Fig. 3. Diaphragmatic pressure–frequency curves obtained in the hypercapnia group. ( A ) Induction of hypercapnia phase (n = 8). Transdiaphragmatic pressure (Pdi) after supramaximal phrenic nerve stimulation at 20, 40, 60, 80, 100, and 120 Hz for each level of arterial carbon dioxide tension (Paco2) obtained in the hypercapnia group. Pdi was significantly lower (* P < 0.05 vs . baseline) between the third (T2), fourth (T3), and fifth (T4) levels of capnia and the initial value of Pdi (T0: normocapnia–baseline). ( B ) Recovery phase (n = 5). Pdi obtained after supramaximal phrenic nerve stimulation at 20, 40, 60, 80, 100, and 120 Hz at baseline (Paco2= 40 mmHg) and at 30 and 60 min after return to normocapnia in the five piglets in which recovery phase has been performed. Pdi was significantly lower (* P < 0.05 vs . baseline) between the recovery phase and the initial value of Pdi (T0: normocapnia–baseline). Data are expressed as mean ± SD. NS = not significant.