Ercheng Zhu

Acute effects of high-dose methylprednisolone on diaphragm muscle function

The time‐ and dose‐dependent effects of acute high‐dose corticosteroids on the diaphragm muscle are poorly defined. This study aimed to examine in rabbits the temporal relationships and dose–response effects of acute high‐dose methylprednisolone succinate on diaphragmatic contractile and structural properties. Animals were assigned to groups receiving: (1) 80 mg/kg/day methylprednisolone (MP80) intramuscularly for 1, 2, and 3 days; (2) 10 mg/kg/day methylprednisolone (MP10, pulse‐dose) for 3 days; or (3) saline (placebo) for 3 days; and (4) a control group. Diaphragmatic in vitro force–frequency and force–velocity relationships, myosin heavy chain (MyHC) isoform protein and mRNA, insulin‐like growth factor‐1 (IGF‐1), muscle atrophy F‐box (MAF‐box) mRNA, and volume density of abnormal myofibrils were measured at each time‐point. MP80 did not affect animal nutritional state or fiber cross‐sectional area as assessed in separate pair‐fed groups receiving methylprednisolone or saline for 3 days. Compared with control values, MP80 decreased diaphragmatic maximum tetanic tension (Po) by 19%, 24%, and 34% after 1, 2, and 3 days (P < 0.05), respectively, whereas MP10 decreased Po modestly (12%; P > 0.05). Vmax and MyHC protein proportions were unchanged in both the MP80 and MP10 groups. Maximum power output decreased after 2 and 3 days of MP80. Suppression of IGF‐1 and overexpression of MAF‐box mRNA occurred in both MP groups. Significant myofibrillar disarray was also observed in both MP groups. The decline in Po was significantly associated with the increased volume density of abnormal myofibrils. Thus, very high‐dose methylprednisolone (MP80) can produce rapid reductions in diaphragmatic function, whereas pulse‐dose methylprednisolone (MP10) produces only modest functional loss. Muscle Nerve, 2008

INTERACTIVE EFFECTS OF CORTICOSTEROID AND MECHANICAL VENTILATION ON DIAPHRAGM MUSCLE FUNCTION

Information on the interactive effects of methylprednisolone, controlled mechanical ventilation (CMV), and assisted mechanical ventilation (AMV) on diaphragm function is sparse. Sedated rabbits received 2 days of CMV, AMV, and spontaneous breathing (SB), with either methylprednisolone (MP; 60 mg/kg/day intravenously) or saline. There was also a control group. In vitro diaphragm force, myofibril ultrastructure, αII‐spectrin proteins, insulin‐like growth factor‐1 (IGF‐1), and muscle atrophy F‐box (MAF‐box) mRNA were measured. Maximal tetanic tension (Po) decreased significantly with CMV. Combined MP plus CMV did not decrease Po further. With AMV, Po was similar to SB and controls. Combined MP plus AMV or MP plus SB decreased Po substantially. Combined MP plus CMV, MP plus AMV, or MP plus SB induced myofibrillar disruption that correlated with the reduced Po. αII‐spectrin increased, IGF‐1 decreased, and MAF‐box mRNA increased in both the CMV group and MP plus CMV group. Short‐term, high‐dose MP had no additive effects on CMV‐induced diaphragm dysfunction. Combined MP plus AMV impaired diaphragm function, but AMV alone did not. We found that acute, high‐dose MP produces diaphragm dysfunction depending on the mode of mechanical ventilation. Muscle Nerve, 2011

Positive end-expiratory airway pressure does not aggravate ventilator-induced diaphragmatic dysfunction in rabbits

IntroductionImmobilization of hindlimb muscles in a shortened position results in an accelerated rate of inactivity-induced muscle atrophy and contractile dysfunction. Similarly, prolonged controlled mechanical ventilation (CMV) results in diaphragm inactivity and induces diaphragm muscle atrophy and contractile dysfunction. Further, the application of positive end-expiratory airway pressure (PEEP) during mechanical ventilation would result in shortened diaphragm muscle fibers throughout the respiratory cycle. Therefore, we tested the hypothesis that, compared to CMV without PEEP, the combination of PEEP and CMV would accelerate CMV-induced diaphragm muscle atrophy and contractile dysfunction. To test this hypothesis, we combined PEEP with CMV or with assist-control mechanical ventilation (AMV) and determined the effects on diaphragm muscle atrophy and contractile properties.MethodsThe PEEP level (8 cmH2O) that did not induce lung overdistension or compromise circulation was determined. In vivo segmental length changes of diaphragm muscle fiber were then measured using sonomicrometry. Sedated rabbits were randomized into seven groups: surgical controls and those receiving CMV, AMV or continuous positive airway pressure (CPAP) with or without PEEP for 2 days. We measured in vitro diaphragmatic force, diaphragm muscle morphometry, myosin heavy-chain (MyHC) protein isoforms, caspase 3, insulin-like growth factor 1 (IGF-1), muscle atrophy F-box (MAFbx) and muscle ring finger protein 1 (MuRF1) mRNA.ResultsPEEP shortened end-expiratory diaphragm muscle length by 15%, 14% and 12% with CMV, AMV and CPAP, respectively. Combined PEEP and CMV reduced tidal excursion of segmental diaphragm muscle length; consequently, tidal volume (VT) decreased. VT was maintained with combined PEEP and AMV. CMV alone decreased maximum tetanic force (Po) production by 35% versus control (P < 0.01). Combined PEEP and CMV did not decrease Po further. Po was preserved with AMV, with or without PEEP. Diaphragm muscle atrophy did not occur in any fiber types. Diaphragm MyHC shifted to the fast isoform in the combined PEEP and CMV group. In both the CMV and combined PEEP and CMV groups compared to controls, IGF-1 mRNAs were suppressed, whereas Caspase-3, MAFbx and MuRF1 mRNA expression were elevated.ConclusionsTwo days of diaphragm muscle fiber shortening with PEEP did not exacerbate CMV-induced diaphragm muscle dysfunction.

