Vital-Sign Synchrony as a Marker for Patient Circadian Rhythms in an Intensive Care Unit

Patient circadian rhythms are often disrupted in an intensive care unit (ICU). This disruption is associated with worsened patient outcomes, thus new methods are needed to quantify patient circadian rhythms. We hypothesise that the cross-correlation between vital-sign circadian profiles will allow us to stratify patients by rhythm strength, without reliance on a prior assumed rhythm profile. We selected from the eICU-CRD and MIMIC-III databases the cohort of patients in their final 24 hours of ICU stay who subsequently recovered. We then calculated the mean cross-correlation (R) between each patient's systolic blood pressure, heart rate, and respiratory rate profiles, and grouped them into ‘high’, ‘mid’, and ‘low’ correlation cohorts. The high-corr. cohorts showed vital-sign profiles that closely resembled those reported in the literature for non-ICU cohorts, with peaks at awakening and in the evening, and a large trough overnight. The mid- and low-corr. cohorts, in contrast, showed less consistent and defined peaks and troughs. Vital-sign peak-nadir excursions in the high-correlation cohorts were greater, and the length of ICU stay significantly shorter (p < 0.05), than those for the low-corr. cohorts.