Monitoring depth of anesthesia

Monitoring depth of anesthesiaOscar Leonardo MosqueraAgendaIntroduction EEG and standard monitoring Cerebral OximetryFMRISimultaneous Electroencephalography and Functional Magnetic Resonance Imaging of General Anesthesia. Purdon et al., Disorders of consciousness:Ann. N.Y. Acad. Sci. 1157: 6170 (2009).Combined fMRI and EEG exploits simultaneously the high spatial resolution of fMRI and the high temporal resolution of EEGRapid prototyping and hardware-in-the-loop simulation.Speedgoat's real-time systems

ELECTROENCEPHALOGRAMAn electroencephalogram (EEG) is a test to measurethe electrical activity of the brain.

Brain electrical activity has a certain number of waves per second (frequencies) that are normal for different levels of alertness.

For example, brain waves are faster when you are awake, and slower in certain stages of sleep. Complexity of the EEG waveform also changes during general anesthesia.

EEG and standard monitoring at clnica universidad de la sabana

Cerebral oximetry

Principle: NIRSBiological Spectroscopic Window exists at the wavelength range 660-940 nm because only a few chromophores like Hb and HbO2 strongly absorb light in this spectra range, allowing light to penetrate tissue to a great distance.

Absorption of this light due to other biological compounds and tissues such as water, lipids, skin, and bone is lower in magnitude, and these biological compounds generally have a flat absorption spectra,*Silvay G, Weinreich A, Owitz S. et al. The cerebral function monitoring during open-heart surgery. Herz. 1978;3:270275McCormick PW, Stewart M, Ray P. et al. Measurement of regional cerebrovascular haemoglobin oxygen saturation in cats using optical spectroscopy. Neurological Res. 1991;13:6570.

In order to guarantee that only cerebral oxygen saturation is being measured most commercially available oximeters minimize extracerebral contamination by equipping the sensors with 2 light detectors located at fixed distances from the light source.

By simply subtracting the measurements obtained from the brain detector from the scalp detector, extracerebral contamination can be minimized.

principle...Regional (Capillary) Oximetry (rSO2) Noninvasive Capillary (venous and arterial) sample Measures the balance between O2 supply and demandbeneath the sensor End-organ oxygenation and perfusion Requires neither pulsatility nor blood flow8Localized Area of MeasurementLED Emitter Distal Detector Proximal Detector Hongo K, Kobayashi S, Okudera H, Hokama M, Nakagawa F. Noninvasive cerebral optical spectroscopy: Depth-resolved measurements of cerebral haemodynamics using indocyanine green. Neurol Res. 1995;17(2):89-93.

Objective #4: ValidationProven brain interrogationProven accuracy

REP DIRECTION: THIS SLIDE IS A SERIES OF 3 CLICKS Lets talk about how this technology was validated.Hongo: peer-review paper Empirically validated performance of our technology in human subjects, not just based on scientific theory. The study validated that our technology:Measures the brainSuppresses surface tissue (ensuring an oxygenation measure of deeper tissues)And does so reliably across test study subjects of different skin color

INVOS technology uses two detectors, allowing light to be processed at two distances from the emitter; proximal and distalHere you see the shallow light path returning to the proximal detector

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9Localized Area of MeasurementLED Emitter Distal Detector Proximal Detector

Hongo K, Kobayashi S, Okudera H, Hokama M, Nakagawa F. Noninvasive cerebral optical spectroscopy: Depth-resolved measurements of cerebral haemodynamics using indocyanine green. Neurol Res. 1995;17(2):89-93.Objective #4: ValidationProven brain interrogationProven accuracy

Here you see the deep light path returning to the distal detectorThe spacing of the light emitter to the detectors defines the depth and length traveled by the light paths.Deep and shallow signals alternate rapidly for separate measurementsThis permits separate processing of oxygen saturation data to localize the area of measurement; a concept known as spatial resolution

CLICK TO NEXT IMAGE

10Localized Area of MeasurementLED Emitter Distal Detector Proximal Detector

Hongo K, Kobayashi S, Okudera H, Hokama M, Nakagawa F. Noninvasive cerebral optical spectroscopy: Depth-resolved measurements of cerebral haemodynamics using indocyanine green. Neurol Res. 1995;17(2):89-93.How deep does it measures?Measures 2.5 3cm beneath the sensor.