Optimal arrangement of magnetic coils for functional magnetic stimulation of the inspiratory muscles in dogs

In an attempt to maximize inspiratory pressure and volume, the optimal position of a single or of dual magnetic coils during functional magnetic stimulation (FMS) of the inspiratory muscles was evaluated in twenty-three dogs. Unilateral phrenic magnetic stimulation (UPMS) or bilateral phrenic magnetic stimulation (BPMS), posterior cervical magnetic stimulation (PCMS), anterior cervical magnetic stimulation (ACMS) as well as a combination of PCMS and ACMS were performed. Trans-diaphragmatic pressure (Pdi), flow, and lung volume changes with an open airway were measured. Transdiaphragmatic pressure was also measured with an occluded airway. Changes in inspiratory parameters during FMS were compared with 1) electrical stimulation of surgically exposed bilateral phrenic nerves (BPES) and 2) ventral root electrical stimulation at C5-C7 (VRES C5-C7). Relative to the Pdi generated by BPES of 36.3/spl plusmn/4.5 cm H/sub 2/O (Mean /spl plusmn/ SEM), occluded Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and a combined PCMS + ACMS were 51.7%, 61.5%, 22.4%, 100.3%, and 104.5% of the maximal Pdi, respectively. Pdi(s) produced by UPMS, BPMS, PCMS, ACMS, and combined ACMS + PCMS were 38.0%, 45.2%, 16.5%, 73.8%, and 76.8%, respectively, of the Pdi induced by VRES (C5-C7) (48.0/spl plusmn/3.9 cm H/sub 2/O). The maximal Pdi(s) generated during ACMS and combined PCMS + ACMS were higher than the maximal Pdi(s) generated during UPMS, BPMS, or PCMS (p<0.05). ACMS alone induced 129.8% of the inspiratory flow (73.0/spl plusmn/9.4 L/min) and 77.5% of the volume (626/spl plusmn/556 ml) induced by BPES. ACMS and combined PCMS + ACMS produce a greater inspiratory pressure than UPMS, BPMS or PCMS. ACMS can be used to generate sufficient inspiratory pressure, flow, and volume for activation of the inspiratory muscles.

Ventilator-induced diaphragmatic vascular dysfunction.

