Rohrer's Constant,K 2,as a Factor of Determining Inspiratory Resistance of Common Adult Endotracheal Tubes

Abstract

The aim of the study was to calculate the in vitro inspiratory resistance (R_ETT) of adult endotracheal tubes (ETT), via the end-inspiratory occlusion method, and to apply this method in vivo in order to estimate R_ETT value in real time. By plotting R_ETT over inspiratory flow (V) and calculating Rohrer's coefficients of linear and nonlinear resistance, K₁ and K₂ respectively, we determined the resistive behaviour of each ETT.

Peak and plateau pressures were recorded at both proximal and distal sites of the ETT after applying a three-second occlusion under constant flow. Distal pressure was obtained via an intraluminal catheter. R_ETT was calculated as (P_peak – P_plateau)/(V;), at both sites. R_ETT value resulted from the difference R_proximal - R_distal. Graph R_ETT over (V;) was plotted and Rohrer's constants were calculated by the method of least squares.

For ETTs with inner diameter 9.0, 8.5, 8.0, 7.5, 7.0 and 6.5 mm, K₂ was 2.42, 3.05, 4.65, 6.01, 9.17 and 12.80 cmH₂O/l/s, respectively. The intraluminal catheter increased R_ETT No.7.0 by an average of 49%. Finally, ten patients with partially obstructed ETTs were tested and K₂ in vivo constants found to be higher than their corresponding in vitro values (P value 0.00012).

Therefore, knowing the performing size of an ETT may help the clinicians identify ETT obstruction and deal with weaning problems.