The Science Journal of the American Association for Respiratory Care

2005 OPEN FORUM Abstracts


Travis Leistiko RRT, Joel Ray RRT, David R Park MD. Respiratory Care Department, Harborview Medical Center; Division of Pulmonary and Critical Care Medicine, University Of WA

BACKGROUND: We have observed severe life-threatening air trapping during nebulized prostacyclin therapy when a disposable HEPA filter was placed in the expiratory limb of the circuit to protect internal ventilator components. We hypothesized that nebulized prostacyclin clogs these filters excessively and we sought a safer alternative.

MATERIALS AND METHODS: A SERVO 300 (Maquet, Inc. Bridgewater, NJ) ventilator was configured with a standard circuit with a concha humidity setup (Hudson RCI, Temecula, Ca) and attached to a universal test lung. Four circuit configurations were tested: 1. A HEPA filter (Hudson RCI, Inc) was placed at the end of the expiratory limb before the condensation bottle (our typical clinical configuration); 2. A HEPA filter was placed between the condensation bottle and the expiratory inlet to the ventilator; 3. A non-disposable high flow STAR filter (Infrasonics medical) was placed between the condensation bottle and ventilator inlet; 4. A disposable high flow STAR-type filter (Air Safety Limited) in place of the non-disposable Star filter. A Novametrix CO2SMO Plus! Monitor (Novametrix Medical Systems, inc. Wallingford, CT.) was attached at the patient wye for airway pressure measurements. Prostacyclin was delivered using the configuration described by Siobal et al (Respir Care. 2003; 48(8): 742-53). A filter was deemed to be clogged when the resistance through the filter increased auto-PEEP to a point that the peak airway pressure exceeded the high limit on the ventilator (triggering an alarm). Finally, a test was run with only humidity and a HEPA filter in line to determine whether a humidity- saturated filter alone could cause expiratory resistance.

RESULTS: In configuration #1 the HEPA filter consistently clogged, at a mean time of 3 hours and 6 minutes. In configuration #2 clogging of the HEPA filter occurred at 2 hours and 25 minutes. With the third and fourth configurations no clogging was observed in the 48 hours of testing. Lastly, using configuration #1, with no medication nebulized; the HEPA filter did not clog with humidification alone for the 48-hour maximum testing period. An additional finding was that the recorded values of auto-PEEP and MAP from the CO2SMO Plus monitor showed signs of filter clogging sooner than were recorded from the ventilator. It was noted during the first 2 hours of each test that the ventilator showed an average of 2 cm H2O increase in MAP's. However, the CO2SMO Plus in the same time frames showed significantly increased MAP's with a mean of 9 cm H2O. The highest ventilator MAP recorded was 19 cm H2O with the CO2SMO Plus reading 45 cm H2O.

CONCLUSION: Nebulized prostacyclin clogs expiratory HEPA filters and produces clinically important levels of auto-PEEP. Because our study showed that they clog after 2 to 3 hours when nebulizing prostacyclin, it would be prudent to change these filters every 2 hours, or sooner if expiratory resistance is observed. A safe alternative to HEPA filters would be the use of a permanent or disposable high flow STAR filter alone. The use of a proximal airway pressure monitor, such as the Novametrix CO2SMO Plus, is useful for early detection of increased expiratory resistance and auto-PEEP caused by filter clogging. Airway pressure measurement at the ventilator itself does not seem to be a reliable method for detecting filter clogging, as it appears to underestimate the magnitude of auto-PEEP, perhaps due to dampening by filter resistance.