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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Crit Care Med. Author manuscript; available in PMC 2010 July 26.
Published in final edited form as:
PMCID: PMC2909674
NIHMSID: NIHMS218840

Prolonged mechanical ventilation alters diaphragmatic structure and function

Abstract

Objective

To review current knowledge about the impact of prolonged mechanical ventilation on diaphragmatic function and biology.

Measurements

Systematic literature review.

Conclusions

Prolonged mechanical ventilation can promote diaphragmatic atrophy and contractile dysfunction. As few as 18 hrs of mechanical ventilation results in diaphragmatic atrophy in both laboratory animals and humans. Prolonged mechanical ventilation is also associated with diaphragmatic contractile dysfunction. Studies using animal models revealed that mechanical ventilation-induced diaphragmatic atrophy is due to increased diaphragmatic protein breakdown and decreased protein synthesis. Recent investigations have identified calpain, caspase-3, and the ubiquitin-proteasome system as key proteases that contribute to mechanical ventilation-induced diaphragmatic proteolysis. The scientific challenge for the future is to delineate the mechanical ventilation-induced signaling pathways that activate these proteases and depress protein synthesis in the diaphragm. Future investigations that define the signaling mechanisms responsible for mechanical ventilation-induced diaphragmatic weakness will provide the knowledge required for the development of new medicines that can maintain diaphragmatic mass and function during prolonged mechanical ventilation.

Keywords: diaphragm, respiratory muscles, mechanical ventilation, oxidative stress, weaning, diaphragm weakness

Mechanical ventilation (MV) is used clinically to sustain pulmonary gas exchange in patients who are incapable of maintaining sufficient alveolar ventilation. Common indications for MV include respiratory failure due to chronic obstructive pulmonary disease, status asthmaticus, and/or heart failure. MV is also required for many patients with neuromuscular diseases, drug overdoses, and during postsurgical recovery.

Although MV is a lifesaving measure, prolonged MV is often associated with complications, such as tracheal injuries, infection, cardiovascular failure, and lung injury. Abundant evidence also indicates that prolonged MV promotes diaphragmatic weakness due to both atrophy and contractile dysfunction. The detrimental impact of prolonged MV on the diaphragm has been termed ventilator-induced diaphragmatic dysfunction (VIDD) and this phenomenon has received increased research attention in recent years. This review will summarize the current knowledge about the impact of prolonged MV on diaphragmatic function and biology. Our approach will be to provide a synopsis of major concepts and highlight the findings of selected individual studies. Furthermore, in hopes of stimulating additional research, we will highlight specific areas where additional work is warranted. We begin with an overview of the clinical significance of VIDD followed with a discussion of the impact of prolonged MV on diaphragm structure, function, and gene expression.

Clinical Significance of VIDD

Although many patients requiring MV can be extubated in <3 days, ~30% of MV patients will experience weaning problems and will require additional time on the ventilator (1). In patients experiencing weaning problems, weaning procedures account for 40% to 60% of the total time on the ventilator (2). Unfortunately, 1% to 5% of all MV patients repeatedly fail attempts to be weaned from MV and therefore may become ventilator dependent (3). Chronic ventilator dependence is a major medical problem and presents significant psychological and financial problems for both patients and families (3).

Difficult weaning has received considerable clinical attention during the past 20 yrs as numerous studies have attempted to determine clinical predictors of weaning problems and to develop strategies to improve the weaning success rate. The pathophysiological problems that contribute to difficult weaning are complex and numerous (4). To further complicate this issue, the factors that contribute to weaning difficulties vary among patients depending on age, existing comorbidities, and nutritional status. Nonetheless, several key factors have been identified as potential contributors to difficult weaning. These include inadequate ventilatory drive, increased work of breathing, cardiac failure, and inspiratory muscle weakness and/or fatigue (4). Of these factors, growing evidence suggests that inspiratory muscle weakness and fatigue are likely significant contributors to weaning problems (59). Therefore, although MV is a lifesaving intervention for countless patients, it seems likely that VIDD is an important contributor to weaning problems in many patients. It follows that developing methods to protect patients against weaning problems based on an understanding of the mechanisms responsible for MV-induced diaphragmatic weakness is an important topic for future research.

MV-Induced Diaphragmatic Atrophy

Controlled mechanical ventilation (CMV) is a mode of MV whereby the ventilator provides all of the work of breathing and patient triggering of the ventilator is not possible. Clinical indications for CMV include general anesthesia, drug overdoses, coma, and spinal cord injuries (10). Numerous studies have demonstrated that prolonged CMV results in a rapid-onset of diaphragmatic atrophy in several species (e.g., rats, rabbits, pigs) (1116). For example, as few as 12 hrs to 18 hrs of CMV results in significant fiber atrophy in both slow and fast muscle fibers of the rat diaphragm (Fig. 1) (13, 16, 17). In contrast, the limb muscles of mechanically ventilated animals show no signs of atrophy after 12 hrs to 18 hrs of inactivity (13, 16). The rapid rate of MV-induced diaphragmatic atrophy greatly exceeds the time course of atrophy observed in locomotor skeletal muscles during periods of disuse (18). Specifically, it would take approximately 96 hrs to achieve the same level of atrophy in unloaded locomotor skeletal muscles as observed in the diaphragm after 12 hrs of CMV (18). Furthermore, the rate of CMV-induced atrophy exceeds that reported for the diaphragm after denervation (19). Hence, CMV-induced diaphragmatic atrophy is an extremely rapid and unique type of skeletal muscle wasting.

