awake, inform him that the vascular occlusion test can cause tingling sensation and not move the arm. Ensure the probe attached. LED indicator on the front panel of the device is shining, and the touch icon in the software is green, which shows that the probe is attached.

Press the protocol time button. It opens a new dialogue box. Enter the subject ID, operator ID and the target pressure of 50 millimeter mercury higher than the systolic blood pressure.

Press okay to start the automated protocol. Real-time data is displayed in the graphs. The protocol starts with the quality control that automatically adjust laser power, checks the photon count, and the interface between modalities.

The quality check is completed within two minutes. Observe the circular icons labeled DRS and DCS, which must turn green at the end of the quality check. The green icons show that the photon count rate is within the desired range.

There is no external light entering the probe. There is no crosstalk between modalities and hence the measurement can be continued. The graphs are reset at the end of quality phase and signals representing patient data are plotted in real time.

Press the stop button to abort the protocol. If the patient is not stable or if the patient requires clinical intervention at any instant during the protocol. Press the extend button to add 30 seconds of pre-occlusion duration.

If the patient moves the arms and does not have a stable baseline signal for any other reason, the operator can press exchange as many times and in any phase required. Each button press will add 30 seconds. The tourniquet automatically inflate to desired pressure to start the vascular occlusion test.

Press plus or minus buttons to increase or decrease the desired occlusion pressure in steps of 5 millimeter mercury if the blood pressure of the patient changes after starting the protocol. The start and stop of the vascular occlusion test are automatically marked with yellow vertical lines. The software is set to continuously acquire data and to automatically perform three minutes of vascular occlusion test.

After three minutes of baseline, the pre-defined standard protocol lasts for six more minutes after the completion of vascular occlusion test to evaluate the recovery after the patient's hyperemic response is over and a stable condition is obtained. Press okay, when the operator is notified at the completion of protocol through a popup notification which marks the successful completion for the protocol. The operator can remove the probe and the cuff from the patient and clean them using an alcohol swab or equivalent.

Write down the clinical and demographic information according to the predefined study protocols, along with the circumference of the arm at the probe location and the thickness of overlying adipose tissue in the patient data form manually. The computation of real-time absolute values of oxygenated, deoxygenated and total hemoglobin and tissue oxygen saturation is achieved by fitting algorithm using the curve sometime resource spectroscopy of both wavelengths. The calculation of blood flow index in real time is achieved by the fitting algorithm using the auto coercion curves from diffuse coercion spectroscopy.

User script written in your favorite language to reopen and visualize recorded boundary data. Using the script, calculate the index of oxygen consumption, rate, and amplitude of deoxygenation, rate of reoxygenation and amplitude and area under the curve of the reactive hyperemic response after the vascular occlusion test. From this protocol, we can continuously measure absolute tissue oxygen saturation, blood flow index, and arterial oxygen saturation.

The combination of these parameters results in acquiring the metabolic rate of oxygen consumption index while performing the vascular occlusion test. When the cuff is inflated, we get the rate of deoxygenation that shows how fast oxygen is being consumed in the probe region. At the end of vascular occlusion test, when the cuff is deflated, we can see the speed at which the tissue is reoxygenated that shows how fast the oxygen is supplied to an oxygen depleted region.

The results show an increase in the deoxygenated hemoglobin and a decrease oxygenated hemoglobin. As oxygen is extracted from hemoglobin and the number of oxygen depleted cells increase. We can observe a declining trend in microvascular oxygen saturation during the vascular occlusion test.

The early rate of this decrease is representative of metabolic rate of oxygen consumption. While the hyperemic peak, and subsequent decay are linked with the endothelial function and microvascular reactivity. Several biomarkers have been used in the literature like the rate of deoxygenation, amount of deoxygenation, rate of reoxygenation, hyperemic peak value, and area under the curve to represent the severity of diseases as well as classification between healthy and patient populations.

Apart from obtaining absolute concentration values, a further advantage of this protocol is the blood flow index. As hyperemic saturation alone does not express the local increase of oxygen, the blood flow index complements gaining insights into the baseline metabolic rate of oxygen consumption and perfusion. Further analysis of blood flow index also provides the per stability index as the vascular device is capable of fast acquisition of diffuse correlation spectroscopy.

When using the protocol, always ensure that quality tests have been passed, which shows that all the device parameters are within acceptable ranges. Therefore, the data being displayed and stored is useful and meaningful. With this protocol, we can non-invasively provide clinicians with absolute values of hemoglobin oxygen saturation and blood flow index using multiple technologies that clinical parameters obtained can be used to evaluate tissue perfusion, endothelial function, microvascular reactivity, and oxygen metabolism.

The fully automated protocols for device calibration as well as human measurements reduce operator base variations and result in more reliable data.