The differences in thickness of skull;and the skin pigmentation do not modify the readings.

Objective #4: ValidationProven brain interrogationProven accuracy

Here you can see the two light paths together and the unique paths that they travel through tissues.Again, this permits separate processing of oxygen saturation data to localize the area of measurement; a concept known as spatial resolution

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Assumptions:-Length of the path remains fixed.Light to the proximal detector traverses through whole of the non brain tissue and not through any brain tissue.The beam of light scatter a very little during its courseThe beam is broad in the centre and thin at the sensor end.

12Uses a standard MRI scanner Acquires a series of images (numbers)Measures changes in blood oxygenationUse non-invasive, non-ionizing radiationCan be repeated many times; can be used for a wide range of subjectsCombines good spatial and reasonable temporal resolutionFunctional magnetic resonance IMAGE

Blood oxygen level dependent contrast (BOLD)(Ogawa et al. 1990)The most common technique used in fMRI.

Takes advantage of the magnetic susceptibility of oxyhemoglobin and deoxyhemoglobin (Pauling 1936). Deoxy Hb has a higher precess magnitization decay rate than does oxy Hb.

During periods of neuronal activity, local blood flow and volume increase with little or no change in oxygen consumption. As a consequence, the oxygen content of the venous bloodis elevated, resulting in an increase in the MR signal.

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An fMRI picture of the brain is made up of thousands of boxes, called voxels

Inside each voxel are thousands of neurons, when a lot of these neurons start to fire, the body rushes in oxygen to help, as the bloox oxygen level increases, the voxels get redder

The f MRI is measuring a BOLD signal because the color is Blood Oxygen Level Dependent

The f MRI machine can detect the color change because blood with a lot of oxygen is less attracted to magnets tan blood without much oxygen Combining fMRIandEEGwouldmake it pos-sible to exploit simultaneously the high spatial resolution of fMRI and the high temporal res-olution of EEG25,26Further- more, combining fMRI with EEG offers the potential to relate the large body of information in the anesthesiology literature on EEGpattern changes under general anesthesia to changes in neural activity in specific brain sites. To

Technical and safety problems to solveMRI employs powerful static magnetic, gradient magnetic, and radiofrequency fields, the EEG acquisition system and electrodes must be designed and constructed to minimize physical interactions with these fields that can result in subject injury or compromise data quality.The same standards of physiological monitoring for general anesthesia administered in the operating room, which includes blood pressure, heart rate, oxygen saturation, oxy- gen delivery, and tidal carbon dioxide, must be maintained during these studies.All of the anesthesia equipment, including the anesthesia machine, must be MRI compatible. Airway management and ventilation must be carried out while the subject is being imaged under general anesthesia with the same standards of care as in the operating room.Prior the start of each study, an evacuation drill was conducted to ensure that, in the event of an emergency, the subject could be removed from the scanner within approximately 90 secondsStudy design

Functional imagin methods have only recently been applied to the problema of general anesthesia. Studies using positrn emission tomography (PET) to measure aneshtesia induce changes in regional cerebral blood flow (rCBF), or regional glucose metabolism (rGMR) revealed dramatic reductions in rCBF and rGMR globally across the entire brain after loss of consciousness, but comparatively smaller interregional differences that made it difficult to identify specific sites of anesthetic action.Cerebrovascular confounds pose a serious challenge in fMRI studies of GA. Inhaled anesthetic are potent cerebral vasodilators, increasing cerebral blood flow by 20 to 40%, potentially saturating the BOLD fMRI response.To minimize cerebrovascular confounds, we have chosen to study the drug propofol, which has been shown to preserve cerebral flow metabolism. Among inhaled aneshtetic, sevofluorane has similar properties.Rapid prototyping and hardware-in-the-loop simulation.

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