Figure 1
Prolonged (18 hrs) mechanical ventilation (MV) in rats results in significant atrophy of all diaphragm muscle fiber types. Values represent mean ± standard error of the mean; *indicates different from control. Data are redrawn from Shanely et ...

Similar to the findings in animal studies, Levine et al confirmed that 18 hrs to 69 hrs of CMV also results in atrophy of the human diaphragm (20). This landmark study demonstrated that, compared with diaphragm biopsy samples from control subjects, diaphragm fibers from mechanically ventilated subjects showed a large decrease in fiber cross-sectional area of both type I and type II fibers. Specifically, 18 hrs to 69 hrs of CMV resulted in a decreased diaphragm fiber cross-sectional area of 57% and 53%, respectively, in type I and type II fibers (Figs. 2 and and3).3). This magnitude of CMV-induced human diaphragmatic atrophy is similar to that reported for the rat diaphragm after 48 hrs of CMV (21).

Figure 2
Microscopic photographs of diaphragm muscle fibers from control and mechanically ventilated patients (case) (18 hrs to 69 hrs of controlled mechanical ventilation [CMV]). Note that the slow-twitch (type I) and fast-twitch (type II) fibers in the CMV diaphragm ...
Figure 3
Prolonged (18 hrs to 69 hrs) controlled mechanical ventilation (MV) in humans results in significant atrophy within slow and fast diaphragm muscle fiber types. Values represent mean ± standard error of the mean; *indicates different from control. ...

Although it is clear that prolonged CMV promotes a rapid onset of diaphragmatic atrophy, it is unknown whether pressure-support MV can retard or prevent ventilator-induced diaphragmatic atrophy. However, two recent reports suggested that MV using a pressure-support mode can limit ventilator-induced protein catabolism (22, 23). Nonetheless, neither of these studies performed measurements of diaphragmatic atrophy.

CMV-Induced Changes in Ultrastructure of Diaphragm Muscle Fibers

Animal studies revealed that CMV results in time-dependent alterations in the ultrastructure of diaphragm muscle fibers (2427). In contrast, inactivity does not promote ultrastructure damage in locomotor muscles of mechanically ventilated rats (24). In reference to MV-induced changes in diaphragmatic structure, CMV results in diverse areas of abnormal diaphragmatic myofibrils as indicated by myofibrillar disarray and alterations in Z-line structure (24). Furthermore, prolonged CMV also promotes focal areas of diaphragm fiber regeneration without signs of inflammation (27, 28). Finally, prolonged CMV (i.e., 3 days) also results in an increase in cytoplasmic lipid vacuoles (26, 27). However, it is unclear whether these vacuoles represent a pathologic or a physiologic adaptive process. In several forms of myopathy, cytoplasmic vacuoles represent secondary lysosomes involved in the autophagic process (29, 30). Currently, the role of autophagy in CMV-induced changes in diaphragmatic ultrastructure remains unknown.

To date, limited information exists regarding the impact of prolonged CMV on respiratory muscles other than the diaphragm. However, recent evidence indicated that prolonged CMV promotes damage to the external intercostal muscles (26). In this regard, the pattern of CMV-induced ultrastructual damage to the external intercostal muscle fibers seems similar to MV-induced diaphragmatic damage as demonstrated by focal areas of fragmented myofibrils along with increases in lipid vacuoles (26).

MV Promotes Diaphragmatic Contractile Dysfunction

The first report that CMV promotes diaphragmatic contractile dysfunction appeared in 1994 (21). Le Bourdelles et al, using a rat model of mechanical ventilation, demonstrated that 48 hrs of CMV results in a large (i.e., ~40%) reduction in diaphragmatic maximal tetanic specific force production (i.e., force per cross-sectional area) (21). Since this initial observation, numerous studies using several animal species (i.e., rats, rabbits, pigs, baboons) have confirmed that prolonged CMV results in diaphragmatic contractile dysfunction (12, 24, 25, 28, 3138). Prolonged CMV promotes a time-dependent and progressive decrease in diaphragmatic specific force production at both submaximal and maximal stimulation frequencies (35) (Fig. 4). Identical to the time course of CMV-induced atrophy, only 12 hrs of CMV is required to promote significant reductions in diaphragmatic-specific force production (35, 37, 38).

Figure 4
Effects of prolonged mechanical ventilation (MV) on diaphragmatic-specific force production (in vitro) in young adult rats. Values represent mean ± standard error of the mean. Compared with diaphragms from control animals, prolonged MV resulted ...

