L.M.A. Heunks

Diaphragm adaptations in patients with COPD

Inspiratory muscle weakness in patients with COPD is of major clinical relevance. For instance, maximum inspiratory pressure generation is an independent determinant of survival in severe COPD. Traditionally, inspiratory muscle weakness has been ascribed to hyperinflation-induced diaphragm shortening. However, more recently, invasive evaluation of diaphragm contractile function, structure, and biochemistry demonstrated that cellular and molecular alterations occur, of which several can be considered pathologic of nature. Whereas the fiber type shift towards oxidative type I fibers in COPD diaphragm is regarded beneficial, rendering the overloaded diaphragm more resistant to fatigue, the reduction of diaphragm fiber force generation in vitro likely contributes to diaphragm weakness. The reduced diaphragm force generation at single fiber level is associated with loss of myosin content in these fibers. Moreover, the diaphragm in COPD is exposed to oxidative stress and sarcomeric injury. This review postulates that the oxidative stress and sarcomeric injury activate proteolytic machinery, leading to contractile protein wasting and, consequently, loss of force generating capacity of diaphragm fibers in patients with COPD. Interestingly, several of these presumed pathologic alterations are already present early in the course of the disease (GOLD I/II), although these patients appear not limited in their daily life activities. Treatment of diaphragm dysfunction in COPD is complex since its etiology is unclear, but recent findings indicate the ubiquitin-proteasome pathway as a prime target to attenuate diaphragm wasting in COPD.

Clinical review: The ABC of weaning failure - a structured approach

About 20% to 30% of patients are difficult to wean from invasive mechanical ventilation. The pathophysiology of difficult weaning is complex. Accordingly, determining the reason for difficult weaning and subsequently developing a treatment strategy require a dedicated clinician with in-depth knowledge of the pathophysiology of weaning failure. This review presents a structural framework (ABCDE) for the assessment and treatment of difficult-to-wean patients. Earlier recognition of the underlying causes may expedite weaning from mechanical ventilation.

Bench-to-bedside review: Hypercapnic acidosis in lung injury - from 'permissive' to 'therapeutic'

Modern ventilation strategies for patients with acute lung injury and acute respiratory distress syndrome frequently result in hypercapnic acidosis (HCA), which is regarded as an acceptable side effect (permissive hypercapnia). Multiple experimental studies have demonstrated advantageous effects of HCA in several lung injury models. To date, however, human trials studying the effect of carbon dioxide per se on outcome in patients with lung injury have not been performed. While significant concerns regarding HCA remain, in particular the possible unfavorable effects on bacterial killing and the inhibition of pulmonary epithelial wound repair, the potential for HCA in attenuating lung injury is promising. The underlying mechanisms by which HCA exerts its protective effects are complex, but dampening of the inflammatory response seems to play a pivotal role. After briefly summarizing the physiological effects of HCA, a critical analysis of the available evidence on the potential beneficial effects of therapeutic HCA from in vitro, ex vivo and in vivo lung injury models and from human studies will be reviewed. In addition, the potential concerns in the clinical setting will be outlined.

Plasma from septic shock patients induces loss of muscle protein

IntroductionICU-acquired muscle weakness commonly occurs in patients with septic shock and is associated with poor outcome. Although atrophy is known to be involved, it is unclear whether ligands in plasma from these patients are responsible for initiating degradation of muscle proteins. The aim of the present study was to investigate if plasma from septic shock patients induces skeletal muscle atrophy and to examine the time course of plasma-induced muscle atrophy during ICU stay.MethodsPlasma was derived from septic shock patients within 24 hours after hospital admission (n = 21) and healthy controls (n = 12). From nine patients with septic shock plasma was additionally derived at two, five and seven days after ICU admission. These plasma samples were added to skeletal myotubes, cultured from murine myoblasts. After incubation for 24 hours, myotubes were harvested and analyzed on myosin content, mRNA expression of E3-ligase and Nuclear Factor Kappa B (NFκB) activity. Plasma samples were analyzed on cytokine concentrations.ResultsMyosin content was approximately 25% lower in myotubes exposed to plasma from septic shock patients than in myotubes exposed to plasma from controls (P < 0.01). Furthermore, patient plasma increased expression of E3-ligases Muscle RING Finger protein-1 (MuRF-1) and Muscle Atrophy F-box protein (MAFbx) (P < 0.01), enhanced NFκB activity (P < 0.05) and elevated levels of ubiquitinated myosin in myotubes. Myosin loss was significantly associated with elevated plasma levels of interleukin (IL)-6 in septic shock patients (P < 0.001). Addition of antiIL-6 to septic shock plasma diminished the loss of myosin in exposed myotubes by approximately 25% (P < 0.05). Patient plasma obtained later during ICU stay did not significantly reduce myosin content compared to controls.ConclusionsPlasma from patients with septic shock induces loss of myosin and activates key regulators of proteolysis in skeletal myotubes. IL-6 is an important player in sepsis-induced muscle atrophy in this model. The potential to induce atrophy is strongest in plasma obtained during the early phase of human sepsis.

