The endotracheal tube in detail

The item discussed here is the Mallinckrodt size 8.5 endotracheal tube with an above-the-cuff suction port.  There being millions of different types, I thought it would be better to just pick a representative style, and to discuss it. The suction port is a bit of a gimmick, and many places don't use this style of tube. Of course, the "representative style" available to me was the one which was already unwrapped, in the ICU nurse educator's office.  There are several structural features of note, which each deserve some brief mention. These features are common to the vast majority of ETTs.

Anatomy of the item

Transparent PVC tubing

Overall, in its simplest form the ETT is a length of tubing about 33cm long. It is made of transparent polyvinyl chloride, which is a considerable improvement on the metal and rubber ETTs of the earlier decades, even though in comparison to them it is rather flammable. Previously to the modern infatuation with plastic, the early tubes were made from commercial rubber hose, in varying diameters from 3/8 to 3/16th of an inch in diameter. In 1946 an alarmed Dr J.U. Human wrote to complain that the ordinary rubber ETTs were too hard, and caused bleeding when used nasally, a complication which he felt might "bring the practice of endotracheal intubation into disrepute". He had to actually soften his rubber tubes in kerosene before he was able to use them safely.

Anatomical curvature

The curvature of the endotracheal tube conforms to the shape of the airway with the head held in the neutral position, and is easier to insert then if the tube was straight. This development, largely, is an accident. The man responsible for the modern ETT was Magill, and his assistant cut the first tubes he used for research from a roll of rubber tubing; his tubing thus inherited the memory of the cylindrical roll in which it was stored. Magill relied on this natural curvature to avoid the use of metal stylet. To this day, the curvature of the ETT is called "the Magill Curve". According to the standard, it is a curvature of approximately 140mm radius, +/- 20mm.

Standard 15mm airway connector

Yes, there is a certain body which determines the size of connectors for airway equipment. The connectors are 15 and 22mm (internal diameters), conforming to ISO5356-1. The reason for this standard is self-evident: all airway equipment should be connectable to all other airway equipment, so that there should never be a nightmarish situation where one without ventilation, while the anaesthetist is scrambling desperately to find the right adaptor.

Air-filled cuff

This is the key feature of the ETT for adults (of course, the pediatric tubes being uncuffed). The cuff, once filled with air, seals the lungs against the liquid secretions sloshing around in the upper airway. And, it ensures that the environment below the cuff can be pressurised and ventilated with a carefully controlled gas mixture.

The first few ETTs did not have cuffs; Magill used two green surgical swabs with ribbon gauze attached, which were packed around the ETT and removed manually after extubation.

Why do I need a cuffed tube

Two main reasons:

- To seal the trachea, so that positive pressure cannot escape from the lower airway

- To seal the upper airway, so that material above the glottis cannot enter the trachea

Positive pressure seal

In the pediatric population the cricoid ring is sufficiently narrow to form a seal all by itself, and thus the tubes can be uncuffed for this population. Of course the seal is imperfect (as the cricoid ring is ellipsoid rather than circular) and there is some argument that cuffed tubes should be more widely available for children - but this is a digression. In the adults, the trachea is considerably wider than that. The ETT inflates to 30mm width to occupy this massive wind tunnel, and it should require about 10ml of air.

Protection from secretions

That upper airway is a pig sty, to be sure. Filth would be forever sliding down into the trachea if it wasn't for the upper airway reflexes. The human mouth is a rancid swamp, particularly after a few days in the ICU. And lets not forget feculent vomit, candida plaques, blood, et etc. Lots of good reasons to protect the trachea from this sort of material. But: does the cuff actually do this? Good question. The change to low-pressure high-volume cuffs has resulted in a cuff which is incompletely expanded, forming folds against the walls of the trachea. These folds are perfect entry points for supraglottic slime. Different cuff materials and different cuff shapes (eg. cylindrical, conical) have a different rate of fluid leakage, at least in the lab. And not only in the lab is this the case. Good bedside evidence exists that microaspiration of upper airway gunk continues in spite of appropriate cuff pressure.

What is an appropriate cuff pressure

Good question. A good BMJ article form 1984 reports direct observations of the tracheal mucosa with different pressure levels; I have represented their findings in another uselessly uninformative graph.

These were human subjects. As the cuff pressure goes up to 30cmH₂O, the mucosa becomes somewhat blanched, suggesting that the capillary perfusion is impaired. As the cuff pressure increases, so the capillary perfusion decreases. At a pressure of 40cmH₂O even mucosal arterioles are no longer visible.