As aging impairs diaphragmatic contractile function (39), it is feasible that the senescent diaphragm may be more sensitive to CMV-induced diaphragmatic dysfunction. To address this issue, Criswell et al investigated the cumulative effects of aging and CMV on diaphragmatic function in young adult and senescent rats. Their findings indicated that aging does not exacerbate the relative CMV-induced impairment in diaphragmatic-specific force production (37). However, compared with nonventilated young adult animals, maximal diaphragmatic-specific force production was 13% lower in senescent rats. Therefore, despite the similar relative responses of young and old diaphragms to CMV, the negative effects of MV are additive to the age-related deficit in diaphragmatic contractile performance (Fig. 5). This additive effect of aging and CMV-induced diaphragmatic contractile dysfunction may explain why patient age is an independent predictor of difficulties in patient weaning (40).

Figure 5
Effects of prolonged mechanical ventilation (MV) on diaphragmatic specific force production (in vitro) in both young adult (4 mos old) and senescent (30 mos old) rats. Values represent means ± standard error of the mean; *indicates significantly ...

The aforementioned animal models of MV have consistently shown that CMV leads to depressed diaphragmatic force production. However, many MV patients are also treated with drugs that can exacerbate the impact of CMV on diaphragmatic function (e.g., neuromuscular blockers and/or glucocorticoids). For example, prolonged administration of non-depolarizing neuromuscular blocking agents are used in ~13% of MV patients (41). These agents are typically used to facilitate MV, assist in the treatment of intracranial pressure, and reduce oxygen consumption (42). A possible complication of using neuromuscular blocking agents in the intensive care unit is skeletal muscle myopathy. In this regard, two recent studies confirmed that use of the neuromuscular blocking agent rocuronium exacerbates CMV-induced contractile dysfunction in rats (31, 43).

Also, many patients suffering from acute respiratory failure are treated with corticoidsteroids for the underlying lung disease or other diseases (44). This is significant because glucocorticoid treatment is associated with steroid-induced myopathy of both locomotor and respiratory muscles (45). To investigate the extent to which corticosteroids contribute to diaphragm weakness in mechanically ventilated rats, Maes et al studied the combined effects of corticosteroid (methylprednisolone) treatment on diaphragm function in rats after 24 hrs of CMV (44). Surprisingly, their results revealed that corticosteroid treatment provided partial protection against CMV-induced diaphragmatic atrophy and contractile dysfunction. This protective action of glucocorticoids seems to be associated with the inhibition of the protease calpain (44).

Although CMV remains a common mode of MV for use with specific patient populations (e.g., drug overdose, surgery), other MV modes are often used in the adult intensive care unit. Specifically, modes of MV that provide partial ventilatory support are commonly used with adult patients inflicted with respiratory failure (10). The key question becomes: Do ventilatory modes that provide partial support for the work of breathing result in the same magnitude of diaphragmatic contractile dysfunction as that observed with CMV? Sassoon et al addressed this issue and reported that, compared with CMV, assist-control MV partially attenuates the diaphragmatic contractile dysfunction associated with CMV (22). Specifically, after 3 days of MV in rabbits, peak diaphragmatic power output decreased 41% with CMV but declined only by 20% with assist-control MV. Hence, compared with CMV, modes of MV that provide partial ventilatory support seem to reduce the magnitude of VIDD.

Further evidence that diaphragmatic inactivity is a key factor in promoting CMV-induced VIDD has been provided by Gayan-Ramirez et al (14). This group demonstrated that short (i.e., 5-min) periods of intermittent spontaneous breathing during CMV can retard the damaging effects of CMV on diaphragmatic contractile dysfunction (14). These findings reinforce the concept that diaphragmatic inactivity is an important factor in promoting VIDD.

To date, there are no published reports that have directly evaluated the impact of prolonged MV on diaphragmatic contractile function in humans. Nonetheless, respiratory muscle weakness is a well-known problem in the intensive care unit (4). Many studies indicated that, compared with controls, maximal inspiratory pressure is markedly lower in patients after prolonged MV (5). Furthermore, patients who fail to wean from the ventilator have significantly weaker inspiratory muscles compared with patients who can be weaned from MV (9). Nonetheless, in patient populations, it is often difficult to determine the direct effects of MV on respiratory muscle function vs. the impact of the patient’s illness (e.g., infection).

MV-Induced Changes in Diaphragmatic Protein Turnover

Several studies using laboratory animals have demonstrated that prolonged CMV depresses diaphragmatic protein synthesis and accelerates protein breakdown. For example, as few as 6 hrs of CMV is associated with a 30% decrease in mixed protein synthesis and a 65% decline in the rate of myosin heavy-chain protein synthesis (46). These depressed levels of protein synthesis remained consistent throughout 18 hrs of CMV.

