National Board Ex- 6 Book .pmd - National Board Of Examination

Monitoring in theperioperative periodplays an important partin the satisfactory outcome of asurgical patients. It assumesparamount significance when thepatients belong to neurosurgeryspecialty because, their conditionmay change rapidly within amatter of few minutes, leadingto poor outcome. Neweradvanced monitoring techniquesare introduced into clinicalpractice for the satisfactoryoutcome of neurosurgicalpatients. The following advancedmonitoring (except SJO 2)technique have still a long wayto go before they becomeroutine in clinical practice.Near Infrared Spectroscopy(NIRS)NIRS is an application of atechnology that has beenavailable for a number of years.It can be used to provideinformation about changes inregional cerebral oxygenation,cerebral blood flow and volumeand oxygen utilization in thebrain.PrinciplesThe principle behind NIRS isbased on the fact that light in thenear infrared (700-1000nm) canpass through skin, bone, andother tissues relatively easily.When a beam of light is passedthrough brain tissue, it is bothscattered and absorbed. Theabsorption of near infrared lightAdvances in Neuroanaesthesia MonitoringParmod BithalDepartment of Neuroanaesthesia, All India Institute Of Medical SciencesNew Delhiis proportional to theconcentration of certainchromophores, notably iron inhaemoglobin, copper incytochrome aa 3. Oxygenated(HbO 2) and deoxygenatedhaemoglobin (Hb) andcytochrome aa 3have differentabsorption spectra, depending onthe substances , oxygenationstatus. The isobestic point ofoxygenated and deoxygenatedhaemoglobin is at about 810 nm.HbO 2has greater lightabsorption above this wavelengthand Hb has greater lightabsorption below 810 nm. Themaximum oxidation/reductionproportion for cytochromeoxidase or cytochrome aa 3, whichis the terminal member of themitochondrial respiratory chain,is at 830 mm. This allows for themeasurement of oxidation status.NIRS interrogates arterial,Venous and capillary blood andtherefore the delivered oxygensaturation is an average value forthese compartments. However,most of the NIRS signal is fromthe venous blood because itcontributes to approximately70% of the intracranial bloodvolume. This techniqueindirectly assesses flow bydetecting changes in venoussaturation and can provideinformation about tissue severalcentimeters below the probe.NIRS has greater tissuepenetration than pulse oximeteryand does not need15RecentAdvancespulsatility.Near infraredinstrument generally consists ofsmall optical probes connected toa monitoring device by a wirebundle. This enables the monitorto be placed at a distance fromthe patient which will facilitate itsuse in ICU and during surgery.The rubber optical probescontain light source consisting ofsmall tungsten light filament ofless than 3 watts and two photodiodes filtered at 760 nm and 850nm The light sources arerecessed so as to prevent directskin contact. A photodiodedetector converts the reflectedlight to a current and then to avoltage for amplification andsignal detection.The probesilluminate upto a volume of 10ml of hemispherical tissue. Theradial depth will depend on theinteroptode distance. Theoptodes are placed on one sideof the forehead with aninterrupted spacing of 4-7 cm.Normal values of the HbO 2arereported to be 60-80% and theischaemic threshold is estimatedto be 47% saturation ( 1).Clinical applicationOne of the major problems withNIRS is the inability to reliabledistinguish between extracranialand intracranial changes in bloodflow and oxygenation whichaffect its reliability as a monitorof brain oxygenation in clinicalpractice. The amount ofextracranial contaminationJournal of Postgraduate Medical Education, Training & Research97