Transdiaphragmatic pressure (Pdi) expressed as percentage of Pdi at normocapnia obtained at 100 Hz stimulation at each stimulation (every 12 hours) in the Normocapnia group (A) ( n = 6) and in the Hypercapnia group (B) ( n = 6). Pdi was significantly lower at 100 Hz compared with H0 after 48 hours of mechanical ventilation in the normocapnia group (A). Conversely, no significant decreases were noted in Pdi between the successive measures in the hypercapnia group (B). Data are expressed as percentage (mean ± SD). NS, not significant. P values refer to between H72 versus baseline.