Difficult weaning from mechanical ventilation

Oxidative stress is required for mechanical ventilation-induced protease activation in the diaphragm

Prolonged mechanical ventilation (MV) results in diaphragmatic weakness due to fiber atrophy and contractile dysfunction. Recent work reveals that activation of the proteases calpain and caspase-3 is required for MV-induced diaphragmatic atrophy and contractile dysfunction. However, the mechanism(s) responsible for activation of these proteases remains unknown. To address this issue, we tested the hypothesis that oxidative stress is essential for the activation of calpain and caspase-3 in the diaphragm during MV. Cause-and-effect was established by prevention of MV-induced diaphragmatic oxidative stress using the antioxidant Trolox. Treatment of animals with Trolox prevented MV-induced protein oxidation and lipid peroxidation in the diaphragm. Importantly, the Trolox-mediated protection from MV-induced oxidative stress prevented the activation of calpain and caspase-3 in the diaphragm during MV. Furthermore, the avoidance of MV-induced oxidative stress not only averted the activation of these proteases but also rescued the diaphragm from MV-induced diaphragmatic myofiber atrophy and contractile dysfunction. Collectively, these findings support the prediction that oxidative stress is required for MV-induced activation of calpain and caspase-3 in the diaphragm and are consistent with the concept that antioxidant therapy can retard MV-induced diaphragmatic weakness.

Diaphragm unloading via controlled mechanical ventilation alters the gene expression profile

RATIONALE

Prolonged controlled mechanical ventilation results in diaphragmatic inactivity and promotes oxidative injury, atrophy, and contractile dysfunction in this important inspiratory muscle. However, the impact of controlled mechanical ventilation on global mRNA alterations in the diaphragm remains unknown.

OBJECTIVES

In these experiments, we used an Affymetrix oligonucleotide array to identify the temporal changes in diaphragmatic gene expression during controlled mechanical ventilation in the rat.

METHODS

Adult Sprague-Dawley rats were assigned to either control or mechanical ventilation groups (n = 5/group). Mechanically ventilated animals were anesthetized, tracheostomized, and ventilated with room air for 6 or 18 h. Animals in the control group were acutely anesthetized but not exposed to mechanical ventilation.

MEASUREMENTS AND MAIN RESULTS

Compared with control diaphragms, microarray analysis identified 354 differentially expressed, unique gene products after 6 and 18 h of mechanical ventilation. In general, genes in the cell growth/cell maintenance, stress response, and nucleic acid metabolism categories showed predominant upregulation, whereas genes in the structural protein and energy metabolism categories were predominantly downregulated.

CONCLUSIONS

We conclude that mechanical ventilation results in rapid changes in diaphragmatic gene expression, and subsequently, many of these changes may contribute to atrophy and muscle fiber remodeling associated with unloading this primary inspiratory muscle. Importantly, this study also provides new insights into why the diaphragm, after the onset of contractile inactivity, atrophies more rapidly than locomotor skeletal muscles and also highlights unique differences that exist between these muscles in the mRNA response to inactivity.

Mechanical ventilation induces alterations of the ubiquitin-proteasome pathway in the diaphragm

