Theory Of Operation - Philips Respironics BiliChek Benutzerhandbuch

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Jaundice
Jaundice is a condition characterized by yellowing of the skin and eyes that is caused by an excess of
bilirubin in the blood. Bilirubin is a normal waste product resulting from the breakdown of red blood
cells and is removed from the body by the liver. Prior to birth, bilirubin in an infant is processed by
the mother's liver and excreted. After birth, an infant must eliminate bilirubin without the mother's
help. It may take the infant's system several days to begin eliminating bilirubin from the blood faster
than it is produced. Infants who are born prematurely, who are underfed, or who belong to certain
ethnic groups are at increased risk of developing jaundice. The initial screening of jaundice is the
observation of yellow skin. This is a subjective determination prone to errors due to differing skin
colors and gestational ages. Bilirubin levels normally peak in full-term infants 4 to 5 days after birth
and may peak later in premature infants. If the infant's serum bilirubin levels continue to rise the
infant is at risk for kernicterus (the deposit of bilirubin in the brain at toxic levels which can cause
permanent neurologic impairment).

Theory of Operation

The BiliChek works by directing white light into the skin of the newborn and measuring the intensity
of the specific wavelengths that are returned. By knowing the spectral properties of the components
within the skin, one can subtract out the interfering components and determine the concentration of
bilirubin. This is called a transcutaneous bilirubin measurement.
Photons of specific wavelengths are preferentially absorbed by certain molecules. By plotting the
absorption against the wavelength, one can visualize characteristic absorption spectra of the particular
molecules. For example, melanin has a near-linear absorption spectrum in the visible spectrum and,
like the scattering phenomenon, there is greater absorption of photons with shorter wavelengths
than in the red region of the spectrum. Conversely, hemoglobin is a much more complicated absorber
which is compounded by the fact that oxyhemoglobin and deoxyhemoglobin have different profiles.
The peak absorption of photons by bilirubin occurs at a wavelength of 460 nm. This is in the blue
portion of the spectrum and is the reason why blue lights are sometimes preferred for phototherapy.
It is also in a region of the spectrum at which hemoglobin absorption is relatively low.
The light reflected from the skin of neonates and collected by the BiliChek is analyzed with a highly
sophisticated, proprietary algorithm to generate a serum bilirubin measurement. The major skin
components (see figure) which impact the spectral reflectance in newborns are (1) dermal maturity,
(2) melanin, (3) hemoglobin, and (4) bilirubin. The intensity of the reflected light is converted to
absorbance units, Optical Density (OD), for analysis. The total OD at each measured wavelength is
equal to the sum of the component ODs. This is represented mathematically as:
OD = OD + OD + OD + OD
total derm mel heme bili
BiliChek user manual