A respiratory monitoring device includes an electronic controller configured to: analyze an audio signal triggered during inspiratory and expiratory phases of a patient receiving mechanical ventilation therapy from a mechanical ventilator, the audio signal being acoustically coupled into the airway of the patient, to determine resonant frequencies of the airway; determine a shift in the resonant frequencies between the inspiratory and expiratory phases to determine a presence of pendelluft inside of a lung of the patient; and output an indication of the presence of pendelluft.