Controlled mechanical ventilation (CMV) induces a profound and rapid reduction in diaphragmatic force-generating capacity, coined as ventilator-induced diaphragmatic dysfunction (VIDD) (1), and has been postulated as a potential cause of weaning failure. In animal studies, using CMV to completely abolish diaphragm muscle contractions, VIDD develops within a few hours to days both in vitro (2) and in vivo (3, 4). VIDD is associated with prominent diaphragm muscle fiber atrophy, muscle fiber injury, and multiple biochemical alterations such as increased production of reactive oxygen species (ROS), decreased protein synthesis, increased proteolysis, and apoptosis (5–7). Recently, similar observations have been demonstrated in human studies of brain-dead organ donors receiving CMV (6). Tremendous endeavors have been invested to explore the mechanisms of VIDD. Unquestionably, ROS is required for the molecular cascades that eventually produce diaphragm myofiber atrophy or injury. However, the trigger of ROS production remains an enigma. Previously, we postulated that CMV decreases regional diaphragm muscle perfusion leading to ROS production and consequently, myofiber injury (4). The elegant study of Davis and colleagues (8) in this issue of Critical Care Medicine confirmed our hypothesis and replicated the observation of Brancatisano et al (9) on the reduced regional diaphragm muscle perfusion with CMV. In addition, the study contributes further knowledge on the impact of CMV on diaphragm muscle O2 delivery (D∙ o2) and O2 uptake (V ∙ o2). Both D ∙ o2 and V∙ o2 decreased rapidly, as early as after 30 mins, and continued to decrease after 6 hrs of CMV compared with spontaneous breathing. Regional hypoxia may disrupt muscle oxidative metabolism and increase mitochondrial ROS production. While the study did not include measurement of mitochondrial ROS, the findings of decreased diaphragmatic microvascular PO2 with CMV are one step forward. Intriguing observation was the inability of the diaphragm muscle to augment blood flow in response to direct electrical stimulation of the diaphragm muscle after 6 hrs, in contrast to after 30 mins of CMV in which blood flow increased threefold. However, this observation deserves careful interpretation. Is the inability to augment blood flow a function of the duration of inactivity or the extremely short duty cycle (ratio of the duration of muscle contractions to total duration of muscle contractions plus inactivity)? Applying 3 mins of direct twitch electrical stimulations to the diaphragm after 6 hrs of passive ventilation yields a duty cycle of 2% (3/183). In contrast, after 30 mins of passive ventilation, the duty cycle was 9%, nearly five-fold greater than after 6 hrs of CMV. In addition to the force generated, the duty cycle of diaphragm muscle contractions is an important determinant of regional blood flow (10). Peak diaphragmatic blood flow was achieved at diaphragmatic tension-time index of 20%. Tension-time index is the product of diaphragmatic force and duty cycle (in this case, duty cycle is defined as inspiratory time to total breath cycle duration), expressed as ratio of maximum diaphragmatic force (10). The study of Davis et al (8) did neither control for diaphragmatic duty cycle nor control the stimulation paradigm to mimic that of spontaneous breathing. Nevertheless, impaired diaphragmatic vasoregulation or ventilator-induced diaphragmatic vascular dysfunction is an additional important consideration of the negative impact of CMV on the diaphragm. Given the direct consequences of diaphragm inactivity with CMV, prevention of VIDD is of utmost importance in the critically ill patients receiving ventilatory support. In view of the deleterious role of ROS, administration of antioxidants appears to protect from VIDD (5, 11, 12). In critically ill surgical patients, administration of antioxidants comprising vitamins E and C decreases the duration of mechanical ventilation (13). In addition to pharmacological intervention, mechanical intervention may complement or independently play a significant role in the prevention of VIDD. Electrical stimulation to the inactive diaphragm early in the course of CMV enhanced blood flow and , suggesting that the maintenance of diaphragm contractions is essential and may prevent VIDD. Indeed, in animal experiments in which the animal triggers the ventilator, and thereby, maintains partial diaphragm muscle contractions, diaphragmatic force-generating capacity is preserved (4, 14). The intensity, duration, or method (via bilateral phrenic nerves stimulation or direct muscle stimulation) of producing diaphragm muscle contractions adequate to preserve diaphragmatic function remain unknown. In sedated rabbits, partial diaphragmatic contractions with assist-control mechanical ventilation of >30% of control appeared to mitigate the decline in diaphragmatic force after 3 days of mechanical ventilation (15). In anesthetized rats, intermittent spontaneous breathing for 5 mins Copyright

Inhibition of Intestinal Thiamin Transport in Rat Model of Sepsis.

Objectives: Thiamin deficiency is highly prevalent in patients with sepsis, but the mechanism by which sepsis induces thiamin deficiency is unknown. This study aimed to determine the influence of various severity of sepsis on carrier-mediated intestinal thiamin uptake, level of expressions of thiamin transporters (thiamin transporter-1 and thiamin transporter-2), and mitochondrial thiamin pyrophosphate transporter. Design: Randomized controlled study. Setting: Research laboratory at a Veterans Affairs Medical Center. Subjects: Twenty-four Sprague-Dawley rats were randomized into controls, mild, moderate, and severe sepsis with equal number of animals in each group. Interventions: Sepsis was induced by cecal ligation and puncture with the cecum ligated below the cecal valve at 25%, 50%, and 75% of cecal length, defined as severe, moderate, and mild sepsis, respectively. Control animals underwent laparotomy only. Measurements and Main Results: After 2 days of induced sepsis, carrier-mediated intestinal thiamin uptake was measured using [3H]thiamin. Expressions of thiamin transporter-1, thiamin transporter-2, and mitochondrial thiamin pyrophosphate transporter proteins and messenger RNA were measured. Proinflammatory cytokines (interleukin-1&bgr; and interleukin-6) and adenosine triphosphate were also measured. Sepsis inhibited [3H]thiamin uptake, and the inhibition was a function of sepsis severity. Both cell membrane thiamin transporters and mitochondrial thiamin pyrophosphate transporter expression levels were suppressed; also levels of adenosine triphosphate in the intestine of animals with moderate and severe sepsis were significantly lower than that of sham-operated controls. Conclusions: For the first time, we demonstrated that sepsis inhibited carrier-mediated intestinal thiamin uptake as a function of sepsis severity, suppressed thiamin transporters and mitochondrial thiamin pyrophosphate transporter, leading to adenosine triphosphate depletion.