The effects of milrinone and its mechanism in the fatigued diaphragm in dogs.

UNLABELLED: We studied the effects of milrinone and its mechanism in nonfatigued and fatigued diaphragms in dogs. In Group Ia (n = 5), animals without fatigue, defined as the inability to sustain muscle force, received only maintenance fluids. In Group Ib (n = 5), dogs without fatigue were given a bolus injection (50 microg/kg) followed by continuous infusion (0.5 microg x kg(-1) x min(-1)) of milrinone. In Groups IIa, IIb, and IIc (n = 8 in each), diaphragmatic fatigue was induced by intermittent supramaximal bilateral electrophrenic stimulation at a frequency of 20 Hz applied for 30 min. After producing fatigue, only maintenance fluids were administered (Group IIa); milrinone (50 microg/kg loading dose plus 0.5 microg x kg(-1) x min(-1) maintenance dose) was administered (Group IIb); or nicardipine 5 microg x kg(-1) x min(-1) was infused simultaneously with milrinone (Group IIc). Diaphragmatic contractility was assessed with transdiaphragmatic pressure (Pdi). No differences in Pdi were observed in Groups Ia and Ib. After the fatigue-producing period, Pdi at low-frequency (20-Hz) stimulation decreased from the prefatigued values in Groups IIa, IIb, and IIc (P < 0.05), whereas the decrease was minimal at high-frequency (100-Hz) stimulation. Compared with Group IIa, Pdi to each stimulus increased during milrinone infusion in Group IIb (P < 0.05). In Group IIc, the augmentation of Pdi in the fatigued diaphragm by milrinone was not abolished with an administration of nicardipine. In conclusion, milrinone improves contractility in the fatigued canine diaphragm but not via its effect on transmembrane calcium movement. IMPLICATIONS: Diaphragmatic fatigue may contribute to the development of respiratory failure. Milrinone increases contractility in the fatigued diaphragm and thereby may have an inotropic action on the improvement of diaphragmatic fatigue.