Furthermore, animal studies have consistently reported that CMV results in increased diaphragmatic proteolysis. Specifically, in vitro measurements of diaphragmatic protein breakdown reveal that 12 hrs to 18 hrs of CMV results in a large increase (i.e., >46%) in diaphragmatic proteolysis (13, 38). Furthermore, numerous studies have demonstrated that prolonged CMV activates several proteases in the diaphragm including calpain, caspase-3, and the ubiquitin proteasome system of proteolysis (13, 16, 22, 33, 38, 47).

At present, no studies have performed direct measurements of CMV-induced diaphragmatic proteolysis in humans. However, Levine et al reported that 18 hrs to 69 hrs of CMV increases caspase-3 activity in the diaphragm along with increased messenger RNA (mRNA) for two key components of the ubiquitin-proteasome system of proteolysis (20). These findings in humans mimic the numerous animal studies and are consistent with the notion that prolonged CMV results in a significant increase in proteolysis in the human diaphragm.

CMV Promotes Oxidative Injury in the Diaphragm

CMV lasting ≥6 hrs results in diaphragmatic redox disturbances as demonstrated by increases in biomarkers of oxidative injury (i.e., protein carbonyls and lipid peroxidation) (13, 38, 48, 49). For example, key contractile proteins, such as actin and myosin, are oxidized in the diaphragm during prolonged CMV (49). This redox disturbance occurs because prolonged CMV results in increased reactive oxygen species (ROS) production and a diminished antioxidant capacity in the diaphragm (48). The pathways responsible for CMV-induced ROS production in the diaphragm remain unclear but nitric oxide synthase and nicotinamide adenine dinucleotide phosphate oxidase are not strong candidates (50, 51). Although xanthine oxidase is a source of ROS production in the diaphragm during CMV, it is clear that additional unknown sources also exist (52). Therefore, by elimination, it is feasible that mitochondria are an important source of diaphragmatic ROS production during prolonged MV.

In addition to increased ROS production, prolonged CMV also results in a diminished total antioxidant capacity in the diaphragm as evidenced by decreases in glutathione levels and diminished glutathione peroxidase and CuZn superoxide dismutase levels (48). Interestingly, the changes in protein abundance of these antioxidant enzymes are not associated with alterations in the mRNA level for either enzyme. Therefore, factors other than the amount of mRNA expression dictate the abundance of these antioxidant proteins in the diaphragm during prolonged CMV. It follows that decreased rates of mRNA translation and/or enhanced degradation of the antioxidant protein could be responsible for the dissociation between mRNA and diaphragmatic protein levels (48).

Growing evidence indicates that prevention of CMV-induced oxidative damage to the diaphragm can retard the deleterious effects of prolonged MV on the diaphragm. Treatment of animals with an antioxidant (i.e., Trolox, water-soluble vitamin E analog) can impede CMV-induced diaphragmatic atrophy and contractile dysfunction (17, 33, 38). Prevention of oxidative stress may protect the diaphragm against atrophy and contractile dysfunction through a variety of signaling mechanisms including the regulation of proteolytic signaling pathways (5355). Furthermore, because the transcription of many genes is under redox control, CMV-induced redox disturbances could have a profound effect on diaphragmatic gene expression.

MV-Induced Changes in Diaphragmatic Gene Expression and Cell Signaling

Recent studies have explored the impact of prolonged MV on diaphragmatic gene expression. Using a gene chip approach in a rat model of MV, Deruisseau et al examined the effect of both short-(6 hrs) and long-term (18 hrs) CMV on diaphragmatic gene expression (56). Compared with diaphragms from control animals, microarray analysis revealed that MV resulted in 354 differentially expressed gene products after 6 hrs to 18 hrs of CMV (56). In general, both stress response and proteolytic genes were up-regulated in the diaphragm, whereas genes in the structural protein and energy metabolism categories were down-regulated.

Diaphragmatic genes that exhibit a large MV-induced increase in expression include heme oxygenase-1 (+19-fold) and metallothionein-1 and -2 (+26-fold) (56). The physiologic significance of these changes remains speculative but it seems likely that both of these genes are up-regulated in response to CMV-induced oxidative stress in the diaphragm. In reference to heme oxygenase-1, this enzyme catalyzes the rate-limiting step in the degradation of heme, resulting in the generation of carbon monoxide, biliverdin, and free iron (Fe2+) (57). Furthermore, biliverdin is then reduced to bilirubin via biliverdin reductase. Both bilirubin and biliverdin exhibit antioxidant effects and, therefore, it is feasible that HO-1 induction in the diaphragm plays a protective role against MV-induced oxidative stress (57).

Metallothionein gene expression is induced by a variety of stimuli including metal exposure and oxidative stress (58, 59). Metallothionein belongs to a family of cysteine-rich, low-molecular weight proteins. These proteins have several physiologic functions including the binding of several metals (e.g., zinc, copper, cadmium) and the scavenging of ROS (58). Therefore, it seems that the CMV-induced expression of metallothionein in the diaphragm may represent an attempt by diaphragm fibers to preserve redox balance by increasing the level of cellular antioxidants.

