Spectral analysis of heart rate variability in spontaneously breathing very preterm infants.

The influence of maturation, sleep state and respiration on heart rate variability was studied in 16 spontaneously breathing preterm infants (< 33 weeks). ECG, respiratory impedance curve and movements were recorded four times a day, during the first three days of life. The power content of selected frequency bands of the R-R interval power spectrum, as well as respiratory frequency and breath amplitude oscillation frequency, were calculated for 3-min periods. An increase in low-frequency heart rate variability with gestational age was found. High-frequency variability increased during early postnatal life. Sleep state influenced very low-frequency heart rate variability. The amount of respiratory sinus arrhythmia and breath amplitude sinus arrhythmia was determined mainly by respiratory rate and breath amplitude oscillation frequency, respectively. The influences of gestational and postnatal age on heart rate variability might be due to an increase in sympathetic tone before birth and a change in parasympathetic-sympathetic balance after birth. Respiration has an important influence on heart rate variability, even in very preterm infants.