Rohrer's constant, k₂, as a factor for determining endotracheal tube obstruction

Introduction

The purpose of the study was to apply a method by which to measure Rohrer's constant, k₂, in order to estimate endotracheal tube (ETT) resistance (RETT). The resistance drop across the ETT is expressed by the equation RETT = k₁ + k₂V', as Rohrer described, where k₁ is the constant of laminar flow (V') and k₂ is the constant of turbulent flow. In our past study we graphed RETT over inspiratory V' for ETTs with inner diameters of 6.5 to 9.0 mm [1]. This graph provided us with k₁ and k₂ constant values, for each ETT size.

Methods

Ten intubated patients with ETTs with difficulty in patency were included in the study. Patients were all fully sedated and mechanically ventilated, by a Siemens Servo 300 ventilator, under constant flow. Pressure data were obtained: at the proximal end of the ETT (P_proximal), reflecting the impedance distally to the proximal end of the ETT; and at the distal end of the ETT (P_distal), reflecting the resistance distally to the distal end of the ETT. P_distal was recorded by an intratracheal catheter, placed 2 cm above the carinal end of the ETT. Each resistance was calculated by dividing ΔP (P_peak - P_plateau) by V', at every point of interest (either proximal or distal sites), using the rapid end-inspiratory occlusion method. RETT resulted from the difference: RETT = R_proximal - R_distal. A two-tailed t test (unpaired with unequal variances) was used to analyse the difference between data and the level of significance P was set at 0.05.

Results

Ten patients (five men), with mean age of 66 ± 17 years, were tested. Figure 1 demonstrates the difference in measured k₂ constant values compared with baseline in vitro values of the corresponding ETT size, for every patient. This is based on the assumption that at the moment of endotracheal intubation, the k₂ constant has approximately the same value as the one measured in vitro. Figure 1 shows that the in vivo values were significantly higher (P = 0.0012).

Figure 1

Comparison of the k2 constant in vivo value with the corresponding in vitro k2 value.

Conclusions

Our data suggest a significant discrepancy between predicted and in situ ETT resistance, raising concern for the presence of unrecognized ETT obstruction. Comparing the k₂ constant, measured in vivo, with its corresponding in vitro value provides an estimation of ETT's resistive behaviour.