Left-sided double-lumen endobronchial tubes are used to manage patients undergoing thoracic surgery with one-lung anaesthesia. It is essential that the tube be placed in optimal position for patient safety and to avoid complications related to its use . Many studies have shown a statistically significant correlation between body height and the optimal depth of insertion for average-sized adult patients [2-5]. However, in these studies only 12 of 101 patients , 8 of 65 patients , and 19 of 140 patients  measured 155 cm or less, which is not uncommon among adult patients in Asia.
We have previously reported a case of ruptured left main-stem bronchus after intubation with a left-sided double-lumen tube using a blind method  in an adult patient who was only 145 cm tall. Since the tube was advanced too far for that patient, we studied whether or not the optimal depth of insertion in below average-sized adult patients can be predicted from body height.
The study was approved by the Ethics Committee of our university and written informed consent was obtained from each patient. All patients with a body height of ≤ 155 cm, requiring one-lung anaesthesia for open thoracic or thoracoscopic surgery, were prospectively studied. Body height was measured in the standing position before surgery. Anaesthesia was induced with intravenous (i.v.) fentanyl (0.1-0.2 mg) and propofol (1-2 mg kg−1).
Monitoring included electrocardiography, non-invasive blood pressure, pulse oximetry, and capnography. The trachea was intubated with a left-sided double-lumen tube (Broncho-Cath™; Mallinckrodt Medical, Athlone, Ireland, or Blue-Line™ Endobronchial tube; SIMS, Inc., Keene, USA). The size of the tube was based on the width of the trachea on a posterior-anterior chest radiograph according to Chow and colleagues . The left main bronchus was intubated under direct vision with fibre-optic bronchoscopy as reported by Shinnick and colleagues . A bronchoscope (Model 3C40; Olympus, Tokyo, Japan, outer diameter: 3.3 mm) was introduced into the left bronchial lumen of the tube after its tip had passed the vocal cords. Under direct visual guidance, the tube was advanced into the left main-stem bronchus until its tip was located just proximal to the orifice of the upper lobe bronchus. Then, the bronchoscope was inserted into the tracheal lumen of the double lumen tube to guide the proximal part of the bronchial cuff to just below the tracheal carina. The depth of insertion was recorded at the upper incisors using external centimetre markings on the tube. The patient’s head was placed on a 5-cm high pillow in a neutral position during measurement of the depth of the tube with the bronchial cuff deflated.
All values are expressed as mean ± standard deviation (SD). The relationship between body height and the optimal depth of insertion was calculated by linear regression analysis. Statistical analysis was done using the regression module of Statview 5.0 (SAS Institute Inc., Cary, NC, USA). A P-value below 0.05 was considered statistically significant.
One hundred and ninety six consecutive adult patients with a body height ≤ 155 cm were enrolled in this study. Table 1 shows the patients characteristics. A 35 F left-sided double-lumen tube was used in 141 patients; a Broncho-Cath™ was used in 128 patients and a Blue-Line™ endobronchial tube in 68 patients.
As shown in Figure 1, there was a statistically significant positive linear correlation between the optimal depth of insertion and patient height (r = 0.61, P < 0.0001). The regression lines and P-values for 57 male and 139 female were y = 0.19x + 0.28 (r = 0.35, P < 0.0001) and y = 0.24x − 8.44 (r = 0.59, P < 0.0001), respectively. One female patient whose height was 140 cm had a calculated depth of insertion of 24.5 cm from the obtained equation; however, the actual optimal depth of insertion in this patient, as verified by bronchoscopy was only 20 cm. Another female patient who was 145 cm tall and whose calculated depth of insertion was 26 cm had an actual optimal depth of insertion of 22 cm. Figure 2 shows the actual optimal depths of insertion for all patients (range: 20-32 cm). The most frequent actual optimal depth of insertion was 27 cm.
We have shown that although there is a statistically significant positive linear correlation between body height and optimal depth of insertion of left-sided double-lumen tubes in adult patients of short stature, it is not clinically applicable due to the wide inter-individual variations.
The relationship between body height and the optimal depth of insertion in adult patients has been studied previously. However, only two studies include more than 100 patients. Brodsky and colleagues  found a statistically significant linear relationship between body height and optimal depth in 101 adult patients with heights ranging from 136 to 194 cm. Only 12 (12%) of those patients were shorter than 155 cm. Dyer and colleagues  could also demonstrate a statistically significant relationship between body height and optimal depth of insertion in 140 adult patients (body height 144-186 cm). Their study only included 19 patients (14%) ≤ 155 cm. Thus, although a statistically significant linear correlation between body height and the optimal depth of insertion has been demonstrated previously in adult patients, this has not been adequately shown for patients of short stature. In our 196 consecutive patients with a body height ≤ 155 cm, there was a statistically significant positive linear correlation between body height and the optimal depth of insertion. However, the actual optimal depth of insertion ranged from 20 to 32 cm. In addition, the actual optimal depth of insertion of one patient was even 4.5 cm shorter than the value calculated from the obtained equation. These findings indicate that clinical application of the optimal depth of insertion of a left-sided double-lumen tube from linear regression analysis is not safe.
There are two methods of intubation with a double lumen tube using fibre-optic bronchosocpy. The first method involves inserting the tube until slight resistance is encountered, after which a bronchoscope is inserted into the tracheal lumen to guide the bronchial cuff to a position just distal to the carina. However, this method can cause bronchial rupture after blind insertion of the tube in patients with unexpectedly short tracheas [6,9,10]. Another method involves placing the tip of the tube into the trachea and then passing the bronchoscope through the bronchial lumen. The carina is detected and the bronchoscope advanced into the left main-stem bronchus. With the bronchoscope as a stylet, the tube is then advanced into the left main-stem bronchus, making sure that it does not occlude the upper lobe bronchus . Our results indicate that a double lumen tube should not be blindly inserted more than 20 cm in patients ≤ 155 cm in height and, thus, this latter technique of insertion is advocated. Using a bronchoscope after the tip of the tube passes the larynx to guide it to the optimal position is the safest method for avoiding complications related to insertion and it shortens the time taken to place the tube in optimal position .
We conclude that although there is a statistically significant positive correlation between body height and the optimal depth of insertion of a left-sided double-lumen tube in below average-sized adult patients, clinical application of the obtained equation cannot be recommended. These tubes should be inserted under direct vision using a fibre-optic bronchoscope.
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