Prolonged mechanical ventilation (MV) results in diaphragmatic atrophy due, in part, to an increase in proteolysis. These experiments tested the hypothesis that MV-induced diaphragmatic proteolysis is accompanied by increased expression of key components of the ubiquitin-proteasome pathway (UPP). To test this postulate, we investigated the effect of prolonged MV on UPP components and determined the trypsin-like and peptidylglutamyl peptide hydrolyzing activities of the 20S proteasome. Adult Sprague-Dawley rats were assigned to either control or 12-h MV groups ( n = 7/group). MV animals were anesthetized, tracheostomized, and ventilated with room air for 12 h. Animals in the control group were acutely anesthetized but not exposed to MV. Compared with controls, MV animals demonstrated increased diaphragmatic mRNA levels of two ubiquitin ligases, muscle atrophy F-box (+8.3-fold) and muscle ring finger 1 (+19.0-fold). However, MV did not alter mRNA levels of 14-kDa ubiquitin-conjugating enzyme, polyubiquitin, proteasome-activating complex PA28, or 20S α-subunit 7. Protein levels of 14-kDa ubiquitin-conjugating enzyme and proteasome-activating complex PA28 were not altered following MV, but 20S α-subunit 7 levels declined (−17.7%). MV increased diaphragmatic trypsin-like activity (+31%) but did not alter peptidylglutamyl peptide hydrolyzing activity. Finally, compared with controls, MV increased ubiquitin-protein conjugates in both the myofibrillar (+24.9%) and cytosolic (+54.7%) fractions of the diaphragm. These results are consistent with the hypothesis that prolonged MV increases diaphragmatic levels of key components within the UPP and that increases in 20S proteasome activity contribute to MV-induced diaphragmatic proteolysis and atrophy.

Cumulative Effects of Aging and Mechanical Ventilation on In Vitro Diaphragm Function

STUDY OBJECTIVE

Unloading the diaphragm, via mechanical ventilation (MV), results in significant diaphragmatic atrophy, contractile dysfunction, and oxidative stress in young adult animals. Since aging increases skeletal muscle susceptibility to atrophy and injury, we tested the hypothesis that MV-induced diaphragmatic contractile dysfunction would be exacerbated in aging rats.

METHODS

Fisher 344/Brown Norway hybrid rats (4 months old [young] and 30 months old [old]) were assigned to either control or MV groups. MV rats were anesthetized, tracheostomized, and ventilated with 21% O(2) for 12 h. Arterial BP, pH, and blood gas homeostasis were maintained in the MV animals throughout the experimental period. Animals in the control group were acutely anesthetized, and the diaphragms were immediately removed. Muscle strips from the mid-costal diaphragm were removed from each experimental animal, and contractile properties were studied in vitro.

RESULTS

Compared to young control animals, aging (old control animals) was associated with a 13% decrease in maximal isometric tension (24.5 N/cm(2) vs 21.3 N/cm(2)). Although, MV induced similar relative losses (24%) in diaphragmatic isometric tension in both young and old animals receiving MV, the combined effects of aging and MV resulted in a 34% decrement in diaphragmatic isometric tension compared to young control animals (24.5 N/cm(2) vs 16.1 N/cm(2)).

CONCLUSIONS

These data do not support the hypothesis that aging exacerbates the relative MV-induced impairment in diaphragmatic isometric tension. Nonetheless, the additive effects of aging and MV have dramatic effects on diaphragmatic force reserve. This could exacerbate weaning difficulties in older individuals receiving MV.

Mechanical ventilation-induced oxidative stress in the diaphragm

Prolonged mechanical ventilation (MV) results in oxidative damage in the diaphragm; however, it is unclear whether this MV-induced oxidative injury occurs rapidly or develops slowly over time. Furthermore, it is unknown whether both soluble (cytosolic) and insoluble (myofibrillar) proteins are equally susceptible to oxidation during MV. These experiments tested two hypotheses: 1). MV-induced oxidative injury in the diaphragm occurs within the first 6 h after the initiation of MV; and 2). MV is associated with oxidative modification of both soluble and insoluble proteins. Adult Sprague-Dawley rats were randomly divided into one of seven experimental groups: 1) control (n = 8); 2) 3-h MV (n = 8); 3). 6-h MV (n = 6); 4). 18-h MV (n = 8); 5). 3-h anesthesia-spontaneous breathing (n = 8); 6). 6-h anesthesia-spontaneous breathing (n = 6); and 7). 18-h anesthesia-spontaneous breathing (n = 8). Markers of oxidative injury in the diaphragm included the measurement of reactive (protein) carbonyl derivatives (RCD) and total lipid hydroperoxides. Three hours of MV did not result in oxidative injury in the diaphragm. In contrast, both 6 and 18 h of MV promoted oxidative injury in the diaphragm, as indicated by increases in both protein RCD and lipid hydroperoxides. Electrophoretic separation of soluble and insoluble proteins indicated that the MV-induced accumulation of RCD was limited to insoluble proteins with molecular masses of approximately 200, 120, 80, and 40 kDa. We conclude that MV results in a rapid onset of oxidative injury in the diaphragm and that insoluble proteins are primary targets of MV-induced protein oxidation.