It is also clear that CMV promotes large increases in the expression of key proteins involved in the ubiquitin-proteasome proteolytic system. CMV results in increased diaphragmatic levels of both atrogin-1 mRNA (eight-fold increase) and muscle ring finger-1 mRNA (19-fold increase) (56). These two muscle-specific E3 ligases are known to play an important role in disuse muscle atrophy (54).

Many diaphragmatic genes involved in energy metabolism (e.g., fat metabolism and oxidative phosphorylation) are down-regulated during CMV (56). Furthermore, diaphragmatic genes involved in calcium homeostasis are also down-regulated during CMV including calsequestrin 2 and sarco(endo)plasmic reticulum calcium ATPase (SERCA) pumps (60). A decrease in the expression of these proteins would likely contribute to increased cytosolic levels of free calcium and, therefore, promote the calcium-induced activation of the calcium-activated proteolytic enzyme calpain (61).

Also, CMV results in decreased insulin-like growth factor-1 mRNA in the diaphragm (15). This is significant because signaling initiated by insulin-like growth factor-1 exerts an important influence on muscle fiber size (62). Specifically, insulin-like growth factor-1 binding to its receptor results in the phosphorylation of phosphatidylinositol 3 kinase and subsequent activation of protein kinase B (Akt) (63). Activated (i.e., phosphorylated) Akt exerts an important regulatory role in the maintenance of skeletal muscle fiber size, in part, because active Akt promotes protein synthesis and represses protein degradation (63). Numerous studies revealed that decreased Akt activation is a hallmark of many forms of locomotor skeletal muscle atrophy (63). In regard to the impact of prolonged CMV on Akt activation in the diaphragm, McClung et al demonstrated that 18 hrs of CMV results in a down-regulation of insulin-like growth factor-1 to Akt signaling (17).

Finally, CMV also results in altered diaphragmatic expression of several myogenic regulatory factors including myogenin (mRNA increased), MyoD (mRNA decreased), and myf-5 (mRNA increased) (60). During embryogenesis, these myogenic regulatory factors stimulate myoblast determination and differentiation (64). Unfortunately, the role of these myogenic factors in adult muscle is not fully established and, therefore, the physiologic importance of CMV-induced changes in these myogenic factors remains unclear. Nonetheless, diaphragmatic contractile function is depressed in MyoD knockout mice and, therefore, the diminished MyoD expression may play a role in CMV-induced diaphragmatic contractile dysfunction (65).

Summary and Future Directions

Prolonged CMV is associated with numerous important structural and functional changes in the diaphragm (Fig. 6). First, CMV results in a rapid onset of diaphragmatic atrophy in both animals and humans. CMV is also associated with a time-dependent induction of diaphragmatic contractile dysfunction, resulting in depressed diaphragmatic-specific force production at both submaximal and maximal stimulation frequencies. Hence, it is feasible that CMV-induced diaphragmatic weakness may play an important role in weaning difficulties.

Figure 6
Prolonged controlled mechanical ventilation results in numerous biochemical, structural, and functional effects on the diaphragm. ROS, reactive oxygen species.

CMV-induced diaphragmatic atrophy in animal models of MV occurs due to both a reduction in protein synthesis and an increase in diaphragmatic protein degradation. MV-induced diaphragmatic atrophy is also associated with ultrastructural changes in the diaphragm including diverse areas of abnormal sarcomere structure and irregular Z-line structure.

Furthermore, CMV results in significant changes in diaphragmatic gene expression as >350 gene products are either increased or decreased during 6 hrs to 18 hrs of CMV. In general, stress responsive genes are up-regulated whereas genes in the structural protein and energy metabolism categories are down-regulated. This alteration in diaphragmatic gene expression contributes to the extensive remodeling that occurs in the diaphragm during prolonged CMV.

Importantly, prolonged CMV results in oxidative damage to the diaphragm as demonstrated by increased protein oxidation and lipid peroxidation. This is significant because redox disturbances in skeletal muscle promote contractile dysfunction and the activation of proteolytic systems. Prevention of CMV-induced oxidative damage in the diaphragm has been demonstrated to retard MV-induced diaphragmatic atrophy and contractile dysfunction in animals.

Although much research progress has been made toward describing the impact of prolonged CMV on diaphragmatic structure and function, numerous unanswered questions remain. An important clinical question that remains unresolved is: Do pressure-assist modes of MV protect the diaphragm against the rapid onset of diaphragmatic atrophy and contractile dysfunction that is associated with CMV? A related question is: Can periodic activation of the diaphragm (i.e., electrical stimulation) protect the diaphragm against CMV-induced diaphragmatic wasting?

Other important mechanistic questions remain unanswered. For example, the role that autophagy plays in CMV-induced diaphragmatic atrophy remains unclear. Furthermore, limited information exists about which diaphragmatic proteases are essential and/or rate-limiting in MV-induced proteolysis in the diaphragm. Also, it is unclear if major proteolytic systems are independently regulated or coordinately controlled by a unifying regulatory system.

