The item discussed here is the Robertshaw 28Fr left-sided double lumen endotracheal tube. Dorsch and Dorsch describe this thing as a "lung isolation device". Of course, then Dorsch and Dorsch get into some indepth discussions of every type of rare and exotic dual lumen tube there is. There is the Carlens, the White, the Robertshaw, the Broncho-Cath, the Sher-I-Bronch, the Silbroncho, and so forth and so on. I question the relevance of these details in the ICU. We are infrequently exposed to a mind-paralyzing array of choice when it comes to dual lumen tubes.
Thus far, several aspects of the DLT have been interrogated in the CICM Part II exam. For example, Question 27.2 from the second paper of 2011 asked about the indications for its use.
Indications for DLT insertion:
Complications of DLT insertion:
Now, in some detail:
There are several species of these devices; the major distinction one needs to be aware of is the very important difference between a right-sided and a left-sided dual lumen tube.
The right and left sided DLTs are so named according to the bronchus they are supposed to enter. The right sided DLT, for instance, is designed to penetrate the right main bronchus. There is one major difference between the design of the right and left sided dual lumen tubes. This difference is derived from the anatomical difference between the right and the left bronchial trees. I will use this stolen Greys Anatomy picture to illustrate.
Viewed side by side, the difference is obvious.
This website by Dr Tilakaratna is the source for the above image. I had to steal it, because my department (understandably) did not let me unwrap and photograph expensive equipment. All credit is therefore due to Dr Tilakaratna, a tireless airway enthusiast.
Yes, these tubes are not sized according to their internal diameter, but rather according to the French catheter gauge (i.e. the diameter in mm. times 3), which corresponds to their external diameter. This is because there are actually two internal diameters, and the lumens are not exactly circular in cross-section, which means the mathematical diameter and the actual air flow rate though the tube are only vaguely related.
There is still some relationship between the external French Gauge and the internal diameter:
|French size||Internal diameter (mm)|
But... how meaningful is this size system?
In fact, even the reported diameter somewhat unreliable.
An excellent article on this matter is available as free full text. Among the many excellent points made, the authors lament the inconsistent and manufacturer-specific relationship between quoted tube size, actual measured tube size, and the bronchial diameter of the patient whom it is used for. A comment is made regarding the tight tolerances of normal endotracheal tubes which comply with ISO standards, and surprise is expressed regarding the lack of such tight standards in the manufacture of tracheobronchial tubes.
That's right, the French gauge of the DLT refers to the almost completely useless measurement of the external tube diameter. Of course, the usefullness of the tube, its entire purpose is related to the external diameter of the bronchial segment. This measurement was often not reported by the manufacturer; however now that the ISO has made it mandatory it should appear on the packaging.
However to know this diameter is vitally important, because the endobronchial segment diameter needs to closely match the diameter of the bronchus in order to actually occlude it.
One can crudely estimate the size required by simply stating that boys need bigger tubes.
Thus, one can make the distinction that the girls get sizes 28-37, and boys get sizes 37-41.
However, this is somewhat unscientific.
One can get all clever about it, and estimate the tube size according to the patient's height, expecting that the length and diameter of the trachea is loosely related to this value. However, it seems that the diameter of the left main bronchus is only related to height among males and that the height-bronchus relationship breaks down among women. It gets even weirder. Among males the left main bronchus seems to dilate with age, increasing in diameter by about 1mm every 25 years.
One can get even more clever and use chest Xrays to directly measure the diameter of the left main bronchus off the film. The magnification actor one applies is about 10%. Then, one would need to go and measure the endobronchial segment of the DLT, because the diameter is not recorded anywhere.
In any case, the selected DLT diameter should be about 1-2mm smaller than that of the calculated left mainstem bronchus diameter.
Like every other sort of endotracheal tube, the DLT has markers of depth along its side, which give one some sort of an impression of how deep one has shoved this thing. It also allows you to make a record of tube position, so one can watch for its migration over time.
