A control unit for controlling a ventilator that provides EMG controlled ventilation to a patient receives EMG signals from an esophageal catheter inserted into the patient and selects, dependent on the EMG signal, at least one signal for controlling the ventilator. A user interface unit operates by the control unit indicates the position of the catheter relative to the patient's diaphragm based on the selected signal. The position information may be presented in relation to signal curves representing the catheter signal or as an elongate vertical shape representing the catheter, the display unit being operated to indicate on the elongate vertical shape the position of the diaphragm.