Reflectance spectroscopy uses light emitters and detectors to non-invasively monitor tissue oxygenation by measuring light absorption and scatter through tissue. Visible light spectroscopy (VLS) uses visible light for shallow penetration up to 16mm and is suitable for small volumes, while near infrared spectroscopy (NIRS) uses infrared light for deeper penetration of several cm and larger tissue volumes. These techniques are used clinically to monitor tissue oxygenation in various organs and procedures, particularly cerebral oxygenation monitored by NIRS probes on the forehead. Limitations include light attenuation from skin pigmentation affecting measured values.
2. WHY..?
Organ or tissue oxygenation- reflects the local balance between O2 supply and demand.
Regional O2 balance can differ among the organs as well as different region within the same
organ.
3. Current non-invasive methods for assessment of microcirculatory oxygenation uses REFLECTANCE
SPECTROSCOPY using light
Either in visible spectrum (VLS) OR
In the near infrared spectrum(NIRS)
Recent technique
Based on Protoporphyrin IX triplet state lifetime. (In vivo)
4. REFLECTANCE SPECTROSCOPY
The probes have light emitters and receivers positioned in line.
Light transmission through the tissue is affected by reflection, absorption and scatter.
Reflection depends upon the angle of incidence & wavelength of the light.
Scatter depends upon the number and type of tissue interfaces.
Beer –Lambert’s law relates the absorption of the light by the tissue to the concentration of tissue
chromophores, the extinction coefficient of each, and the path length of the light through the
tissue.
The predominant tissue chromophore is haemoglobin.
5. The path length is affected by both reflection & scatter; so it should be estimated.
Most of the detected photons travel in an arc between the two detectors.
The depth of penetration of arc into the tissue is proportional to the wavelength of light and the
distance btwn transmitter and detector.
6. VLS Vs NIRS:
VLS NIRS
Uses white light Uses infra red light
Wavelength 500-800 nm Wavelength 700-1100 nm
Lesser depth of penetration Deeper penetration
-upto 16 mm - Depth of several cm
Therefore, suitable for small subsurface volume Larger volume of tissue
The volume includes arteries, capillaries, veins
Has a predominantlycenous weighting.
7. CLINICAL APPLICATION:
VLS:
Buccal microvascular Hb saturation- associated with survival in patients with sepsis.
Monitoring flap viability following reconstructive surgery.
Monitoring post operative anastomotic complications in gastrointestinal and oesophageal
surgeries.
Endoscopic VLS –identifies btwn normal and ischemic areas of colon., for diagnosis of mesenteric
ischemia.
Mucosal O2 saturation of the gastric conduit following esophagectomy is useful and explores the
benefits of ischemic preconditioning.
8. NIRS
Mostly in CEREBRAL OXIMETRY.
Probes placed on forehead to measure frontal cortical oxygenation (rSO2).
No gold std for cerebral oximetry.
Typical value of rSO2 range from 51% to 82% with mean baseline of 66%.
Reduction of rSO2 below 20-25% of the baseline or lower than absolute value of 50%- calls for Intervention.
9. USES:
In cardiovascular, abdominal, thoracic and orthopedic.
During carotid endarterectomy, intraoperative reduction in rSO2 corelate well with
changes in transcranial doppler measurements, electroencephalographic waveforms,
stump pressure which are consistent with ischemia.
During and following ICU open thoracoabdominal aortic aneurysm repair for continuous
monitoring of spinal cord oxygenation.
In shock- when patients have inadequate regional perfusion in the setting of normal
global perfusion parameters. (Applied to thenar eminence- StO2)
10. In patients with major trauma, who present in shock, StO2 values can identify patients who
proceed to develop multiorgan dysfunction.