Titin and diaphragm dysfunction in mechanically ventilated rats.

PurposeDiaphragm weakness induced by mechanical ventilation may contribute to difficult weaning from the ventilator. For optimal force generation the muscle proteins myosin and titin are indispensable. The present study investigated if myosin and titin loss or dysfunction are involved in mechanical ventilation-induced diaphragm weakness.MethodsMale Wistar rats were either assigned to a control group (nxa0=xa010) or submitted to 18xa0h of mechanical ventilation (MV, nxa0=xa010). At the end of the experiment, diaphragm and soleus muscle were excised for functional and biochemical analysis.ResultsMaximal specific active force generation of muscle fibers isolated from the diaphragm of MV rats was lower than controls (128xa0±xa09 vs. 165xa0±xa013xa0mN/mm2, pxa0=xa00.02) and was accompanied by a proportional reduction of myosin heavy chain concentration in these fibers. Passive force generation upon stretch was significantly reduced in diaphragm fibers from MV rats by ca. 35%. Yet, titin content was not significantly different between control and MV diaphragm. In vitro pre-incubation with phosphatase-1 decreased passive force generation upon stretch in diaphragm fibers from control, but not from MV rats. Mechanical ventilation did not affect active or passive force generation in the soleus muscle.ConclusionsMechanical ventilation leads to impaired diaphragm fiber active force-generating capacity and passive force generation upon stretch. Loss of myosin contributes to reduced active force generation, whereas reduced passive force generation is likely to result from a decreased phosphorylation status of titin. These impairments were not discernable in the soleus muscle of 18xa0h mechanically ventilated rats.

Heart failure decreases passive tension generation of rat diaphragm fibers.

BACKGROUNDnDiaphragm dysfunction is well-known to limit quality of life and prognosis of patients with heart failure (HF), but its underlying mechanisms are not well understood. In an animal model for HF we recently showed that impaired diaphragm contractility arises at the single fiber level and is associated with sarcomeric injuries. For optimal muscle function and sarcomeric stability passive elastic structures, like titin, are indispensable. The current study aimed to investigate if impaired passive elasticity contributes to diaphragm dysfunction in rats with heart failure.nnnMETHODSnSkinned muscle fibers were isolated from the diaphragm and soleus of rats with chronic HF, induced by left coronary artery ligation and of sham-operated rats. Passive tension-length relationships were determined by applying segmental extension tests. Immunofluorescence was performed on muscle cryosections using antibodies (T12) against a titin epitope near the Z-line. Titin content was determined by SDS-agarose-gel electrophoresis. Titins mobility on gel was studied to detect changes in titin size.nnnRESULTSnPassive tension generation upon stretch was significantly reduced (>35%) in HF diaphragm fibers compared to sham. Immunostaining intensities against titin were reduced in diaphragm cryosections of HF rats compared to sham. Soleus fibers from HF and sham rats did not display differences, neither in passive tension nor in immunostaining. No differences in titins size were detected in HF and sham diaphragm. Titin content, however, was significantly reduced ( approximately 25%) in HF diaphragm.nnnDISCUSSIONnWe conclude that in the diaphragm of HF rats, passive elasticity is impaired, mainly resulting from titin loss.

Isoflurane attenuates pulmonary interleukin‐1β and systemic tumor necrosis factor‐α following mechanical ventilation in healthy mice

Background: Mechanical ventilation (MV) induces an inflammatory response in healthy lungs. The resulting pro‐inflammatory state is a risk factor for ventilator‐induced lung injury and peripheral organ dysfunction. Isoflurane is known to have protective immunological effects on different organ systems. We tested the hypothesis that the MV‐induced inflammatory response in healthy lungs is reduced by isoflurane.

Time course of diaphragm function recovery after controlled mechanical ventilation in rats

