The respiratory and circulatory conditions in high frequency positive pressure ventilation (HFPPV) in the dog have been investigated previously. By means of expiratory resistance a positive intratracheal pressure was maintained throughout the respiratory cycle. Adequate ventilation was achieved at low intratracheal and transpulmonary pressures.A clinical test of this form of ventilation was conducted in connection with routine surgery in 15 patients; in 11 patients neuroleptic analgesia and in four patients thiopental sodium anaesthesia and pethidine analgesia was combined with oxygen-nitrous oxide ventilation (30 % 0, in N 2 0 ) . Muscle relaxants were given to all patients except one.The patients were ventilated, with the same gas mixtures, either manually (anaesthetic apparatus), with the Engstrom Respirator (ER-300) or by HFPPV. HFPPV was studied at different frequencies and different relative insufflation times ( 15-35 % of the period time). Expiratory minute volumes, intratracheal pressures (HFPPV) and tubing pressures (ER-300), arterial pH, Pco;?, Po2, base excess and standard bicarbonate were determined.During HFPPV there was a positive intratracheal pressure throughout the respiratory cycle. I n all patients adequate alveolar ventilation was obtained with HFPPV. The changes both in the blood gases and acid-base status, and in the intratracheal pressures and expiratory volumes, in relation to frequency and insufflation time, agreed in principle with those observed in the previous animal experiments.During HFPPV, i. e. with low intratracheal pressures, the patients maintained an adequate arterial Po2 and to judge from the pH and Pcoe this did not necessitate hyperventilation.

The low pulmonary compliance, the high airway resistance and the “rapid” breathing pattern of neonatal and paediatric patients make it necessary to design special ventilators to match the pulmonary physiology of infants.A ventilator system which also in small children has a negligible compression volume was evaluated in a lung model and during repair of cleft lip and palate in 16 patients under general anaesthesia and in two other infants during other operations. High‐frequency positive‐pressure ventilation (HFPPV) was given with an insufflation frequency (f) of 60 per min and a relative insufflation time (t%) of 32%. In addition, two neonates treated postoperatively with HFPPV are reported.Despite the “open” character of the ventilator system both intra‐ and postoperative ventilation were uneventful in all patients. The arterial oxygenation was good in all cases, as judged from clinical signs or blood gas analyses. Postoperative ventilation required conventional clinical observation and intermittent analyses of blood.HFPPV has been shown to depress, or abolish, spontaneous respiration via reflex mechanisms. In all patients in this investigation respiratory movements were absent at normo‐ or slight hyperventilation during HFPPV.