QUANTUM YIELD AND SKIN FILTERING EFFECTS ON THE FORMATION RATE OF LUMIRUBIN

Photocyclization of bilirubin to lumirubin in the skin of jaundiced infants exposed to blue-green light irradiation is considered to be the most important process for bilirubin elimination from the organism. The quantum yield phi(LR) of the bilirubin --> lumirubin photoreaction has been recently measured and found to vary with the excitation wavelength, with a peak at about 520 nm. The quantum yield phi(ZE) for the strongly competing reversible configurational photoisomerization of bilirubin has also been recently shown to be wavelength dependent and to decrease significantly in the long-wavelength part of the absorption band of bilirubin. These new data are taken into account to model the bilirubin photochemistry in vivo by using a simplified skin optical model based on the Kubelka-Munk theory. The rate k(LR) of formation of lumirubin has been evaluated for the case of a four-layer skin and for monochromatic and narrow-band coloured fluorescent lamps. The effects of long-wavelength increase in phi(LR), decrease in phi(ZE) and skin optical losses all combine to shift significantly the optimal rate of formation of lumirubin towards the green. These results suggest that a significant improvement in phototherapy might be obtained with the introduction of new lamps emitting in the blue-green spectral region between 490 and 510 nm.