The effects of intermittent positive pressure breathing on the intrapulmonary distribution of inspired air

Abstract

The effect of intermittent positive pressure on the intrapulmonary mixing of inspired gas was studied in twelve normal subjects and nine emphysematous patients by the method of nitrogen washout. The over-all efficiency of ventilation, as measured by Becklake's lung clearance index, was not improved by intermittent positive pressure breathing in the normal subjects and only to a small degree in the emphysematous patients. The evenness of ventilation as measured by Fowler's percentage of lung clearance delay was improved in both the normal subjects and to an even greater degree in the emphysematous patients. In all the normal subjects and in those emphysematous patients who could do so, a voluntary increase in tidal volume comparable to that produced by intermittent positive pressure breathing effected the changes in the over-all efficiency of ventilation. It is concluded that the beneficial effects of intermittent positive pressure breathing on the intrapulmonary mixing of gases are due to the increase in tidal volume obtained, and are of particular value in those patients who are unable to maintain an effective tidal volume or to increase it. There was no evidence that the flow rates produced in practice with intermittent positive pressure breathing were high enough to alter the time constants of the airways and so accentuate any initial unevenness of ventilation.