Effective dead space of differently shaped airways during high-frequency ventilation of a CO2-producing lung model

A simple lung model is described which simulates the mechanical properties and CO2 production of the lungs of a small animal, and which can be used with high-frequency ventilators. The model was ventilated by a rotary-valve ventilator and the system was used to determine the increment in 'physiological' (Bohr) dead space produced at various ventilatory frequencies between 0.5 and 15 Hz by the insertion of various additional volumes and configurations of 'anatomical' dead space in its 'upper airway'. It was found that the insertion of a straight tube with an internal diameter similar to that of the ventilator outlet tube produced an increment in 'physiological' dead space that remained commensurate with the volume of the added tube at all combinations of the tube volume and ventilatory frequency. The insertion of similar volumes of dead space in the form of tubes with smoothly expanded central portions produced increments in 'physiological' dead space which were commensurate with the volume of the added tube only at the lowest frequencies, and actually became negative at higher frequencies and volumes. These findings provide further evidence that the volume and configuration of the external portion of a high-frequency ventilator system may be of at least as much importance in determining its efficiency as is the lung structure of the animal or patient being ventilated.