Abstract: A method of determining a patient's respiratory effort and related patient-ventilator asynchrony comprises acquiring first impedance data representative of a first region of the lungs of the patient, the first region comprising at least a dependent region of the lungs, during the applied positive expiratory pressure, optionally acquiring second impedance data representative of a second region of the lungs, and comparing the first impedance data with one or more of the second impedance data, a flow rate within a breathing circuit of the patient, a pressure within the breathing circuit, historical impedance data of the first region, and stored patterns of impedance data of the first region. Related systems for determining a respiratory effort of a patient are also disclosed.