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


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.


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 (Pproximal), reflecting the impedance distally to the proximal end of the ETT; and at the distal end of the ETT (Pdistal), reflecting the resistance distally to the distal end of the ETT. Pdistal was recorded by an intratracheal catheter, placed 2 cm above the carinal end of the ETT. Each resistance was calculated by dividing ΔP (Ppeak - Pplateau) 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 = Rproximal - Rdistal. 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.


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
figure 1

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


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.


  1. Flevari A, et al.: Intensive Care Med. 2010,36(Suppl 2):S213.

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Flevari, A., Maniatis, N., Dimitriadou, E. et al. Rohrer's constant, k2, as a factor for determining endotracheal tube obstruction. Crit Care 15 (Suppl 1), P156 (2011).

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