The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

EFFECTS OF IMPEDANCE TO MECHANICAL VENTILATION ON RESISTANCE MEASUREMENTS

Frank Dennison, MEd, RRT, RPFT, Kitty Hernlen, MBA, RRT, Rick Hall, MS, RRT, RPFT, Arthur Taft, PhD, RRT; Medical College of Georgia, Augusta, GA, and Jason Hunter, RRT, Medical Center of Central Georgia, Macon, GA

Measurement of total respiratory system resistance (RTRS) is a clinically useful tool for assessing Raw. However, it includes measuring resistance through the ventilator circuit, which changes (i.e., expands) during positive pressure ventilation. To determine if the ventilator circuit contributes to errors in resistance measurement, we designed a bench study to evaluate RTRS measurement (PIP --Ppause/Flow) using a mechanical model so that airways and circuit resistance were fixed (constant) at a given flow rate. Research question: will changes in lung compliance (CL) affect the accuracy of resistance measurements as currently practiced? Method: We used a Bird 8400sti to ventilate a Michigan Instruments Training Test Lung (TTL) at a set VT (1.0 L) using dry air, conventional circuit, HME and size 8 ETT. RTRS was calculated from measured flows, PIP and Ppause, which were made using a Timeter RT200. Alterations in lung impedance were created by changing total CL (0.1 to 0.01 L/cm H2O) and altering airway resistance using 2 Pneuflo Resistors (Rp20 and Rp5) in line to the TTL. Measurements were repeated (n = 10) at flows of 30, 60 and 90 L/min with and without the TTL in line for comparison. ANOVA for repeated measures was used for statistical analysis of data. Resistance measurements with no impedance (None in Table) to ventilation are accurate and set the standard for comparison.

RTRS (cm H2O/L/s) 30 L/min RTRS (cm H2O/L/s) 60 L/min RTRS (cm H2O/L/s) 90 L/min
CL Rp20 Rp5 Rp20 Rp5 Rp20 Rp5
None 18.1 11.1 30.5 15.8 43.5 21.3
0.10 L/cm H2O 16.2 9.9 27.1 14.6 38.6 19.6
0.07 L/cm H2O 15.7 9.6 26.8 14.0 37.8 18.9
0.04 L/cm H2O 14.9 9.1 24.6 13.2 35.3 17.8
0.01 L/cm H2O 9.7 5.9 15.9 7.9 22.3 10.2

Results clearly demonstrate that RTRS is reduced as CL is reduced (P < 0.001). Data from the study (not shown) suggest that this decrease in resistance is caused by a reduction in flow as a result of positive pressure expanding circuit tubing. As the circuit expands, volume over time (flow) is reduced. Thus, reduction in flow causes the reduction in the RTRS measurements. The clinical importance of this bench study is that pathophysiologic changes in airways (i.e., inflammation or obstruction), which cause increased Raw, may be obscured by the decreases in the measured circuit resistance included in the RTRS measurements when there is a decrease in CL.

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