Short-duration mechanical ventilation enhances diaphragmatic fatigue resistance but impairs force production

STUDY OBJECTIVE

s: Mechanical ventilation (MV) is a life-support measure for patients who cannot maintain adequate alveolar ventilation. Following prolonged MV, difficulty in weaning patients from the ventilator can occur, and it has been postulated that difficult weaning is linked to respiratory muscle dysfunction. We tested the hypothesis that 18 h of controlled MV will diminish diaphragmatic maximal tetanic specific tension (force per cross-sectional area of muscle) without impairing diaphragmatic fatigue resistance.

DESIGN

To test this postulate, adult Sprague-Dawley rats were randomly classified into one of two experimental groups: (1) control group (n = 8), and (2) 18-h MV group (n = 6). MV-treated animals were anesthetized, tracheostomized, and received room air ventilation. Animals in the control group were acutely anesthetized but did not receive MV. Muscle strips from the mid-costal diaphragm were removed from both experimental groups, and contractile properties were studied in vitro to determine the effects of MV on diaphragmatic endurance and maximal force production. Diaphragmatic endurance was investigated by measuring tension development during repeated contractions throughout a 30-min fatigue protocol.

RESULTS

MV resulted in a reduction (p < 0.05) in diaphragmatic maximal specific tension (control group, 26.8 +/- 0.2 Newtons/cm(2) vs MV group, 21.3 +/- 0.6 Newtons/cm(2)). Compared to the control group, diaphragms from MV-treated animals maintained higher (p < 0.05) percentages of the initial force production throughout the fatigue protocol. The observed improvement in fatigue resistance was associated with an increase in diaphragmatic oxidative and antioxidant capacity as evidenced by increases (p < 0.05) in both citrate synthase and superoxide dismutase activities. However, by comparison to the control group, diaphragms from MV-treated animals generated less (p < 0.05) absolute specific force throughout the fatigue protocol.

CONCLUSIONS

These data indicate that 18 h of MV enhances diaphragmatic fatigue resistance but impairs diaphragmatic specific tension.

Mechanical ventilation results in progressive contractile dysfunction in the diaphragm

These experiments tested the hypothesis that a relatively short duration of controlled mechanical ventilation (MV) will impair diaphragmatic maximal specific force generation (specific P(o)) and that this force deficit will be exacerbated with increased time on the ventilator. To test this postulate, adult Sprague-Dawley rats were randomly divided into one of six experimental groups: 1) control (n = 12); 2) 12 h of MV (n = 4); 3) 18 h of MV (n = 4); 4) 18 h of anesthesia and spontaneous breathing (n = 4); 5) 24 h of MV (n = 7); and 6) 24 h of anesthesia and spontaneous breathing (n = 4). MV animals were anesthetized, tracheostomized, and ventilated with room air. Animals in the control group were acutely anesthetized but were not exposed to MV. Animals in two spontaneous breathing groups were anesthetized and breathed spontaneously for either 18 or 24 h. No differences (P > 0.05) existed in diaphragmatic specific P(o) between control and the two spontaneous breathing groups. In contrast, compared with control, all durations of MV resulted in a reduction (P < 0.05) in diaphragmatic specific tension at stimulation frequencies ranging from 15 to 160 Hz. Furthermore, the MV-induced decrease in diaphragmatic specific P(o) was time dependent, with specific P(o) being approximately 18 and approximately 46% lower (P < 0.05) in animals mechanically ventilated for 12 and 24 h, respectively. These data support the hypothesis that relatively short-term MV impairs diaphragmatic contractile function and that the magnitude of MV-induced force deficit increases with time on the ventilator.