Finally, the principal ROS producing pathways in the diaphragm that are active during prolonged CMV remain to be identified. This is important because CMV-induced diaphragmatic ROS production is a required upstream signal that promotes diaphragmatic atrophy and contractile dysfunction. Furthermore, the specific signaling pathways that link diaphragmatic ROS production to depressed protein synthesis and/or increased proteolysis in the diaphragm remain unresolved. This is a critical area for future research because understanding the cellular signaling pathways that are responsible for MV-induced diaphragmatic atrophy and contractile dysfunction is a prerequisite to develop methods to protect patients against MV-mediated diaphragmatic weakness.

Acknowledgments

The research reported in the article was supported, in part, by Grants R01HL062361 and R01HL072789 from the National Institutes of Health (SKP).

Footnotes

The authors have not disclosed any potential conflicts of interest.

References

1. Esteban A, Frutos F, Tobin MJ, et al. A comparison of four methods of weaning patients from mechanical ventilation. Spanish Lung Failure Collaborative Group. N Engl J Med. 1995;332:345–350. [PubMed]
2. Esteban A, Alía I, Ibañez J, et al. Modes of mechanical ventilation and weaning. A national survey of Spanish hospitals. The Spanish Lung Failure Collaborative Group. Chest. 1994;106:1188–1193. [PubMed]
3. Celli B. Home mechanical ventilation. In: Tobin M, editor. Principles and Practice of Mechanical Ventilation. New York: McGraw Hill; 1994. pp. 619–629.
4. Gayan-Ramirez G, Decramer M. Effects of mechanical ventilation on diaphragm function and biology. Eur Respir J. 2002;20:1579–1586. [PubMed]
5. Vassilakopoulos T, Zakynthinos S, Roussos C. The tension-time index and the frequency/tidal volume ratio are the major pathophysiologic determinants of weaning failure and success. Am J Respir Crit Care Med. 1998;158:378–385. [PubMed]
6. Goldstone JC, Green M, Moxham J. Maximum relaxation rate of the diaphragm during weaning from mechanical ventilation. Thorax. 1994;49:54–60. [PMC free article] [PubMed]
7. Vallverdú I, Calaf N, Subirana M, et al. Clinical characteristics, respiratory functional parameters, and outcome of a two-hour T-piece trial in patients weaning from mechanical ventilation. Am J Respir Crit Care Med. 1998;158:1855–1862. [PubMed]
8. Krieger BP, Ershowsky PF, Becker DA, et al. Evaluation of conventional criteria for predicting successful weaning from mechanical ventilatory support in elderly patients. Crit Care Med. 1989;17:858–861. [PubMed]
9. Purro A, Appendini L, De Gaetano A, et al. Physiologic determinants of ventilator dependence in long-term mechanically ventilated patients. Am J Respir Crit Care Med. 2000;161:1115–1123. [PubMed]
10. Hess D, Kacmarek R. Essentials of Mechanical Ventilation. New York: McGraw-Hill; 1996.
11. Capdevila X, Lopez S, Bernard N, et al. Effects of controlled mechanical ventilation on respiratory muscle contractile properties in rabbits. Intensive Care Med. 2003;29:103–110. [PubMed]
12. Anzueto A, Peters JI, Tobin MJ, et al. Effects of prolonged controlled mechanical ventilation on diaphragmatic function in healthy adult baboons. Crit Care Med. 1997;25:1187–1190. [PubMed]
13. Shanely RA, Zergerogly MA, Lennon SL, et al. Mechanical ventilation-induced diaphragmatic atrophy is associated with oxidative injury and increased proteolytic activity. Am J Respir Crit Care Med. 2002;166:1369–1374. [PubMed]
14. Gayan-Ramirez G, Testelmans D, Maes K, et al. Intermittent spontaneous breathing protects the rat diaphragm from mechanical ventilation effects. Crit Care Med. 2005;33:2804–2809. [PubMed]
15. Gayan-Ramirez G, de Paepe K, Cadot P, et al. Detrimental effects of short-term mechanical ventilation on diaphragm function and IGF-I mRNA in rats. Intensive Care Med. 2003;29:825–833. [PubMed]
16. McClung JM, Kavazis AN, DeRuisseau KC, et al. Caspase-3 regulation of diaphragm myonuclear domain during mechanical ventilation-induced atrophy. Am J Respir Crit Care Med. 2007;175:150–159. Epub 2006 Nov 2. [PMC free article] [PubMed]
17. McClung JM, Kavazis AN, Whidden MA, et al. Antioxidant administration attenuates mechanical ventilation-induced rat diaphragm muscle atrophy independent of protein kinase B (PKB Akt) signalling. J Physiol. 2007;585:203–215. [PubMed]
