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

Photocyclization of bilirubin to lumirubin in the skin of jaundiced in fants exposed to blue-green light irradiation is considered to be the most important process for bilirubin elimination from the organism. Th e quantum yield phi(LR) of the bilirubin --> lumirubin photoreaction h as been recently measured and found to vary with the excitation wavele ngth, with a peak at about 520 nm. The quantum yield phi(ZE) for the s trongly competing reversible configurational photoisomerization of bil irubin has also been recently shown to be wavelength dependent and to decrease significantly in the long-wavelength part of the absorption b and of bilirubin. These new data are taken into account to model the b ilirubin photochemistry in vivo by using a simplified skin optical mod el based on the Kubelka-Munk theory. The rate k(LR) of formation of lu mirubin has been evaluated for the case of a four-layer skin and for m onochromatic and narrow-band coloured fluorescent lamps. The effects o f long-wavelength increase in phi(LR), decrease in phi(ZE) and skin op tical losses all combine to shift significantly the optimal rate of fo rmation of lumirubin towards the green. These results suggest that a s ignificant improvement in phototherapy might be obtained with the intr oduction of new lamps emitting in the blue-green spectral region betwe en 490 and 510 nm.