Clinical review: ventilator-induced diaphragmatic dysfunction--human studies confirm animal model findings!

Abstract

Diaphragmatic function is a major determinant of the ability to successfully wean patients from mechanical ventilation. However, the use of controlled mechanical ventilation in animal models results in a major reduction of diaphragmatic force-generating capacity together with structural injury and atrophy of diaphragm muscle fibers, a condition termed ventilator-induced diaphragmatic dysfunction (VIDD). Increased oxidative stress and exaggerated proteolysis in the diaphragm have been linked to the development of VIDD in animal models, but much less is known about the extent to which these phenomena occur in humans undergoing mechanical ventilation in the ICU. In the present review, we first briefly summarize the large body of evidence demonstrating the existence of VIDD in animal models, and outline the major cellular mechanisms that have been implicated in this process. We then relate these findings to very recently published data in critically ill patients, which have thus far been found to exhibit a remarkable degree of similarity with the animal model data. Hence, the human studies to date have indicated that mechanical ventilation is associated with increased oxidative stress, atrophy, and injury of diaphragmatic muscle fibers along with a rapid loss of diaphragmatic force production. These changes are, to a large extent, directly proportional to the duration of mechanical ventilation. In the context of these human data, we also review the methods that can be used in the clinical setting to diagnose and/or monitor the development of VIDD in critically ill patients. Finally, we discuss the potential for using different mechanical ventilation strategies and pharmacological approaches to prevent and/or to treat VIDD and suggest promising avenues for future research in this area.

Figure 1

Mechanistic pathways implicated in the human studies of ventilator-induced diaphragmatic dysfunction in critically ill patients. Diaphragmatic inactivity induced by mechanical ventilation is associated with an increased generation of reactive oxygen species (ROS) and a downregulation of the Akt pathway. This leads to increased proteolysis in the diaphragm via activation of the caspase, calpain, ubiquitin-proteasome, and lysosomal-autophagic pathways. In addition, downregulation of the Akt pathway also inhibits protein synthesis (dotted line). Collectively, all of the above contribute to atrophy and injury of diaphragmatic muscle fibers, resulting in functional impairment of the diaphragm. Akt-P, phosphorylated (activated) form of protein kinase B.