The choice of mechanical ventilation and other technical measures for positive-pressure ventilation plays a central role in the clinical management of an increasing number of serious conditions. It is a well known fact that the circulatory effects of positive-pressure ventilation are often closely linked to the elevated mean intrathoracic pressure with which it is associated. Thus, a controversy arises between physics and physiology in the designing of ventilators to match the breathing patterns and pulmonary and car-diocirculatory physiology of patients with seriously impaired vital functions.A ventilator system utilizing a pneumatic valve principle for pressure/flow-generated, time-cycled high-frequency positive-pressure ventilation (HFPPV) for use during bronchoscopy and laryngoscopy is described. Bronchoscopic and laryngoscopic HFPPV represent new and clinically established applications of high-frequency ventilation. They may be regarded as unequivocal improvements over previous methods of ventilation during bronchoscopy and laryngoscopy and, in this respect, HFPPV has substantially increased the possibilities of microlaryngeal and tracheal surgery.A prototype of a ventilator system for pressure/flow-generated, time-cycled, volume-controlled ventilation, functionally without a compression volume, has also been developed. In this system for volume-controlled ventilation, inspiratory flow has a decelerating character, intrapulmonary gas distribution is improved in relation to that with use of a conventional ventilator system, and volume is the primary (independent) variable, while pressure becomes a secondary (dependent) variable. Such ventilation set at a high ventilatory frequency would seem to cause less interference with cardiocirculatory functions.The major characteristics of the ventilatory pattern of volume-controlled HFPPV are (1) a ventilatory frequency of about 60–100/min and an inspiration: expiration ratio of less than 0.3, (2) smaller tidal volumes, and thereby lower maximal and mean airway-thus, lower transpulmonary pressures, yet a higher functional residual capacity than in conventional IPPV/CPPV, (3) positive intratracheal and negative intrapleural pressures throughout the ventilatory cycle, (4) less circulatory interference than in conventional IPPV/CPPV, (5) reflex suppression of spontaneous respiratory rhythmicity (under certain conditions) during normoventilation, (6) decelerating inspiratory flow without an end-inspiratory plateau, and (7) more efficient pulmonary gas distribution than in conventional IPPV/CPPV. This means that the conventional opinion of how alveolar ventilation takes place during mechanical ventilation is no longer adequate for explaining how alveolar ventilation occurs during ventilation with low-compressive systems and HFPPV.Although the functional characteristics of volume-controlled HFPPV would appear to have considerable impact on the pulmonary and cardiocirculatory physiology of patients with seriously impaired vital functions, the results of the present and more extensive clinical evaluation in critically ill patients must be awaited before the merits of low-compressive, volume-controlled ventilation of high frequencies can be satisfactorily compared with those of the traditional techniques and ventilatory rates.