The Science Journal of the American Association for Respiratory Care

2007 OPEN FORUM Abstracts

BENCH STUDY TO EVALUATE THE USE OF ACOUSTIC REFLECTOMETRY TO MEASURE ENDOTRACHEAL TUBE OBSTRUCTION

R. Stone1, S. S. Bricknell1, O. Morejon2


Background: It is well-known that over time endotracheal tubes (ETT) will become coated with inspisated secretions and biofilm. Clinical evaluation of ETT occlusion is limited to bronchoscopic evaluation or indirectly measuring increases in work of breathing. Acoustic reflectometry (AR) uses amplitude and arrival times of acoustic returns to construct the area-distance function, a plot of cross-sectional area of the airway as a function of distance from the airway opening. We conducted this bench study to evaluate the accuracy and limitations of AR as a tool to measure cross-sectional area of ETTs under simulated conditions.

Method: Four pairs of ETTs (size 6, 7, 8 and 9 mm) were uniformly cut at the 26.5cm mark and lumen comparisons were made over a 19-centimeter length (starting at the 21-centimeter mark). New ETTs were first studied with AR (Eccovision Acoustic Pharyngometer #2495, Hood Laboratories). A matching set of ETTs were then treated by injecting 3 ml of epoxy resin (Loctite Epoxy, Henkel Consumer Adhesives) in the distal end and then passing of a 14 Fr suction catheter to distribute the resin thru the distal 1/3rd of the ETTs. Once cured, these ETTs were studied with AR. All ETTs were then cross-sectioned at one-cm intervals and each section imaged using a digital camera. The cross-sectional ETT lumens on the printed images were measured using an image digitizing tablet (SummaSketch MM III 1201, Summagraphics) and surface area calculation software (Excel Area Digi 1.01, The Logic Group). Specifically, the lumens of the untreated and treated ETTs were traced to determine the degree of obstruction for the latter. Each individual AR and photograph analysis (PA) data point was measured three times and the average taken as the final reading. AR readings were compared to the values measured using the PA technique. We compared average cross-sectional obstruction (%), maximum obstruction % and the corresponding location on the ETT.
Results: (table)

Conclusion: For ETTs of nearly full length, AR measures average obstruction within 5 percentage points of the same obstructions measured using PA. Although AR approximates the location of maximum ETT obstruction, the degree of obstruction appears to be underestimated by AR compared to PA.

Comparison of Digital Photograph Analysis Measurements to Acoustic Reflectometry Readings
ETT Size Average Obstruction Maximum Obstruction ETT Location of Maximum Obstruction
 PA AR PA AR PA AR
6 mm 22% 20% 53% 35% 17 cm 15.0 cm
7 mm 23% 23% 39% 34% 15 cm 12.5 cm
8 mm 29% 24% 48% 36% 15 cm 13.1 cm
9 mm 21% 23% 35% 33% 15 cm 7.4 cm



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