Abstract
Mechanical ventilation in the intensive care unit (MV) is a rescue measure for patients who have a temporary disability to provide adequate tissue oxygenation through their ventilatory function. It may, however, be the cause of a ventilator-induced diaphragmatic dysfunction (VIDD), increasing the patient's dependence on his ventilator and his length of stay in intensive care unit. Sudden resting of the diaphragm induced by ventilation can cause energy stress as well as mechanical stress participating in the initiation of oxidative stress at the base of an VIDD. Thus, the aim of this thesis is to evaluate the physiopathological mechanisms involved in the genesis of this oxidative stress and to make the mechanistic link with the contractile dysfunction of the diaphragm ventilated on a murine model. The understanding of these mechanisms will allow us to propose new therapeutic approaches to limit the negative impact of this pathology on the future of patients treated in intensive care by mechanical ventilation.In our first study, we showed for the first time, in the diaphragm of mechanically ventilated mice, the presence of a mitochondrial oxidative stress at 6 hours of ventilation, responsible for an early biochemical remodeling of the calcium release channel of the sarcoplasmic reticulum RyR1, leading to disorders of calcium homeostasis at the base of alteration of excitation-contraction coupling, involved in the VIDD. We have also shown that with a longer period of MV, this alteration of calcium homeostasis contributes to a secondary activation of the calcium-dependent protease system linked to the disassembly of actomyosin complexes and to atrophy participating in the VIDD. Finally, we have shown that by pharmacologically targeting RyR1, in order to reduce calcium leak, we could prevent VIDD.Resting the diaphragm early results in a mitochondrial fission phenomenon that has been linked to metabolic stress. In our second study, we show that this early mitochondrial fission is responsible for mitochondrial oxidative stress and that its pharmacological inhibition during MV prevents the production of ROS of mitochondrial origin as well as the remodeling of the RyR1 calcium channel with good therapeutic efficacy on the prevention of VIDD.Mechanical ventilation, by the genesis of assynchrony between the patient and his ventilator can induce a mechanical stress whose involvement in the VIDD is poorly known. In a dystrophic murine model (mdx mice), whose diaphragm is more sensitive to mechanical stress due to a deficiency in dystrophin, we modeled this stress during mechanical ventilation by in vitro application, after 6 hours of ventilation, eccentric contractions. Thus, in our third study, we found that 6 hours of MV in mdx mice increase the susceptibility of the diaphragm to eccentric contractions involving biochemical remodeling of RyR1 channels and the presence of intracellular Ca2 + leakage. We demonstrate on this dystrophic murine model, that a mechanical stress applied in vitro after 6 hours of MV, could activate the NOX2, at the base of an oxidative stress coming to aggravate the remodeling of the diaphragm RyR1 already altered by the MV. We have also shown the pharmacological inhibition of NOX2 to erase the deleterious effects of this mechanical stress imposed in vitro.Finally, we also show in this last work, that a respiratory breathing preventive training in the mdx mice attenuates the VIDD as well as the susceptibility to the eccentric contraction induced by MV. This preliminary work should be continued in order to evaluate the mechanisms of this improvement induced by this training.Thus during this thesis we were able to identify the mechanisms involved in