Controlled mechanical ventilation (CMV) is known to result in rapid and severe diaphragmatic dysfunction, but the recovery response of the diaphragm to normal function after CMV is unknown. Therefore, we examined the time course of diaphragm function recovery in an animal model of CMV. Healthy rats were submitted to CMV for 24-27 h (n = 16), or to 24-h CMV followed by either 1 h (CMV + 1 h SB, n = 9), 2 h (CMV + 2 h SB, n = 9), 3 h (CMV + 3 h SB, n = 9), or 4-7 h (CMV + 4-7 h SB, n = 9) of spontaneous breathing (SB). At the end of the experiment, the diaphragm muscle was excised for functional and biochemical analysis. The in vitro diaphragm force was significantly improved in the CMV + 3 h SB and CMV + 4-7 h SB groups compared with CMV (maximal tetanic force: +27%, P < 0.05, and +59%, P < 0.001, respectively). This was associated with an increase in the type IIx/b fiber dimensions (P < 0.05). Neutrophil influx was increased in the CMV + 4-7 h SB group (P < 0.05), while macrophage numbers remained unchanged. Markers of protein synthesis (phosphorylated Akt and eukaryotic initiation factor 4E binding protein 1) were significantly increased (±40%, P < 0.001, and ±52%, P < 0.01, respectively) in the CMV + 3 h SB and CMV + 4-7 h SB groups and were positively correlated with diaphragm force (P < 0.05). Finally, also the maximal specific force generation of skinned single diaphragm fibers was increased in the CMV + 4-7 h SB group compared with CMV (+45%, P < 0.05). In rats, reloading the diaphragm for 3 h after CMV is sufficient to improve diaphragm function, while complete recovery occurs after longer periods of reloading. Enhanced muscle fiber dimensions, increased protein synthesis, and improved intrinsic contractile properties of diaphragm muscle fibers may have contributed to diaphragm function recovery.

Role of nitric oxide in isometric contraction properties of rat diaphragm during hypoxia.

Abstract. Hypoxia disturbs Ca2+ regulation and increases the intracellular Ca2+ concentration ([Ca2+]i), which may in turn activate the nitric oxide synthase (NOS) regulated by [Ca2+]i. Since nitric oxide (NO) reduces the isometric contractility of rat diaphragm in vitro, we hypothesized that NO contributes to the impaired force generation of an hypoxic diaphragm. The effects of different concentrations of the NOS inhibitor, NG-monomethyl-L-arginine (L-NMMA), the NO scavenger haemoglobin (150xa0µmol·l–1) and the NO donor spermine NONOate (Sp-NO; 1xa0mmol·l–1) were determined on isometric contractility during hypoxia [partial pressure of oxygen, PO2, about 7xa0kPa (about 54xa0mmHg)] and hyperoxia [PO2 about 83xa0kPa (about 639xa0mmHg)]. Hypoxia significantly reduced maximal twitch force (Ft), and submaximal tetanic force (30xa0Hz, F30) in all L-NMMA groups. A low concentration of L-NMMA (30xa0µmol·l–1) increased F30 but a high concentration (1,000xa0µmol·l–1) reduced F30 during hypoxia. The effects of L-NMMA on force generation were more pronounced during hypoxia compared to hyperoxia. Peak increases in F30 and Ft were observed at a concentration of 30xa0µmol·l–1 L-NMMA during hypoxia, but with 10xa0µmol·l–1 L-NMMA during hyperoxia. The same concentration of haemoglobin increased F30 and Ft less during hypoxia compared to hyperoxia. The Sp-NO reduced Ft, F30 and maximal tetanic force (F0) during hypoxia; these effects were abolished in the presence of haemoglobin. The Sp-NO did not alter Ft, F30 and F0 during hyperoxia. We conclude that NO plays a more prominent role during hypoxia and that NO contributes to the depression of force generation in the hypoxic rat diaphragm in vitro. This change may be related to an elevated NO generation within the hypoxic diaphragm.

Nitric oxide modulates neuromuscular transmission during hypoxia in rat diaphragm

Hypoxia impairs neuromuscular transmission in the rat diaphragm. In previous studies, we have shown that nitric oxide (NO) plays a role in force modulation of the diaphragm under hypoxic conditions. The role of NO, a neurotransmitter, on neurotransmission in skeletal muscle under hypoxic conditions is unknown. The effects of the NO synthase (NOS) inhibitor nomega‐nitro‐L‐arginine (L‐NNA, 1 mM) and the NO donor spermine NONOate (Sp‐NO, 1 mM) were evaluated on neurotransmission failure during nonfatiguing and fatiguing contractions of the rat diaphragm under hypoxic (PO2 ∼ 5.8 kPa) and hyperoxic conditions (PO2 ∼ 64.0 kPa). Hypoxia impaired force generated by both muscle stimulation at 40 HZ (P40M) and by nerve stimulation at 40 HZ (P40N). The effect of hypoxia in the latter was more pronounced. L‐NNA increased P40N whereas Sp‐NO decreased P40N during hypoxia. In contrast, neither L‐NNA nor Sp‐NO affected P40N during hyperoxia. L‐NNA only slightly reduced neurotransmission failure during fatiguing contractions under hyperoxic conditions. Consequently, neurotransmission failure assessed by comparing force loss during repetitive nerve simulation and superimposed direct muscle stimulation was more pronounced in hypoxia, which was alleviated by L‐NNA and aggravated by Sp‐NO. These data provide insight in the underlying mechanisms of hypoxia‐induced neurotransmission failure. This is important as respiratory muscle failure may result from hypoxia in vivo. Muscle Nerve, 2005