Factors influencing mechanical performance of neonatal high-frequency ventilators

Factors influencing the mechanical performance of neonatal high-frequency ventilators of diverse design were assessed under controlled conditions. Each of eight ventilators was coupled to in vitro models of the neonatal respiratory system simulating disease of varying severity. The principal performance characteristics examined were frequency dependence and load dependence of tidal volume delivered, peak inspiratory flow rate, and waveforms of pressure at either end of the endotracheal tube. Despite wide diversity of ventilator designs, including jets, flow interrupters, and oscillators, common features emerged. In almost all devices tidal volume increased with endotracheal tube size, was invariant with respiratory system compliance, and decreased with frequency of oscillation. Peak inspiratory flow rates for a given tidal volume and frequency were smallest in the group of oscillators compared with jets and flow interrupters. Proximal pressure was a poor indicator of distal pressure. These findings suggest that delivered tidal volume may be sensitive to endotracheal tube size and airway patency but relatively insensitive to changes in lung tissue or chest wall mechanical properties. In these regards high-frequency ventilation differs from pressure-limited conventional mechanical ventilation. Comparison of data obtained at different clinical centers using high-frequency ventilators of varying design may be possible by taking these factors into account.