Interaction between ventilator and patient's breathing control represents a challenging field of research. The present study presents the comparison among varying level of assistance under three different modalities of mechanical ventilation (i.e., NAVA, PSV and PCV) on a group of fifteen patients with prolonged weaning. Integrated effort of (i) diaphragmatic electromyography and (ii) inspiratory/expiratory time delays under the three modalities were computed. Integrated effort of inspiratory time delay was lower under NAVA compared to PSV and PCV at less comfortable level of assistance. Integrated effort of expiratory time delay was lower under NAVA compared to PCV and lower under PSV compared to NAVA, both at most and less comfortable level of assistance. Our findings showed better patient-ventilator interaction under assisted ventilator support modes, especially under NAVA, compared to PCV.