Since 1980, Siemens have been involved in development of equipment for high frequency ventilation (HFV) and research on the clinical application of HFV. Several prototypes have been developed. Most of these models have also been tested by experimental and clinical studies in institutes around the world. At an early stage of development, we examined a technology where HFV was combined with other modes of ventilation (CHFV), as HFV alone did not appear to produce significant improvement in ventilatory status. Other reasons were that important functions such as safety, reliability, monitoring capability and means for controlled gas delivery were difficult to secure in the traditional type of open circuit HFV gas delivery system. Some of the advantages of using a combined ventilation technique were: access to monitoring, alarms and safety functions of the Servo Ventilator; immediate access to alternative modes of ventilation if HFV fails; the possibility of altering the proportion of gas delivered by HFV and conventional modes; the possibility of using the ventilator for clinical research without compromising safety.