Mechanical ventilation is used to improve gas exchange and unload the respiratory muscles allowing for their rest and recovery, which require good synchronization between the patient and the ventilator. Spontaneous respiratory effort is generally preferred because it reduces atelectasis, improves oxygenation, and may prevent disuse diaphragm atrophy. Nevertheless, vigorous spontaneous effort can cause both lung injury and diaphragm injury (myotrauma). These injuries lead to prolonged ventilation, difficult weaning, and increased morbidity and mortality. Normal expiration is passive due to the recoil of the lungs and chest wall. In mechanical ventilation, during expiration the ventilator controls the pressure (ie, the target value is PEEP), therefore, we must look at the flow and volume waveforms to see the physiology and patient-ventilator interactions. In expiration the patient-ventilation interaction is not characterized by timing but by work. Expiratory effort (ie, negative Pmus) will deform the flow waveform in a negative direction (away from baseline). Keywords: synchronization, spontaneous effort, lung injury, myotrauma, expiratory effort

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