Influence of end-expiratory lung volume on carbon dioxide elimination during high frequency ventilation in dogs

We have examined the effect of varying end-expiratory lung volume on carbon dioxide elimination in 10 mongrel dogs undergoing conventional mechanical ventilation at 12 b.p.m. and forced diffusion ventilation (FDV) at 6 Hz and 50 Hz and continuous flow. End-expiratory volumes were altered by changing the pressure in a plethysmographic box in which the dogs underwent ventilation. The pressures studied were atmospheric, sub-atmospheric (box pressure -1.0 kPa) and increased atmospheric (box pressure + 0.5 kPa). The results indicated that more carbon dioxide was eliminated at low lung volumes and this was most pronounced with HFV at 50 Hz and continuous flow. It is postulated that changes in airway geometry and different lung volumes may alter the distance between the gas interface in the conductive airways and the respiratory zone and so alter the efficiency of ventilation during FDV.