(1363) External Jet Nebulization and Measured Ventilator Performance

Introduction: During invasive ventilation, external flow jet nebulization results in increases in displayed exhaled volumes (VTe).  We hypothesized that the magnitude of the increase is inaccurate.  An ASL-5000 simulator measured multiple ventilatory parameters over a wide range of adult settings; actual tidal volume (VTasl), peak inspiratory pressure (PIP), and time to minimum pressure, (TPmin).

Methods: Ventilators with internal and external flow sensors were tested using a variety of volume and pressure control modes (target tidal volume was 420mL).  Patient conditions (normal, COPD, ARDS) defined on the ASL were assessed at baseline and with 3.5 or 8L/min of added external flow.  Patient triggering was tested by reducing muscle effort to the level resulting in backup ventilation and by changing ventilator sensitivity to the point of auto triggering.

Results: Results are reported as % change from baseline.  For ventilators with internal flow sensors, across all modes changes in VTe ranged from 41 to 118%.  For volume-control, addition of 3.5 or 8L/min external flow resulted in actual increases in VTasl and PIP of 4.4-12% (P=.031) and 12-24% (P=.031) respectively.  Changes in VTasl correlated closely with changes in PIP (P <.0001, R2=.682). For pressure-control, VTasl decreased by 3.2-4.6% (P=.031) and 3.6-9.0% (P=.031), with 3.5 and 8L/min, PIP was unchanged.  With flow sensors at the distal Y junction, volume and pressure changes were statistically insignificant.   TPmin was found to increase at most by 8.2% (P=.015) across all modes and ventilators.  Muscle pressure effects were minimal (~1cm H2O) and ventilator sensitivity effects were nearly undetectable.

Conclusions: External flow jet nebulization results in smaller changes in volume than indicated by the ventilator display. Statistically significant effects were confined primarily to machines with internal flow sensors.  Differences approached the manufacturer reported variation in ventilator baseline performance.  Therapists can assess shifts in VT qualitatively by monitoring PIP during nebulizer therapy.