To the question, "how deep do you shove it", one can respond with the sensible statement that insertion depth is determined by appropriate bronchial cuff position, which is confirmed by bronchoscopy. However, there may be some relationship between patient size and correct tube placement depth. For instance, one study among 130 patients found that 170cm humans typically required an insertion depth of 29cm, and that for every 10cm of height difference the tube depth varied by 1cm. However, the relationship was not strong enough. The variation was so wide that the authors could not recommend this as a means of confirming appropriate tube position.
The endobronchial lumen is traditionally blue, and all of its connectors and extensions are by convention blue as well. This is a safety feature; the blue cuff also has a blue pilot balloon, with a blue valve, so it is obvious which is the endobronchial and which is the endotracheal ventilator circuits. The blueness extends to the ventilator connections, so that you know which circuit to clamp.
Why did they settle on blue? It may be easier to see in the bronchus, compared to other colours. The reason for the vivid blue is the ease with which the balloon is visualised on direct bronchoscopy.
The blue balloon is significantly smaller than the normal tracheal balloon. Typically, no more than 3ml of air should be required to inflate it fully.
This is a "double adaptor" which connects the two lumens to the same ventilator. Of course, it does not cater for the slightly insane practice of ventilating each lung independently.
The connector has two "soft" segments, which are made of silicone, softer and springier than the PVC. These are for your clamp. By clamping one of these segments, one is able to accomplish lung isolation.
The silicone is soft to ensure that it can be clamped safely without fracturing. A normal ETT is made of PVC, if clamped and re-clamped frequently, may develop little cracks.
There is, at the back of both airway connectors, a small port with a one-way valve. This port is designed as the point of entry for the bronchoscope. The valve permits bronchoscopy with minimal loss of pressure (and, with minimal anaesthetic gas blasting into the face of the anaesthetist who does the bronchoscopy).
The function of the DLT is childishly simple. Having each lung connect to its own tube gives one the power to only ventilate one lung at a time, allowing the other lung to deflate (as in thoracic surgery) or fill up with blood, as the case may be.
This avoids the open pneumothorax problem of thoracic surgery, where the operator is constantly frustrated either by the inflated lung of positive pressure ventilation, or by the creepy asymmetry of "pendelluft" ventilation (where the collapsed lung expands in expiration and contracts in inspiration, in a spontaneously breathing patient). Brodsky and Lemmens give a detailed account of what thoracic anaesthesia was like before the first Carlens tubes.
At first, Carlens had intended his dual-lumen tube to be used for bronchospirometery, that is the measurement of the respiratory function of each lung independently. Its adaptation as a conduit for anaesthetic gases and lung isolation device came soon thereafter.
The left main bronchus is about 50-60mm long, whereas the right main bronchus is only about 20-25mm long. The left sided tubes have a much greater margin of safety, as they are much easier to insert. Thus, it is generally said that the only indication for the insertion of a right-sided tube is an operation which is performed on the left main bronchus. In every other case, a left sided tube is preferred.
Let us not get bogged down with contraindications to intubation in general.
Let us focus the contraindications to intubating with a dual-lumen tube.
Again, intubation in general and the various methods of DLT intubation have been covered extensively.
I will not digress into debating the merits of the different methods, except in brief.
Once this complex dance is complete, one might be tempted to ask, "am I actually isolating the lung? Is there a lung-to-lung leak?"
This question can be answered by the measurement of expired CO2 in the clamped tracheal lumen.
Observe: if the lung were truly isolated, then surely it would receive none of the contralateral lung's CO2, and thus have no waveform. Of course, in practice, a small leak will probably not be detected by this method. But one would be much less concerned about a small leak. The core goal would still be achieved - huge gobs of infected mucus or bloody clot will not be rolling constantly into the healthy lung.
Yes, the DLT shares all of the safety features of the single lumen ETT, which are as follows:
However, the dual lumen tube has a few added extras:
These complications are unique to the dual lumen tube:
These complications are common to both the dual lumen tube and the normal ETT: