Carbon dioxide clearance during high frequency jet ventilation. Effect of deadspace in a lung model

The effects of the volume and length of deadspace on the clearance of carbon dioxide from a lung model have been investigated during high frequency jet ventilation (HFJV) at 1, 3 and 5 Hz. At 1 Hz, increasing the volume of the deadspace without changing the length caused a reduction in the clearance of carbon dioxide. At 5 Hz, an increase in the length of deadspace decreased carbon dioxide clearance, whilst an increase in volume had no effect. Since the delivered tidal volume was less than the volume of the morphological deadspace at this frequency, the elimination of carbon dioxide must have been accomplished by mechanisms which are not considered important at normal tidal volumes and frequencies. Furthermore, the clearance of carbon dioxide at 5 Hz was very inefficient compared with that at 1 Hz. It is concluded that, during HFJV, carbon dioxide is cleared most efficiently when the frequency is low enough for the delivered tidal volume to be greater than the volume of the morphological deadspace.