Effects of controlled mechanical ventilation on respiratory muscle contractile properties in rabbits

Objective: We examined in rabbits the effects of more than 48 h of mechanical ventilation on the contractile properties and fiber type adaptations of the respiratory muscles. Design and setting: Experimental prospective study in a university laboratory. Animals and interventions: Nineteen rabbits were randomly allocated to two groups: control (n=10) or mechanically ventilated (MV; n=9) for 51 +/- 3 h. Measurements and results: Respiratory muscles contractile properties were analyzed before and after a fatigue protocol using in vivo isometric 1-s tetanic contraction characteristics in both muscles: peak tetanic force, contraction time, relaxation time, and total contraction time. Both muscle fiber type proportions, diameter, and cross-sectional areas were measured using ATPase staining. The MV rabbits showed significant weight loss in both muscles, accompanied by a reduced peak tetanic force (9.96 +/- 3.2 vs. 7.44 +/- 2.2 N for diaphragm of control and MV animals respectively), fatigue resistance index, and increased relaxation time (57.5 +/- 8.7 vs. 85.8 +/- 9.4 ms for diaphragm of control and MV animals) and contraction time. These impairments in the MV group worsened after the fatigue runs. Both muscle showed a significant atrophy of type IIa and IIb fibers but a stability in type I fibers cross-sectional area. Conclusions: Mechanical ventilation in rabbits produces alterations in contractile properties of the diaphragm and 5th external intercostal muscle, increases both muscles fatigue, and promotes atrophy of type II fibers.