To study the influence of artificial ventilation rate on neonatal heart rate variability (HRV), ECG and respiratory impedance curves were recorded four times a day in 20 preterm infants (<33 wk) during the first 3 d after birth while the infants were ventilated at a wide range of ventilator rates. The contents of selected frequency bands within the R-R interval power spectrum were calculated for 3-min periods. Respiratory distress syndrome severity was assessed at each measurement. Respiratory sinus arrhythmia (RSA) induced by the ventilator appeared to mimic spontaneous RSA. As in spontaneous respiration, the amount of RSA (power in a frequency band around the respiratory rate) increases as the ventilation rate decreases. This phenomenon is most probably due to entrainment with baroreflex-related fluctuations in the heart rate. Although the artificial ventilation rate influences RSA and thus high-frequency HRV, an increase in respiratory distress syndrome severity results in a decrease in low-frequency HRV. Thus, the attenuation of low-frequency HRV by respiratory distress syndrome is not likely to be due to artificial ventilation. (Pediatr Res 37: 124-130, 1995) Abbreviations aRSA, artificial respiratory sinus arrhythmia aVR, artificial ventilation rate HF, high-frequency HRV, heart rate variability LF, low-frequency RDS, respiratory distress syndrome RSA, respiratory sinus arrhythmia VLF, very LF HRV, i.e. the variation in beat-to-beat R-R interval length, is influenced by maturational (e.g. gestational age), physiologic (e.g. respiration), and clinical factors (1, 2). HRV can be assessed by spectral analysis to study the frequency-specific oscillations in the heart rate signal. In the newborn, the following heart rate oscillations may be present. HF oscillations have a frequency equal to the respiratory rate and are known as RSA. LF oscillations are due to intrinsic oscillations of the baroreceptor reflex loop with a frequency of 0.07 Hz (3). A second kind of LF oscillation may result from periodic fluctuations in respiratory depth or tidal volume (4). The last type of HRV features VLF oscillations with a frequency below 0.04 Hz. These are ascribed to thermoregulation. H F oscillations are mediated by the parasympathetic system. LF and VLF oscillations are mediated by both the sympathetic and parasympathetic systems (5). Thus, studying neonatal HRV can give an insight into the maturation of the autonomic nervous system.In newborns suffering from RDS, HRV appears to be atten- HRV returns to normal (2). Several explanations have been proposed for this reversible effect of RDS on HRV. For instance, it has been ascribed to transient depression of the medulla oblongata by hypercarbia (2). The possible influence of artificial ventilation as such on neonatal HRV has not been taken into account in studies regarding the influence of RDS. For the correct interpretation of HRV in infants with RDS, insight into the effect of artificial ventilation on HRV is needed. This influence cannot be studied by simply comparing...