Light and the outcome of the critically ill: an observational cohort study

Abstract

Introduction: Light before and during acute illness has been associated with both benefit and harm in animal models and small human studies. Our objective was to determine the associations of light duration (photoperiod) and intensity (insolation) before and during critical illness with hospital mortality in ICU patients. Based on the 'winter immunoenhancement' theory, we tested the hypothesis that a shorter photoperiod before critical illness is associated with improved survival.

Methods: We analyzed data from 11,439 patients admitted to 8 ICUs at the University of Pittsburgh Medical Center between June 30, 1999 and July 31, 2004. Daily photoperiod and insolation prior to and after ICU admission were estimated for each patient by using data provided by the United States Naval Observatory and National Aeronautics and Space Administration and direct measurement of light gradient from outside to bedside for each ICU room. Our primary outcome was hospital mortality. The association between light and risk of death was analyzed using multivariate analyses, adjusting for potential confounders, including severity of illness, case mix, and ICU type.

Results: The cohort had an average APACHE III of 52.9 and a hospital mortality of 10.7%. In total, 128 ICU beds were analyzed; 108 (84%) had windows. Pre-illness photoperiod ranged from 259 to 421 hours in the prior month. A shorter photoperiod was associated with a reduced risk of death: for each 1-hour decrease, the adjusted OR was 0.997 (0.994 to 0.999, p = 0.03). In the ICU, there was near complete (99.6%) degradation of natural light from outside to the ICU bed. Thus, light exposure once in the ICU approached zero; the 24-hour insolation was 0.005 ± 0.003 kWh/m² with little diurnal variation. There was no association between ICU photoperiod or insolation and mortality.

Conclusions: Consistent with the winter immunoenhancement theory, a shorter photoperiod in the month before critical illness is associated with a reduced risk of death. Once in the ICU, patients are exposed to near negligible natural light despite the presence of windows. Further studies are warranted to determine the underlying mechanisms and whether manipulating light exposure, before or during ICU admission, can enhance survival.

Figures

Figure 1

Characteristics of insolation and photoperiod. (a) Daily insolation and photoperiod from 30 June 1999 to 31 July 2004 (+40.447°N -79.9517°W). (b) Cumulative photoperiod before intensive care unit (ICU) admission, according to season. (c) Degradation of light signal (lux) from outdoors to ICU bed on sunny and cloudy days. (d) Adjusted cumulative ICU insolation (expressed in kilowatt hours per square meter) according to season.

Figure 2

Association between cumulative 28-day photoperiod and mortality. The association between cumulative 28-day pre-admission photoperiod and in-hospital mortality was examined by using multivariate logistic regression and the fractional polynomial method. The adjusted odds ratio of death with decreasing cumulative 28-day pre-admission photoperiod is depicted. The reference category is the highest cumulative 28-day pre-admission photoperiod.