The Impact of Imposed Expiratory Resistance in Neonatal Mechanical Ventilation: A Laboratory Evaluation

BACKGROUND: Small endotracheal tubes (ETTs) and neonatal ventilators can impact gas exchange, work of breathing, and lung-mechanics measurements in infants, by increasing the expiratory resistance (R_E) to gas flow.

METHODS: We tested two each of the Babylog 8000plus, Avea, Carestation, and Servo-i ventilators. In the first phase of the study we evaluated (1) the imposed R_E of an ETT and ventilator system during simulated passive breathing at various tidal volume (V_T), positive end-expiratory pressure (PEEP), and frequency settings, and (2) the intrinsic PEEP at various ventilator settings. In the second phase of this study we evaluated the imposed expiratory work of breathing (WOB) of the ETT and ventilator system at various PEEP levels during simulated spontaneous breathing using an infant lung model. Pressure and flow were measured continuously, and we calculated the imposed R_E of the ETT and each ventilator, and the intrinsic PEEP with various PEEP, V_T, and frequency settings. We measured the imposed expiratory WOB with several PEEP levels during a simulated spontaneous breathing pattern.

RESULTS: The ventilator's contribution to the imposed R_E was greater than that of the ETT with nearly all of the ventilators tested. There were significant differences in ventilator-imposed R_E between the ventilator brands at various PEEP, V_T, and frequency settings. The Babylog 8000plus consistently had the lowest ventilator-imposed R_E in the majority of the test conditions. There was no intrinsic PEEP (> 1 cm H₂O) in any of the test conditions with any ventilator brand. There were also no significant differences in the imposed expiratory WOB between ventilator brands during simulated spontaneous breathing.

CONCLUSIONS: The major cause of R_E appears to be the ventilator exhalation valve. Neonatal ventilators that use a set constant flow during inhalation and exhalation appear to have less R_E than ventilators that use a variable bias flow during exhalation. Clinical studies are needed to determine whether the imposed R_E of these ventilator designs impacts gas exchange, lung mechanics, or ventilator weaning.