The Effective Diameter and Airflow Resistance of the Individual Lumens of Left Polyvinyl Chloride Double-Lumen Endobronchial Tubes

Abstract

Knowledge of the internal diameter and airflow resistance of each individual lumen of a doublelumen endobronchial tube (DLT) will help clinicians choose appropriate ventilatory settings and assess the airway resistance during one-lung ventilation [1,2]. This information will also help predict the chances of successful weaning from mechanical ventilation and extubation of the trachea after pneumonectomy when spontaneous ventilation is conducted through one lumen of a DLT. This information is not currently available for modern polyvinyl chloride DLTs. Therefore, this study sought to identify the equivalent (effective) diameter of the D-shaped lumens of differently sized left-sided DLTs (Sheridan, Argyle, NY) and to measure their airflow resistance. Methods Measurements were performed on the two lumens of five left-sided Sheridan DLTs for each of the adult sizes: 35 Fr, 37 Fr, 39 Fr, and 41 Fr. To measure the effective diameter, a 20-cm straight segment of the bronchial and tracheal lumens of each DLT was closed at one end. The volume of water necessary to fill each lumen was measured. The effective inner diameter of the lumen was then calculated from the volume and length of the corresponding cylinder, according to the following equation Equation 1 where V = volume, L = length, pi = pi. Airflow resistance of each lumen was calculated after measuring the pressure drop across the whole length of the tube at 0.33, 0.66, and 1.0 L/s flow rates. Airflow was measured using a calibrated Wright respirometer (Mark 8) (Ferraris Medical, London, England). A calibrated Magnehelic differential pressure transducer (Dwyer, Michigan, IN) measured the upstream pressure proximal to the tube entrance and measured the downstream pressure just inside the distal segment of the tube. The same measurements were performed on different sized Sheridan single-lumen endotracheal tubes (ETT) in order to validate the accuracy of our measurements of the effective diameter, and to provide a familiar reference against which to compare DLT resistance. Analysis of variance followed by the Tukey's test for multiple comparisons was used to compare the mean resistance of each of the tracheal and bronchial lumens of the different DLT sizes at each flow rate, and the mean resistance of each lumen at different flow rates. A P value of 0.05 or less was considered to be statistically significant. Results Data for DLTs and ETTs are summarized in Table 1 and Table 2, respectively. In most of the Tukey comparisons of mean tube resistance of each DLT side at each of the three flow rates the resistance was significantly higher the smaller the tube size (P < 0.05), with the exception of the differences in resistance between the tube sizes summarized in Table 3. Resistance of all tubes was significantly higher at the higher flow rates (P < 0.05). There were differences in the diameters and resistances between the tracheal and bronchial sides of the same DLT. The bronchial lumens had a 0.2-0.4 mm larger diameter than the tracheal lumen. At flow rates 0.66 and 1.0 L/s airflow resistance of the bronchial lumens were lower than that of the tracheal lumens. The differences in resistance between the two sides at these flow rates which reached statistical significance (P < 0.05) are summarized in Table 1.Table 1: The Effective Diameter and Airflow Resistance at Different Flow Rates of the Individual Lumens of Size 35 Fr Through 41 Fr Double-Lumen Endobronchial TubesTable 2: The Measured Diameter and Airflow Resistances at Different Flow Rates of Size 6.0 Through 7.5 mm ID Single-Lumen Endotracheal TubesTable 3: The Double-Lumen Tube Sizes Where the Difference in Airflow Resistance Did Not Reach Statistical SignificanceDiscussion Our data show that the effective diameter and airflow resistance of each individual lumen of size 35 Fr through 41 Fr DLTs are comparable to those of size 6.0 mm through 7.0 mm (inside diameter [ID]) ETTs. Small breathing tubes lead to higher peak inflation pressures, compromise the chances of successful weaning from mechanical ventilation, predispose to inadvertent positive end-expiratory pressure (auto-PEEP), and interfere with fiberoptic bronchoscopy and pulmonary toilet [3]. Based on our measurements, these potential problems need to be considered when controlled or spontaneous ventilation is performed through one side of a DLT. The higher resistance of small breathing tubes results in increased work of breathing which can adversely affect the spontaneously breathing patient. Shapiro et al. [4] measured the work of breathing and tension-time index in healthy volunteers breathing through different sized ETTs as minute ventilation (VE) was increased from 5 to 30 L/min. As tube diameter decreased, work and the tension-time index increased. The changes were magnified and the tension-time index critical fatigue level of 0.15 was approached or exceeded at the higher VE through the 6.0- and 7.0-mm ID ETTs. These findings suggest that breathing through size 6.0- or 7.0-mm ID tubes at normal VE should be tolerated by normal subjects. The authors cautioned, however, that in postoperative patients, and in patients who have underlying pulmonary or neuromuscular disease, the critical fatigue level may be reached at lower VE values than in healthy subjects, and that in these patients any increased work may be significant during weaning from mechanical ventilation. Based on these considerations, it would be prudent to replace a small DLT with an ETT of larger internal diameter before allowing a patient to breathe spontaneously after pneumonectomy when all of the patient's ventilation is conducted through one lumen of the DLT. Alternatively, pressure support ventilation can be used to decrease the imposed extrinsic work of breathing during spontaneous ventilation through the small tube [5]. At high VE, the use of a small breathing tube can lead to auto-PEEP. This results from the limited time for expiration associated with the high VE which prevents complete exhalation through the small tube [6]. If excessive, auto-PEEP can decrease cardiac output and lung compliance, and may lead to barotrauma [6]. The risk of developing auto-PEEP when a DLT is in use can be significant during fiberoptic bronchoscopy if the bronchoscopy is performed during one-lung ventilation through the ventilated side, as the lumen of the DLT will be severely compromised by the size of the bronchoscope. A single number cannot describe the flow resistance of a breathing tube because of the curvilinear pressure/volume relationship over a wide range of flows [2]. This fact was clearly shown by our data, which demonstrated the significant influence exerted on tube resistance by changes in flow rate. With low flow rates during quiet breathing, the impact of tube resistance is likely to be minimal. On the other hand, during weaning from mechanical ventilation, irritation of the tracheobronchial tree by a DLT may result in rapid breathing associated with high flow rates which can markedly increase the tube resistance. In addition to flow rate, other factors affect tube resistance including the type of flow: continuous versus sinusoidal, and, if the distal pressure is assumed to be atmospheric, whether or not the tube was placed inside a trachea-sized tube [2]. Values of ETT resistance measured in vivo are generally higher than those derived from in vitro measurements, because of secretions, head or neck position, tube deformation, or increased turbulence [7]. Sahn et al. [8] and Sullivan et al. [9] presented resistance figures which were up to fourfold different for similar ETT sizes measured under the same constant flow rate. The discrepancies between their data were attributed to the different methods used to measure the fall in pressure across the tube [10]. While one investigator measured the downstream pressure inside the tube itself, the other investigator referenced downstream pressure outside the tubes's distal end. Because of the effect of this multitude of variables on tube resistance, and because most studies on the resistance of breathing tubes were performed on single-lumen ETTs, we measured the flow resistive properties of DLTs as well as ETTs under identical conditions to provide a familiar device to compare DLT resistance to. The D shape of the lumens of modern polyvinyl chloride DLTs is based on the design of the original red rubber Robertshaw DLTs. Compared with the older Carlens' red rubber DLTs, which had oval shaped lumens, Robertshaw DLTs were designed with D-shaped lumens in order to have larger cross-sectional areas, overall size being equal [2]. Reliability of our method of measurement of the effective diameter of these D-shaped lumens is confirmed by the fact that measurement of the diameters of ETTs was accurate to within 0.1-0.2 mm of their predicted diameter. The difference between the size of the tracheal and bronchial lumens is most likely a result of the manufacturing process. Interestingly, Chiaranda et al. [2] also found that the bronchial lumen of most Robertshaw DLTs offered less resistance than the tracheal lumen. In conclusion, the effective diameter and airway resistance of each lumen of sizes 35 Fr through 41 Fr DLTs are comparable to those of sizes 6.0 mm through 7.0 mm ID ETTs. It is important to be aware of these size limitations when ventilation is performed through one side of a DLT. The authors would like to thank Tisa Reeves for her help in the preparation of this manuscript.