Coaxial Tubing Systems Increase Artificial Airway Resistance and Work of Breathing

Pressure-Flow Characteristics

Pressure-flow characteristics were measured by streaming the tube under test with a defined gas flow (Fig. 2). To generate a reproducible curvature of the respective tube, it was mounted with a defined sagging of 33% of its total length. When applicable, foldable tubes were measured at their full lengths (Table 1). To cover a clinically relevant flow range, the model was streamed with a sinusoidal flow of ±1.2 L/s using a piston pump driven by a control unit (E1100, LinMot, Spreitenbach, Switzerland). Flow (V̇) was measured using a pneumotachograph (Fleisch 2, Dr Fenyves & Gut, Hechingen, Germany) connected to a differential pressure transducer (CP 100, Hoffrichter, Schwerin, Germany). Airway pressures were measured at both ends of the respective tubing system using piezoresistive pressure transducers (Type 4, SI-Special Instruments GmbH, Nördlingen, Germany). Proximal and distal pressure measuring sites were of identical inner diameter to prevent the Bernoulli effect from influencing the measurements. All instruments were calibrated daily before the beginning of the experiments, after interruption of an experiment exceeding 1 h, or after a visible offset in baseline values.

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Fig. 2.

Flow model, exemplarily shown for measurements across the inspiratory lumen of a coaxial tubing system. Sinusoidal flow was applied using a piston pump driven by a linear motor (Flow generator). Flow (V̇) was measured at the tubing system's ventilator end. Airway pressures were measured at the proximal (P_prox) and the distal end (P_dist) of the tubing system. Please note that the tube's sagging was standardized to 33% of the total length.

We calculated the flow-dependent pressure gradient across the tubing systems (ΔP) by means of Rohrer's approach,² reflecting originally a linear and a quadratic relation between flow and pressure gradient. In preliminary experiments, we found that in tubing systems, the linear term does not contribute to ΔP to any relevant extent. Therefore, we used a simplified equation containing only the coefficient of the quadratic flow term (K₂) and inertance (I).³ (1)

From the continuously measured ΔP, flow, and derived volume acceleration (V̈), K₂ and inertance I were determined for each tubing system by linear regression of Equation 1. For easier comparison between the different tubing systems, ΔP was calculated for an exemplary flow of 600 mL/s (ΔP₆₀₀).

All measurements were performed for each lumen separately and repeated 6 times after dismantling and reassembling of the model setup. Flow and pressure signals were sampled at a rate of 200 Hz. Twelve ventilation cycles were recorded for each condition.

Additional Work of Breathing

To quantify the tubing systems' load for a patient, we determined the additional work of breathing caused by the tubing systems during quiet breathing. In addition, to investigate whether differences in breathing resistance between the different tubing systems would be of clinical relevance, we evaluated the perception of tubing resistance in healthy volunteers when breathing via the different types of tubing systems. The study was approved by the ethics committee of the University of Freiburg, and written informed consent was obtained by each volunteer before the experiments. Eighteen volunteers (female, n = 5; male, n = 13; age, mean [range] 37 y [24–62 y]; weight, mean [range] 76 kg [54–125 kg]; height, mean [range] 176 cm [162–192 cm]) with no known history of lung disease were asked to breathe via a mouthpiece through a pneumotachograph (as above). Initially, 20 quiet breaths were taken in a sitting position without any tubing attached to the measurement system.

To eliminate effects of breathing variability during the different trials and to prevent adaptation of the breathing pattern in response to the current load, additional work of breathing (WOB) was calculated instead of directly measured. Additional WOB was calculated from the data measured during the subjects' quiet breathing (without tubing) by integrating the product of ΔP and V̇ over the time (t) of 1 min.⁴ (2)

For that purpose, ΔP was calculated from the measured flow and the previously determined respective K₂ and I, following Equation 1. Additional WOB/L of ventilation was calculated by dividing additional WOB by minute ventilation.⁵

Perceived Breathing Resistance

After the initial measurement, a conventional, a disposable, and a coaxial tubing system (the latter two from Dräger Medical) were attached successively to the measuring system, and the volunteers were asked to take another 20 breaths through each device, respectively. To achieve directed flow within the tubing systems, their ends were connected to passive inspiration and expiration valves (resistance < 0.7 cm H₂O × s/L at 1.0 L/s). The tubing systems were presented in randomized order, blinded to the volunteers who were given a 1-min rest between 2 consecutive tests. Subsequently, the volunteers were asked to specify the one tubing system they had perceived to provide the highest breathing resistance, respectively.

Statistical Analysis

Data are presented as means ± SD if not indicated otherwise. Pressure and flow measurements were recorded using LabView 7.1 (National Instruments, Austin, Texas) and analyzed using MATLAB 2014.R1 (The MathWorks, Natick, Massachusetts). For comparisons of ΔP₆₀₀, ordinary one-way analysis of variance was calculated, followed by Sidak post hoc tests if appropriate (GraphPad PRISM 6.02, GraphPad Software, La Jolla, California). The number of volunteers was calculated in an a priori power analysis based on a chi-square test. From our experimental findings, we expected a very large effect size of 0.75. Under this assumption, 18 subjects were required to achieve a power of 0.8 for identifying a difference in breathing resistance between the 3 types of tubing systems. For comparison of additional WOB, one-way analysis of variance was calculated, followed by the Dunnett multiple-comparisons test. The ratings for breathing resistance were compared using the Kruskal-Wallis test, followed by Dunn tests. A P value < .05 was considered significant.