18. Thomason DB, Biggs RB, Booth FW. Protein metabolism and beta-myosin heavy-chain mRNA in unweighted soleus muscle. Am J Physiol. 1989;257:R300–R305. [PubMed]
19. Geiger PC, Bailey JP, Zhan WZ, et al. Denervation-induced changes in myosin heavy chain expression in the rat diaphragm muscle. J Appl Physiol. 2003;95:611–619. Epub 2003 Apr 18. [PubMed]
20. Levine S, Nguyen T, Taylor N, et al. Rapid disuse atrophy of diaphragm fibers in mechanically ventilated humans. N Engl J Med. 2008;358:1327–1335. [PubMed]
21. Le Bourdelles G, Viires N, Boczkowski J, et al. Effects of mechanical ventilation on diaphragmatic contractile properties in rats. Am J Respir Crit Care Med. 1994;149:1539–1544. [PubMed]
22. Sassoon CS, Zhu E, Caiozzo VJ. Assist-control mechanical ventilation attenuates ventilator-induced diaphragmatic dysfunction. Am J Respir Crit Care Med. 2004;170:626–632. [PubMed]
23. Futier E, Constantin JM, Combaret L, et al. Pressure support ventilation attenuates ventilator-induced protein modifications in the diaphragm. Crit Care. 2008;12:R116. [PMC free article] [PubMed]
24. Sassoon CS, Caiozzo VJ, Manka A, et al. Altered diaphragm contractile properties with controlled mechanical ventilation. J Appl Physiol. 2002;92:2585–2595. [PubMed]
25. Zhu E, Sassoon CS, Nelson R, et al. Early effects of mechanical ventilation on isotonic contractile properties and MAF-box gene expression in the diaphragm. J Appl Physiol. 2005;99:747–756. [PubMed]
26. Bernard N, Matecki S, Py G, et al. Effects of prolonged mechanical ventilation on respiratory muscle ultrastructure and mitochondrial respiration in rabbits. Intensive Care Med. 2003;29:111–118. [PubMed]
27. Radell P, Edström L, Stibler H, et al. Changes in diaphragm structure following prolonged mechanical ventilation in piglets. Acta Anaesthesiol Scand. 2004;48:430–437. [PubMed]
28. Van Gammeren D, Falk DJ, DeRuisseau KC, et al. Reloading the diaphragm following mechanical ventilation does not promote injury. Chest. 2005;127:2204–2210. [PubMed]
29. Marzella L, Glaumann H. Biogenesis, translocation, and function of lysosomal enzymes. Int Rev Exp Pathol. 1983;25:239–278. [PubMed]
30. Malicdan MC, Noguchi S, Nonaka I, et al. Lysosomal myopathies: An excessive build-up in autophagosomes is too much to handle. Neuromuscul Disord. 2008;18:521–529. [PubMed]
31. Testelmans D, Maes K, Wouters P, et al. Rocuronium exacerbates mechanical ventilation-induced diaphragm dysfunction in rats. Crit Care Med. 2006;34:3018–3023. [PubMed]
32. Yang L, Luo J, Bourdon J, et al. Controlled mechanical ventilation leads to remodeling of the rat diaphragm. Am J Respir Crit Care Med. 2002;166:1135–1140. [PubMed]
33. McClung JM, Whidden MA, Kavazis AN, et al. Redox regulation of diaphragm proteolysis during mechanical ventilation. Am J Physiol Regul Integr Comp Physiol. 2008;294:R1608–R1617. [PubMed]
34. Shanely RA, Coombes JS, Zergeroglu AM, et al. Short-duration mechanical ventilation enhances diaphragmatic fatigue resistance but impairs force production. Chest. 2003;123:195–201. [PubMed]
35. Powers SK, Shanely RA, Coombes JS, et al. Mechanical ventilation results in progressive contractile dysfunction in the diaphragm. J Appl Physiol. 2002;92:1851–1858. [PubMed]
36. Radell PJ, Remahl S, Nichols DG, et al. Effects of prolonged mechanical ventilation and inactivity on piglet diaphragm function. Intensive Care Med. 2002;28:358–364. Epub 2002 Feb 6. [PubMed]
37. Criswell DS, Shanely RA, Betters JJ, et al. Cumulative effects of aging and mechanical ventilation on in vitro diaphragm function. Chest. 2003;124:2302–2308. [PubMed]
38. Betters JL, Criswell DS, Shanely RA, et al. Trolox attenuates mechanical ventilation-induced diaphragmatic dysfunction and proteolysis. Am J Respir Crit Care Med. 2004;170:1179–1184. [PubMed]
39. Criswell DS, Powers SK, Herb RA, et al. Mechanism of specific force deficit in the senescent rat diaphragm. Respir Physiol. 1997;107:149–155. [PubMed]
40. Doering LV, Imperial-Perez F, Monsein S, et al. Preoperative and postoperative predictors of early and delayed extubation after coronary artery bypass surgery. Am J Crit Care. 1998;7:37–44. [PubMed]
