Identifying an optimal epoch length for spectral analysis of heart rate of critically-ill infants.

BACKGROUND AND OBJECTIVE

To identify the optimal epoch length for power spectral analysis of cardiac beat-to-beat intervals (BBi) in critically ill newborns.

MATERIALS AND METHOD

BBi of 49 term newborns undergoing therapeutic hypothermia for hypoxic-ischemic encephalopathy with well-defined outcomes (good outcome (n = 28): no or mild brain injury and adverse outcome (n = 21): moderate or severe brain injury or death) served as test population. A power spectrum of BBi was calculated with an autoregressive model in three different epoch lengths: 2 min, 5 min, and 10 min. Spectral power was quantified in three different frequency bands: very low-frequency (0.016-0.04 Hz), low-frequency (0.05-0.25 Hz), and high-frequency (0.3-1 Hz). In each frequency band, the absolute power and the normalized power were calculated. Furthermore, standard deviation (SDNN) of BBi was calculated. These metrics were compared between the outcome groups with a receiver operator characteristic (ROC) analysis in 3-h windows. The ROC curve area >0.7 was regarded as a significant separation.

RESULTS

The absolute spectral powers in all three epoch lengths in all three frequency bands and SDNN distinguished the two outcome groups consistently for most time points. The spectral metrics calculated with a 2-min epoch length performed as well as the five- and 10-min epoch lengths (paired t-test P < 0.05).

CONCLUSION

Spectral analysis of BBi in 2-min epoch shows a similar discriminatory power as longer epoch lengths. A shorter epoch also has clinical advantages for translation into a continuous real-time bedside monitor of heart rate variability in the intensive care unit.