Does the Presence of a Tracheal Bronchus Affect the Margin of Safety of Double-Lumen Tube Placement?

Abstract

During double-lumen tube (DLT) placement, the anesthesiologist must be mindful of the margin of safety. We determined how this margin is affected by the presence of a tracheal bronchus by elucidating the mathematical relationship between some relevant physical dimensions of the trachea, bronchi, and DLT. Our results suggest that a tracheal bronchus only rarely affects the intrinsic margin of safety of DLT placement. When the tracheal bronchus is located much higher than its most frequently seen location (within 2 cm from the carina), however, there is increased risk that it could be blocked by the tracheal cuff of a left-sided DLT.

In Brief

IMPLICATIONS: An orifice arising on the right tracheal wall to supply all or part of the right upper lobe occurs with a frequency of ≈0.5%. Because tracheal bronchi usually arise at within 2 cm of the carina, they should only rarely reduce the intrinsic margin of safety of left-sided double-lumen tubes.

An ectopic bronchus arising from the trachea to supply one or all of the segments of the right upper lobe (RUL) is an uncommon anomaly that has implications on lung isolation. Benumof et al.’s (¹) data of the margin of safety of double-lumen tube (DLT) placement have been a cornerstone of thoracic anesthesia practice. How these margins of safety are affected by the presence of a tracheal bronchus has never been examined.

Methods

We reviewed papers in English searched through MEDLINE between 1966 and November 2003 using the following keywords: tracheal bronchus, margin of safety, double-lumen tube. The objectives were to review the anatomic variations of the tracheal bronchus anomaly and to examine previous work on the margin of safety of DLT placement. We defined the margin of safety without a tracheal bronchus as determined by Benumof et al. (¹) as MS. We also defined the distance between the tracheal cuff and the tracheal bronchus when the DLT is at its most proximal allowable position as D (Fig. 1). If D <MS, then the margin of safety has been affected by the presence of the tracheal bronchus, and effectively becomes D. If not, then the margin of safety is still what Benumof et al. (¹) had determined; in other words, it has not been altered by the presence of the tracheal tube. Other variables were CB (the distance between the carina and the proximal edge of the tracheal bronchial orifice), LMS (the length of the left main bronchus), and TT (the distance between the tips of the two lumens of the DLT) (Fig. 1). Next we determined the relationship between the upper tracheobronchial tree in the presence of a tracheal bronchus and the established margins of safety of DLTs (Fig. 1). Finally, we took relevant measurements of adult-sized Mallinkrodt (Broncho-Cath, St. Louis, MO) and the Rüsch (Endobronchial tubes, AG; Kernen, Germany) DLTs.

Figure 1.: Left-sided double-lumen tube (DLT) in situ at its most proximal allowable position (¹). Any further withdrawal of the tube will result in the bronchial cuff progressively filling the space above the carina (¹). Normal margin of safety as defined by Benumof et al. (¹) (without tracheal bronchus) = MS. The distance between the DLT’s most proximal position and the point at which its tracheal cuff begins to occlude the tracheal bronchus = D. Distance between the carina and the proximal edge of the tracheal bronchus = CB. The further away from the carina the tracheal bronchus is, the smaller D becomes. If D < 0, left-sided DLT for bilateral or right-lung ventilation is contraindicated. If D < MS, the presence of the tracheal cuff has altered the intrinsic margin of safety of DLT placement in this patient. If D > MS, the margin of safety has not been altered. Drawing is not to scale; specifically, the clearance between the endobronchial tube exterior surface and the left main bronchus is small (¹).

As illustrated in Figure 1, D + CB + LMS = MS + TT + 14 mm. Therefore D = (MS + TT) + 14 mm − (LMS + CB). Note that to simplify calculations, we have used only the mean of the distance between the distal tip of the tracheal lumen and the distal bronchial cuff margin.

Results

le Roux (²) found 5 cases among 1000 (0.5%) right bronchograms in which the tracheal bronchus originated from the right lateral tracheal wall, all within 1 cm of the main carina. However, tracheal orifices much further from the carina have also been reported. Ikeno et al. (³) reported one adult case in which the tracheal bronchial orifice was 3.5 cm above the carina and a 2-year-old girl with a tracheal bronchus 1 cm above the carina. Fifteen pediatric tracheal bronchi were identified, some of which had relatively proximal tracheal bronchial orifice (⁴). Conacher (⁵) described a teenager whose tracheal bronchus was between T2 and T3. O’Sullivan et al. (⁶) reported a tracheal bronchus 1 cm above the carina in a 32-week-old infant. Venkateswarlu et al. (⁷) found a tracheal bronchus arising approximately 2.5 cm proximal to the carina in a 2-year-old girl. Siegel et al. (⁸) reported a tracheal bronchus 6 cm above the carina (at the level of the second thoracic vertebra) supplying the posterior segment of the RUL in a 6-year-old child. In summary, most tracheal bronchi are within 2 cm of the carina but, but some are much more proximal. Apparently, no location other than at 3 o’clock in the bronchoscopic view (Fig. 1) exists.

The average lengths (ranges) of MS, TT, and LMS in adults are 17.5 mm (16–19 mm), 72 mm (70–74 mm), and 48 mm (29–63 mm), respectively (¹). The distance between the distal tip of the tracheal opening and the distal margin of the tracheal cuff of the Mallinkrodt and Rüsch left-sided DLTs for sizes 32F, 35F, 37F, and 39F is 14 ± 2 mm. As illustrated in Figure 1, D = (MS + TT) + 14 mm −(LMS + CB). The shortest D (worst-case scenario) is when (MS + TT) is the smallest and (LMS + CB) is the largest. By referring to the ranges of MS, TT, and LMS (see above), one can calculate that D (shortest) is equaled to (37 −CB) mm. In the worst-case scenario, MS is 16 mm. CB must be >21 cm for D to be smaller than the corresponding MS of 16 mm. In other words, the margin of safety of a left-sided DLT will only be reduced if CB is >21 cm, and simultaneously, TT is the shortest (70 mm) and LMS is the longest (63 mm). Otherwise, the margin of safety of a left-sided DLT will be MS, as determined by Benumof et al. (¹) and unaffected by the tracheal bronchus. Table 1 summarizes the values of CB above which the MS of a left-sided DLT becomes less than D.

Table 1: Summary of Values of CB

Discussion

The presence of a tracheal bronchus should only rarely affect the intrinsic margin of safety of a left-sided DLT because most tracheal bronchi are within 2 cm of the carina. It takes a combination of a tracheal bronchus more than 2 cm above the carina and the worst-case scenario to reduce the margin of safety to below that determined by Benumof et al. (¹). Nevertheless, as tracheal bronchi do uncommonly arise at more proximal levels, the possibility exists that the margin of safety of a left-sided DLT could be significantly reduced. Naturally, this would only be a problem when a left-sided DLT is used for right-lung or bilateral ventilation (when the orifice of the tracheal bronchus could be blocked by the tracheal cuff).

In the event that nonventilation of the right lung using a bronchial blocker is desired, an extra blocker would be required to block the tracheal bronchus (⁹), unless the tracheal bronchus happens, in those rare situations as mentioned above, to originate at a proximal enough location that it can be blocked by the cuff of a tracheal tube (¹⁰).

A tracheal bronchus is detectable on computer tomography scan and, occasionally, on plain chest radiograph. Identification before surgery is useful, as its presence precludes the choice of a right-sided DLT, and may, on rare occasions, make left-sided DLT placement problematic. Careful chest auscultation and fiberoptic bronchoscopic verification, therefore, remain necessary steps after every DLT placement.