Rapidly progressive diaphragmatic weakness and injury during mechanical ventilation in humans

Abstract

Rationale: Diaphragmatic function is a major determinant of the ability to successfully wean patients from mechanical ventilation (MV). Paradoxically, MV itself results in a rapid loss of diaphragmatic strength in animals. However, very little is known about the time course or mechanistic basis for such a phenomenon in humans. Objectives: To determine in a prospective fashion the time course for development of diaphragmatic weakness during MV; and the relationship between MV duration and diaphragmatic injury or atrophy, and the status of candidate cellular pathways implicated in these phenomena. Methods: Airway occlusion pressure (TwPtr) generated by the diaphragm during phrenic nerve stimulation was measured in short-term (0.5 h; n = 6) and long-term (>5 d; n = 6) MV groups. Diaphragmatic biopsies obtained during thoracic surgery (MV for 2-3 h; n = 10) and from brain-dead organ donors (MV for 24-249h; n =15 ) were analyzed for ultrastructural injury, atrophy, and expression of proteolysis-related proteins (ubiquitin, nuclear factor-κB, and calpains). Measurements and Main Results: TwPtr decreased progressively during MV, with amean reduction of 32 ± 6% after 6 days. Longer periodsof MV were associated with significantly greater ultrastructural fiber injury (26.2 ± 4.8 vs. 4.7 ± 0.6% area), decreased cross-sectional area of muscle fibers (1,904 ± 220 vs. 3,100 ± 329 μm2), an increase of ubiquitinated proteins (+19%), higher expression of p65 nuclear factor-κB (+77%), and greater levels of the calcium-activated proteases calpain-1, -2, and-3 (+104%,+432%,and +266%, respectively) in the diaphragm. Conclusions: Diaphragmatic weakness, injury,andatrophyoccur rapidly in critically ill patients during MV, and are significantly correlated with the duration of ventilator support.