Variable-flow nasal CPAP ( nCPAP ) , bubble CPAP ( bCPAP ) at pressures of 5 , 7 , and 9 cmH2O , nasal bi-level positive airway pressure ( nBiPAP ) system at pressures of 5 , 7/5 , and 9/5 cmH2O , and unsupported breathing were studied .