Review of the Physiological Rationale for and Development of High‐Frequency Positive‐Pressure Ventilation—HFPPV

In 1967, a method of artificial positive‐pressure ventilation without circulatory effects synchronous with respiration was required for experimental studies and for this purpose high‐frequency positive‐pressure ventilation (HFPPV) was developed. The original rationale for the HFPPV technique was ( a ) by means of endotracheal insufflation a reduction in dead space should make it possible, with smaller tidal volumes and higher ventilatory frequencies, to provide adequate alveolar ventilation at lower maximal and lower mean airway pressures than those required in conventional IPPV; ( b ) under insufflation with a high frequency and a short insufflation period the inertia characteristics of the lungs should be able to achieve critical suppression of the circulatory effects of the ventilatory pattern. In order to compensate for the increased V d V t with intermittent positive‐pressure ventilation of high frequency, a ventilator system whose compressible volume and internal compliance were negligible was required. In dog experiments the HFPPV technique was found to provide the anticipated circulatory conditions. In addition it was observed that when air was intermittently supplied by endotracheal insufflation at a frequency of 60–80 per min the spontaneous breathing ceased almost instantaneously. As arterial blood gas analyses verified that this suppression occurred even at normoventilation, it was assumed that it most probably resulted from a suppression of the reflexogenic spontaneous respiratory rhythm. In comparative experimental studies the original expectations were verified, i.e. in normal dogs under general anaesthesia HFPPV gave adequate ventilation with the aid of relatively low transpulmonary pressures and with only slight circulatory or other systemic effects. This HFPPV technique was therefore investigated clinically and in all patients adequate alveolar ventilation was achieved with insufflation frequencies of 60–100 per min and with only small airway pressure variations. A ventilator system for HFPPV in which the insufflation catheter is excluded from the patient circuit was considered. This requires a modified insufflation technique which essentially gives no increase in the compressible volume or internal compliance of the ventilator system, or in the physiological dead space of the patient. A pneumatic valve principle utilizing the Coanda effect was found to fulfil these requirements. The ventilator systems designed according to these principles comprised a gas‐conditioning unit and a patient circuit. The gas‐conditioning unit supplied gas pressures which remained constant regardless of the ventilatory phase and provided gas mixtures (C 2 /N 2 O or O 2 /air) of desired temperature and relative humidity. The conditioned gas was delivered under high pressure to the side‐arm of the pneumatic valve connector of the patient circuit. These new systems for HFPPV were studied in a lung model and dog experiments. They were then applied in clinical practice for bronchoscopy (broncho‐scopic HFPPV) and laryngoscopy (laryngoscopy HFPPV) under general anaesthesia and for paediatric anaesthesia and neonatal respiratory care. Owing to their “open” character these ventilator systems and techniques lacked the capacity to give a volume‐controlled ventilation. Further technical development was thus motivated, and is the subject of a subsequent article.