1999 OPEN FORUM Abstracts

CAN AN MDI DEVICE AT THE ET TUBE CLOG AN HCH?

Scott Foss BS, Jean Keppel PhD, David Sladek CRTT, Thayer Medical Corporation, Tucson AZ.

Background: Passive humidifiers, such as hygroscopic condenser humidifiers (HCH) have been used increasingly in ventilator circuits since the mid-1980s. Because the HCH is placed between the patient wye and the ET tube, it would need to be removed from the circuit before an MDI drug could be delivered from a location on the inspiratory limb of the circuit. To avoid breaking the circuit for an MDI treatment, it would be desirable to place the MDI device between the HCH and the ET tube, as long as it could be shown that the MDI drug would not clog the HCH even under non-ideal conditions.

Method: A Bear 1 ventilator (V_T = 700 mL, peak flow = 45 L/min, 6 breaths/min) was attached to a patient wye, which was followed by a Baxter Adult Filtered HCH, then a Thayer dual-spray 15-mm ID/OD MiniSpacerÒ MDI device, then the ET tube with filter on the end. The filter was followed by a humidifier which exposed the ET tube and HCH to 100% humidity. A test lung was attached to the humidifier by heated-wire tubing kept at 36 ± 1 °C. At the start of each test run, a new HCH was put into the circuit and the air flow resistance across the HCH was measured at peak flow. Two types of tests were run: (a) A 72-hour test, with ?worst-case? drug delivery, that is, the MDI was actuated at the start of the lung's exhalation. The circuit was run continuously for 3 days, throughout which 72 doses each of VentolinÒ (albuterol) and AlupentÒ (metaproterenol sulfate) were delivered, for a total of 144 actuations. A control was run with the same setup but no drug delivery. The air flow resistance across the HCH was measured periodically in both the control and the test run. (b) Ten sets of 72-minute tests (and controls), with the same two drugs actuated 72 times each, either correctly (at the start of inhalation) or incorrectly (at the start of exhalation or the start of the pause between breaths). For these 10 sets, ANOVA could be run to check the significance of the resistance results.

Results: The graph shows the air flow resistance of the HCH (inches H₂O) measured periodically during the 72-hour test and control. Error bars show the measurement uncertainty in the air flow resistance. Results for the 72-minute tests are not shown here but are similar: In no case did the HCH resistance change. The two-factor ANOVA showed statistical equivalence among all results with time.

Conclusions: MDI drugs delivered at the ET tube are unlikely to clog an HCH, even if the drug is somewhat sticky (like Alupent) and even if the MDI is actuated at the wrong part of the breath cycle. There are other reasons (dose output) to actuate the MDI at the start of inhalation, and there are other substances (secretions) that may clog an HCH, but the MDI drug is not implicated as a clogging agent even in the ?worst case.?

(See Original for Figure)

OF-99-222