- Poster presentation
- Open access
- Published:
Rohrer's constant, k2, as a factor for determining endotracheal tube obstruction
Critical Care volume 15, Article number: P156 (2011)
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 (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.
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).
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.
References
Flevari A, et al.: Intensive Care Med. 2010,36(Suppl 2):S213.
Author information
Authors and Affiliations
Rights and permissions
This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.
About this article
Cite this article
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). https://doi.org/10.1186/cc9576
Published:
DOI: https://doi.org/10.1186/cc9576