This observational, prospective and explorative study primarily aims to explore the effect of NNS, applied by means of a pacifier, on acid and non-acid GER features, evaluated in symptomatic preterm infants (gestational age ≤32 weeks) undergoing a diagnostic combined pH and multiple intraluminal impedance (pH-MII). The secondary aim of this study is to evaluate, in the subgroup of patients with recurrent GER-related apneas, the effect of NNS on cardiorespiratory events, defined as bradycardias and total, central, obstructive, mixed apneas and detected by a simultaneous polysomnographic monitoring.