Effect of different phototherapy lights on incubator characteristics and dynamics under three modes of servocontrol

Phototherapy is one of the most common therapeutic interventions in modern neonatal medicine. Analysis of physiological responses to phototherapy requires understanding the physical effects of incident light under different modes of environmental control. In this study, we tested the effect of initiation of phototherapy under laboratory conditions in a convective incubator maintained in three clinically used servocontrol modes. Under conditions of air servocontrol, the initiation of phototherapy resulted in an abrupt sustained increase in the surface temperature of an infant simulator (a blackened aluminum disc). In contrast, when the surface (skin) temperature was the controlled variable, there was a profound drop in the incubator air temperature. An algorithm that simultaneously controlled both the skin and the air temperatures showed intermediate effects. Under conditions of skin servocontrol, fluctuations in air and environmental temperatures were observed for a period of 3 hours before steady state was reached. These findings are most clearly demonstrated using phase plane plotting techniques. We conclude that wide, abrupt, and sustained changes in the thermal environment of an incubator occur after phototherapy is initiated. Such changes must be anticipated in physiological studies using phototherapy.