Choice of Mechanical Ventilator: Influence on Nebulizer Performance

Abstract

STUDY QUESTION: Do the nebulizer-drive characteristics of mechanical ventilators influence aerosol delivery? MATERIALS & METHODS: Using a test bench protocol incorporating an ETT and test lung, we measured the aerosol generated by two different jet nebulizers charged with radiolabeled (technetium pertechnetate, ^99mTc) saline solution. A low-resistance absolute filter was used to collect the radioaerosol delivered to the distal tip of the ETT, the quantity of which was expressed as a percentage of the nebulizer charge (inhaled mass%). Airlife and Hudson jet medication nebulizers were selected for testing, and four contemporary ventilators (BEAR 1000, Hamilton Veolar, Infrasonics Adult Star, and Puritan-Bennett 7200ae) were set at f = 15/min, V_T = 1,000 mL, square inspiratory flow pattern, inspiratory flowrate = 45 L/min (I:E = 1:2; inspiratory time fraction = 33%) for all experiments. Humidification was not used because it has been previously shown to reduce aerosol delivery. Serial measurements over time of filter radioactivity were made with each nebulizer until cumulative radioactivity reached a plateau; the results at each interval were graphed and expressed as a percentage of the activity initially placed in the nebulizer. In addition to aerosol delivery, we measured the pressure generated in the nebulizer-drive line proximal to the nebulizer during inspiration and the gas flow through the nebulizer, using a pneumotachograph and recorder. RESULTS: Aerosol delivery to the ETT filter varied over a wide range with values from 3.2% to 3.6% for the Hamilton and 9.6% to 14.7% for the other ventilators. In parallel with these observations, driving pressure and flow varied in a similar manner, with values measured for the Hamilton lower than the other brands. The time to complete nebulization appeared related to driving pressure, with the high pressure ventilators completing the treatment in one third the time (30 vs 90 min). CONCLUSION: The choice of mechanical ventilator has an important influence upon the efficiency of jet nebulization within a ventilator circuit. Driving pressure and flow to the nebulizer can significantly affect nebulizer performance and treatment duration. Therefore, in designing a protocol to deliver a nebulized drug to a ventilated patient, the choice of ventilator must also be taken into account together with previously identified variables that include choice of nebulizer, ventilator settings, and humidification.