Rationale: The clinical significance of diaphragm weakness in critically ill patients is evident: it prolongs ventilator dependency, and increases morbidity and duration of hospital stay. To date, the nature of diaphragm weakness and its underlying pathophysiologic mechanisms are poorly understood.Objectives: We hypothesized that diaphragm muscle fibers of mechanically ventilated critically ill patients display atrophy and contractile weakness, and that the ubiquitin-proteasome pathway is activated in the diaphragm.Methods: We obtained diaphragm muscle biopsies from 22 critically ill patients who received mechanical ventilation before surgery and compared these with biopsies obtained from patients during thoracic surgery for resection of a suspected early lung malignancy (control subjects). In a proof-of-concept study in a muscle-specific ring finger protein-1 (MuRF-1) knockout mouse model, we evaluated the role of the ubiquitin-proteasome pathway in the development of contractile weakness during mechanical ventilation.Measurements and Main Results: Both slow-and fast-twitch diaphragm muscle fibers of critically ill patients had approximately 25% smaller cross-sectional area, and had contractile force reduced by half or more. Markers of the ubiquitin-proteasome pathway were significantly up-regulated in the diaphragm of critically ill patients. Finally, MuRF-1 knockout mice were protected against the development of diaphragm contractile weakness during mechanical ventilation.Conclusions: These findings show that diaphragm muscle fibers of critically ill patients display atrophy and severe contractile weakness, and in the diaphragm of critically ill patients the ubiquitin-proteasome pathway is activated. This study provides rationale for the development of treatment strategies that target the contractility of diaphragm fibers to facilitate weaning.

In patients with idiopathic pulmonary arterial hypertension (iPAH), iron deficiency is common and has been associated with reduced exercise capacity and worse survival. Previous studies have shown beneficial effects of intravenous iron administration. In this study, we investigated the use of intravenous iron therapy in iron-deficient iPAH patients in terms of safety and effects on exercise capacity, and we studied whether altered exercise capacity resulted from changes in right ventricular (RV) function and skeletal muscle oxygen handling. Fifteen patients with iPAH and iron deficiency were included. Patients underwent a 6-minute walk test, cardiopulmonary exercise tests, cardiac magnetic resonance imaging, and a quadriceps muscle biopsy and completed a quality-of-life questionnaire before and 12 weeks after receiving a high dose of intravenous iron. The primary end point, 6-minute walk distance, was not significantly changed after 12 weeks (409 ± 110 m before vs. 428 ± 94 m after; P = 0.07). Secondary end points showed that intravenous iron administration was well tolerated and increased body iron stores in all patients. In addition, exercise endurance time (P < 0.001) and aerobic capacity (P < 0.001) increased significantly after iron therapy. This coincided with improved oxygen handling in quadriceps muscle cells, although cardiac function at rest and maximal V : O 2 were unchanged. Furthermore, iron treatment was associated with improved quality of life (P < 0.05). In conclusion, intravenous iron therapy in iron-deficient iPAH patients improves exercise endurance capacity. This could not be explained by improved RV function; however, increased quadriceps muscle oxygen handling may play a role. (Trial registration: ClinicalTrials.gov identifier NCT01288651)

Deep tissue injury (DTI) is a severe form of pressure ulcer where tissue damage starts in deep tissues underneath intact skin. In the present study, the contributions of deformation, ischemia, and reperfusion to skeletal muscle damage development were examined in a rat model during a 6-h period. Magnetic resonance imaging (MRI) was used to study perfusion (contrast-enhanced MRI) and tissue integrity (T2-weighted MRI). The levels of tissue deformation were estimated using finite element models. Complete ischemia caused a gradual homogeneous increase in T2 (∼20% during the 6-h period). The effect of reperfusion on T2 was highly variable, depending on the anatomical location. In experiments involving deformation, inevitably associated with partial ischemia, a variable T2 increase (17-66% during the 6-h period) was observed reflecting the significant variation in deformation (with two-dimensional strain energies of 0.60-1.51 J/mm) and ischemia (50.8-99.8% of the leg) between experiments. These results imply that deformation, ischemia, and reperfusion all contribute to the damage process during prolonged loading, although their importance varies with time. The critical deformation threshold and period of ischemia that cause muscle damage will certainly vary between individuals. These variations are related to intrinsic factors, such as pathological state, which partly explain the individual susceptibility to the development of DTI and highlight the need for regular assessments of individual subjects.

The contractile function and the CSA of LV cardiomyocytes is substantially reduced in PAH patients. We propose that these changes contribute to the reduced in vivo contractility of the LV in PAH patients.

Manders E, de Man FS, Handoko ML, Westerhof N, van Hees HW, Stienen GJ, Vonk-Noordegraaf A, Ottenheijm CA. Diaphragm weakness in pulmonary arterial hypertension: role of sarcomeric dysfunction. Am J Physiol Lung Cell Mol Physiol 303: L1070-L1078, 2012. First published September 7, 2012 doi:10.1152/ajplung.00135.2012.-We previously demonstrated that diaphragm muscle weakness is present in experimental pulmonary arterial hypertension (PH). However, the nature of this diaphragm weakness is still unknown. Therefore, the aim of this study was to investigate whether changes at the sarcomeric level contribute to diaphragm weakness in PH. For this purpose, in control rats and rats with monocrotaline-induced PH, contractile performance and myosin heavy chain content of demembranated single diaphragm fibers were determined. We observed a reduced maximal tension of 20% (P Ͻ 0.05), whereas tension cost was preserved in type 2X and 2B diaphragm fibers in PH compared with control. The reduced maximal tension was associated with a reduction of force generated per half-sarcomeric myosin heavy chain content. Additionally, reduced Ca 2ϩ sensitivity of force generation was found in type 2X fibers compared with control, which could exacerbate diaphragm muscle weakness at submaximal activation. No changes in maximal tension and Ca 2ϩ sensitivity of force generation were observed in fibers from the nonrespiratory extensor digitorum longus muscle. Together, these findings indicate that diaphragm weakness in PH is at least partly caused by sarcomeric dysfunction, which appears to be specific for the diaphragm. single fiber; cross bridge cycling kinetics; Ca 2ϩ sensitivity; myosin heavy chain PULMONARY ARTERIAL HYPERTENSION (PH) is characterized by a progressive increase in pulmonary vascular resistance, ultimately leading to right heart failure and death. Patients with PH hyperventilate at rest, during exercise, and sometimes even during sleep (19,27). As a consequence, inspiratory muscle activity increases substantially, which may ultimately lead to overloading of the inspiratory muscles and respiratory muscle weakness.Recent studies have found markedly lower maximal inspiratory pressures in patients with PH compared with control subjects (19,25). This indicates that the force-generating capacity of the inspiratory muscles is impaired. The presence of inspiratory muscle weakness in PH was further supported by recent work from our group on a rat model for PH, which revealed a significant reduction in twitch and maximal tetanic force generation of the diaphragm, the main muscle of inspiration (8). Data from De Man et al. (8) also suggests a significant reduction of maximal tension in diaphragm fibers of two patients with PH.Thus evidence is accumulating that the diaphragm is weakened in PH. However, the nature of this diaphragm weakness is unknown and might involve changes at the level of sarcoplasmic reticulum calcium cycling or content or at levels downstream in the excitation-contraction coupling process, i.e., at the sarcomeric le...