41. Arroliga A, Frutos-Vivar F, Hall J, et al. Use of sedatives and neuromuscular blockers in a cohort of patients receiving mechanical ventilation. Chest. 2005;128:496–506. [PubMed]
42. Murray MJ, Cowen J, DeBlock H, et al. Clinical practice guidelines for sustained neuromuscular blockade in the adult critically ill patient. Crit Care Med. 2002;30:142–156. [PubMed]
43. Testelmans D, Maes K, Wouters P, et al. Infusions of rocuronium and cisatracurium exert different effects on rat diaphragm function. Intensive Care Med. 2007;33:872–879. [PubMed]
44. Maes K, Testelmans D, Cadot P, et al. Effects of acute administration of corticosteroids during mechanical ventilation on rat diaphragm. Am J Respir Crit Care Med. 2008;178:1219–1226. [PubMed]
45. Lieu FK, Powers SK, Herb RA, et al. Exercise and glucocorticoid-induced diaphragmatic myopathy. J Appl Physiol. 1993;75:763–771. [PubMed]
46. Shanely RA, Van Gammeren D, Deruisseau KC, et al. Mechanical ventilation depresses protein synthesis in the rat diaphragm. Am J Respir Crit Care Med. 2004;170:994–999. [PubMed]
47. Maes KGG-R, Testelmans D, DeRuisseau K, et al. Leupeptin inhibits ventilator-induced diaphragmatic dysfunction in rats. Am J Respir Crit Care Med. 2007;175:1134–1138. [PubMed]
48. Falk DJ, Deruisseau KC, Van Gammeren DL, et al. Mechanical ventilation promotes redox status alterations in the diaphragm. J Appl Physiol. 2006;101:1017–1024. [PubMed]
49. Zergeroglu MA, McKenzie MJ, Shanely RA, et al. Mechanical ventilation-induced oxidative stress in the diaphragm. J Appl Physiol. 2003;95:1116–1124. [PubMed]
50. Van Gammeren D, Falk DJ, Deering MA, et al. Diaphragmatic nitric oxide synthase is not induced during mechanical ventilation. J Appl Physiol. 2007;102:157–162. [PubMed]
51. McClung JM, Van Gammeren D, Whidden MA, et al. Apocynin attenuates diaphragm oxidative stress and protease activation during prolonged mechanical ventilation. Crit Care Med. 2009;37:1373–1379. [PMC free article] [PubMed]
52. Whidden MA, McClung JM, Falk DJ, et al. Xanthine oxidase contributes to mechanical ventilation-induced diaphragmatic oxidative stress and contractile dysfunction. J Appl Physiol. 2009;106:385–394. [PubMed]
53. Powers SK, Jackson MJ. Exercise-induced oxidative stress: Cellular mechanisms and impact on muscle force production. Physiol Rev. 2008;88:1243–1276. [PMC free article] [PubMed]
54. Powers SK, Kavazis AN, DeRuisseau KC. Mechanisms of disuse muscle atrophy: Role of oxidative stress. Am J Physiol Regul Integr Comp Physiol. 2005;288:R337–344. [PubMed]
55. Powers SK, Kavazis AN, McClung JM. Oxidative stress and disuse muscle atrophy. J Appl Physiol. 2007;102:2389–2397. [PubMed]
56. DeRuisseau KC, Shanely RA, Akunuri N, et al. Diaphragm unloading via controlled mechanical ventilation alters the gene expression profile. Am J Respir Crit Care Med. 2005;172:1267–1275. [PMC free article] [PubMed]
57. Ryter SW, Alam J, Choi AM. Heme oxygenase-1/carbon monoxide: from basic science to therapeutic applications. Physiol Rev. 2006;86:583–650. [PubMed]
58. West AK, Chuah MI, Vickers JC, et al. Protective role of metallothioneins in the injured mammalian brain. Rev Neurosci. 2004;15:157–166. [PubMed]
59. Cousins RJ, Liuzzi JP, Lichten LA. Mammalian zinc transport, trafficking, and signals. J Biol Chem. 2006;281:24085–24089. [PubMed]
60. Rácz GZ, Gayan-Ramirez G, Testelmans D, et al. Early changes in rat diaphragm biology with mechanical ventilation. Am J Respir Crit Care Med. 2003;168:297–304. [PubMed]
61. French JP, Hamilton KL, Quindry JC, et al. Exercise-induced protection against myocardial apoptosis and necrosis: MnSOD, calcium-handling proteins, and calpain. FASEB J. 2008;22:2862–2871. [PubMed]
62. Velloso CP. Regulation of muscle mass by growth hormone and IGF-I. Br J Pharmacol. 2008;154:557–568. [PMC free article] [PubMed]
63. Kandarian SC, Jackman RW. Intracellular signaling during skeletal muscle atrophy. Muscle Nerve. 2006;33:155–165. [PubMed]
64. Pownall ME, Gustafsson MK, Emerson CP., Jr Myogenic regulatory factors and the specification of muscle progenitors in vertebrate embryos. Annu Rev Cell Dev Biol. 2002;18:747–783. [PubMed]
65. Staib JL, Swoap SJ, Powers SK. Diaphragm contractile dysfunction in MyoD gene-inactivated mice. Am J Physiol Regul Integr Comp Physiol. 2002;283:R583–R590. [PubMed]