External Jet Nebulization and Measured Ventilator Performance

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

Ventilators with internal and external flow sensors were tested by using a variety of volume and pressure control modes (the target V_T was 420 mL). Patient conditions (normal, COPD, ARDS) defined on the ASL 5000 were assessed at baseline and with 3.5 or 8 L/min of added external flow. Patient-triggering was assessed by reducing muscle effort to the level that resulted in backup ventilation and by changing ventilator sensitivity to the point of auto-triggering.

Results are reported as percentage change from baseline after addition of 3.5 or 8 L/min external flow. For ventilators with internal flow sensors, changes in displayed exhaled V_T ranged from 10% to 118%, however, when using volume control, actual increases in actual V_T and PIP were only 4%-21% (P = .063, .031) and 6%-24% (P = .25, .031), respectively. Changes in actual V_T correlated closely with changes in PIP (P < .001; R² = 0.68). For pressure control, actual V_T decreased by 3%-5% (P = .031) and 4%-9% (P = .031) with 3.5 and 8 L/min respectively, PIP was unchanged. With external flow sensors at the distal Y-piece junction, volume and pressure changes were statistically insignificant. The time to minimum pressure increased at most by 8% (P = .02) across all modes and ventilators. The effects on muscle pressure were minimal (∼1 cm H₂O), and ventilator sensitivity effects were nearly undetectable.

External flow jet nebulization resulted in much 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. During nebulizer therapy, effects on V_T can be estimated at the bedside by monitoring PIP.