Factors Affecting Lung Volume Changes during Newborn Mechanical Ventilation: A Bench Study

Abstract

BACKGROUND: The functional residual capacity and tidal volume of mechanically ventilated infants recovering from respiratory distress syndrome may increase as lung compliance improves, perhaps with deleterious effects. I conducted a bench study using 5 infant ventilators in current use to determine factors affecting those volume increases. MATERIALS & METHODS: A lung simulator set at 1.0 mL/cm H2O [0.01 L/kPa] was attached to each of the 5 infant ventilators tested. Ventilator flow was set at 20 L/min, inspiratory time at 0.66 seconds, and peak inspiratory pressure at 30 cm H2O. Simulator lung compliance was then increased to 3 mL/cm H2O [0.03 L/kPa], without any change in ventilator settings. Changes in baseline end-expiratory volume and tidal volume were recorded, and the sum of these two volume changes (called maximum lung volume, or MaxV) was determined. This procedure was repeated, but with the ventilator flow set at the minimum needed to produce PIP of 30 cm H2O. Then, both procedures were repeated with inspiratory time at 0.17 s. Additional trials were conducted on ventilators offering differing pressure waveform or flow modes. Finally, these experiments were repeated with two ventilators, but with the ventilator's circuit compliance increased. RESULTS: All ventilators produced substantial increases in MaxV when simulator lung compliance increased. The magnitude of change varied widely with ventilator brand and settings (range 210% to 13%, or 4 to 63 mL). Reduction in ventilator flow substantially reduced the degree of volume increase. When inspiratory time was reduced, lung volume increase was limited for most ventilators, but not always as much as when flow was reduced. Increased ventilator-circuit compliance also contributed to volume increase. Tidal volume delivered to the lung simulator prior to compliance increase was affected by reductions in inspiratory time or flow in some but not all ventilators. CONCLUSION: The lung volume change produced as a consequence of an increase in lung compliance varied significantly among the ventilators tested.