There is still uncertainty about safety and effectiveness of HFNC. The aim of this study is to investigate the preterm infants respiratory effort by measuring electrical activity in diaphragm (Edi max and Edi min), respiratory parameters and a clinical observation using a scoring system inspired by Silverman- Andersen retraction score. It is expected that measured electrical activity in the diaphragm, measured respiratory parameters combined with bedside observations provide applicable knowledge about preterm infants respiratory effort in transition from CPAP to HFNC.