Introduction There are approximately 19 000 admissions to paediatric intensive care units in the United Kingdom and Ireland each year. Continuous heart rate monitoring is used to guide treatment, with the aim to ‘normalise’ heart rate. However, ‘abnormal’ heart rates may be an adaptive response to critical illness. In order to decide what is acceptable, we must first define the distribution.

Aims 1.) To describe the distribution of heart rate in intensive care patients during the first 24 hours post-admission

2.) To investigate whether heart rate is associated with clinical outcomes.

Methods 8 94 578 heart rate data from admissions to the paediatric, neonatal and cardiac intensive care units at a tertiary centre in 2013 and 2014 were used in this study. Patients aged 0–18 years were included. We calculated age-standardised heart rates (Z-scores) for each admission over the first 24 hours on the intensive care unit. Two summary statistics were devised - area under the curve and mean z-score over 24 hours – to analyse the relationships between heart rate, length of stay, length of ventilation and discharge status.

Results The mean crude heart rate of the study population was 130 beats per minute. The mean z-score was +0.36. A decrease in mean z-score over the 24 hour period was observed: the mean z-score at hour 0 of +0.58 dropped to +0.25 by hour 23. There was insufficient evidence to suggest a significant relationship between standardised heart rate scores and outcomes.

Conclusions Despite the role of heart rate as an indicator of clinical deterioration, this is the first study to investigate heart rate distribution on the intensive care unit. On average, critically ill children have higher heart rates than their healthy age-equivalents over the first 24 hours, even though heart rate decreases towards the end of this time period.