Unraveling the Force of Inspiration: Pathophysiological Mechanisms of Diaphragm Dysfunction in Mechanically Ventilated Critically Ill Patients.

The pathophysiology of diaphragm dysfunction in mechanically ventilated critically ill patients is complex and multifactorial. This thesis aimed to gain more insight into these pathophysiological mechanisms. To that end, we performed contractility, histological and biochemical experiments on unique diaphragm biopsies and tissue obtained during animal studies. We discovered that contractile dysfunction of diaphragm myofibers is caused by atrophy accompanied by replacement fibrosis and dysfunction on the myofilament level. Myosin heads in myofibers of critically ill patients are folded back onto the myosin backbone; a conformation change called the super-relaxed state (SRX). SRX increases the distance between the myosin head and actin filament, which reduces calcium sensitivity, hampers cross-bridge formation, and, consequently, force production. Contractile dysfunction was restored by treatment with a troponin activator. Additionally, we discovered that diaphragm myofibers adapt to a shortened position caused by mechanical ventilation with positive-end expiratory pressure (PEEP). To maintain an optimal thin- and thick filament overlap during PEEP ventilation, myofibers adapt by absorbing sarcomeres in series. This process is called longitudinal atrophy and may be modulated by mechanosensing through the spring-like myofilament titin and its titin binding proteins such as Muscle Ankyrin Repeat Protein 1 (MARP1). MARP1 is highly upregulated in diaphragm myofibers of critically ill patients, increases the titin filament's stiffness, and may provide sarcomere stability by locking titin-N2A to the thin filament. Data in this thesis suggest that mitochondrial dysfunction and oxidative stress do not play a causal role and that protective mechanisms are at play. Future research should explore these mechanisms to prevent diaphragm weakness by improving ventilation strategies, discovering new therapeutic targets, and optimizing weaning strategies to accelerate liberation from the ventilator.