The pattern of bilirubin response to phototherapy for neonatal hyperbilirubinaemia

The pattern of bilirubin response to phototherapy was studied using lamps with an emission spectrum closely similar to the bilirubin absorption spectrum. A dose-response relationship was demonstrated, with increased intensity of phototherapy resulting in greater response; however, the rate of bilirubin decline progressively decreased with increasing radiance till a 'saturation' point was reached, beyond which no further response occurred with further increase in radiance, suggesting a curvilinear relationship of the nature of an asymptotic regression. At the 'saturation' point, a 24-h decline of about 50% was achieved. With extended periods of phototherapy, a gradually decreasing response to phototherapy with declining bilirubin levels was observed, both with maximal or moderate intensity phototherapy; an exponential regression was suggested. At about 5 mg/dl little further decline was observed with continued phototherapy. The rate of the 24-h decline evaluated at 6-h intervals was observed to be proportional to the bilirubin level at the start of the 24-h period.