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Acquired diaphragm muscle weakness is a key feature in several chronic conditions, including chronic obstructive pulmonary disease, congestive heart failure, and difficult weaning from mechanical ventilation. No drugs are available to improve respiratory muscle function in these patients. Recently, we have shown that the calcium sensitizer levosimendan enhances the force-generating capacity of isolated diaphragm fibers.To investigate the effects of the calcium sensitizer levosimendan onin vivohuman diaphragm function.In a double-blind, randomized, crossover design, 30 healthy subjects performed two identical inspiratory loading tasks. After the first loading task, subjects received levosimendan (40 μg/kg bolus followed by 0.1/0.2 μg/kg/min continuous infusion) or placebo. Transdiaphragmatic pressure, diaphragm electrical activity, and their relationship (neuromechanical efficiency) were measured during loading. Magnetic phrenic nerve stimulation was performed before the first loading task and after bolus administration to assess twitch contractility. Center frequency of diaphragm electrical activity was evaluated to study the effects of levosimendan on muscle fiber conduction velocity.The placebo group showed a 9% (P= 0.01) loss of twitch contractility after loaded breathing, whereas no loss in contractility was observed in the levosimendan group. Neuro-mechanical efficiency of the diaphragm during loading improved by 21% (P< 0.05) in the levosimendan group. Baseline center frequency of diaphragm electrical activity was reduced after levosimendan administration (P< 0.05).The calcium sensitizer levosimendan improves neuromechanical efficiency and contractile function of the human diaphragm. Our findings suggest a new therapeutic approach to improve respiratory muscle function in patients with respiratory failure.