In short, mucosal necrosis risk increases with increasing ETT cuff pressure.

How do we check the cuff pressure

With a manual manometer.

You can both measure AND adjust the cuff pressure.

Yes, the one in the picture has a huge hole smashed through the dial.

My cuff pressure is zero. What has happened?

Most common answers to this question include:

• The cuff has ruptured, or the pilot balloon tube has been chewed through; either way cuff pressure is lost.
  Now the ventilator circuit is loudly alarming, reporting a leak.
• The balloon inflation lumen is occluded. Ventilation parameters are eerily stable in spite of a "zero" reading.

My cuff pressure is too high. What will happen?

A few things could happen:

• Acute tracheal rupture.
• Tracheal mucosal necrosis, which leads to a tracheoesophageal fistula if you re unlucky

And good evidence exists that overinflating the balloon to 50cmH₂O still doesn't prevent microaspiration.

So, when one says "overfilled" and "overpressurised" this implies some sort of gratuitous excess. Has anyone ever wondered how much air you could inject into an ETT cuff before it explodes?

Well. 150ml, that's how much.

After the normal 10ml the balloon only has a pressure of 4cmH₂O. But - accidentally inject 20ml of air, and the manometer needle actually maxes out. That is above 100cmH₂O. With each subsequent air injection the pressure cannot be measured. The balloon continues to expand. It grows to about twice the normal diameter. Then, it pops with a surprisingly loud high-pitched report, which may cause some of the more faint-hearted ICU staff to come over and start wondering what you are doing.

Pilot balloon

This little sack of air is your guide to the integrity of your cuff. If it feels flaccid, chances are the ETT is flaccid too. It is connected to the cuff with a narrow lumen, which is not very strong, and can be easily bitten through.

Additionally, it has a spring-powered one way valve which can be either factory-faulty or can break in the process of frequent use (eg. frequent obsessive cuff pressure measurements). Finally, even the best of us (and by this I mean a series of experienced emergency physicians) are completely useless at deriving any useful information about cuff pressure from the palpation of pilot balloons.

That is not to say that its palpation is totally without merit. An article from 1995 reports a technique of "balloting the cuff", where gentle pressure on the trachea just above the suprasternal notch was transmitted via the ETT cuff to the pilot balloon. Thus, the operator was able to establish correct ETT tip position by confirming that the cuff was at the level of the clavicular heads.

An additional safety feature is the size label on the pilot balloon.

Subglottic " Above the Cuff" Suction Port

This added feature enables the aspiration of secretions which collect above the tube cuff. Those secretions which pool there are a gross infected mixture of saliva and nasal mucus, stewing in the steamy environment of the plugged larynx. Surely, it must be somehow beneficial to aspirate that stuff, so it doesn't slide down into the lungs?

Well, perhaps it is. According to a review of the available evidence in Revista Brasileira de Terapia Intensiva one should not expect very much improvement in mortality or duration of ventilation. The risk of VAP is somewhat reduced, but - to what extent? if it does little to speed up the ventilation wean, its real role is in reducing the healthcare costs associated with VAP, because the tube is cheaper than a course of Tazocin.

The disadvantage of suctioning above the cuff is mucosal damage. The sucker applies 100mmHg pressure to the tracheal wall. This sort of pressure- though considered "low wall suction" - can still strip mucosa off the walls of the trachea. This, it seems, is predominantly a risk associated with continuous rather than intermittent subglottic suction.

Suggested vocal cords marker

Helpfully, the ETT has a marker on its left side which you can still see once the cuff balloon disappears beyond the cords. The marker is placed at a point in the ETT which conforms to some sort of standard idea of the depth from the vocal cords to the trachea. Obviously the human trachea is not manufactured to the same precise specifications. And - this marker seems to be placed on the tube at a totally arbitrary position. There is no standard, and all the manufacturers seem to disagree among themselves as to what kind of marker to put, how many of them, and precisely where to put them.

These issues lead to a disagreement between the vocal cord placement marker and the intensivist.

The intensivist tends to win.

Internal diameter

The internal diameter determines the maximum rate of gas flow for any given pressure gradient, according to Pouiseuille's law. (Lets just forget that air moving through the tube is not an ideal incompressible liquid in a laminar flow pattern). This is usually not encountered as a problem.

The rare situation in which one might discover the importance of this relationship is a nightmarish cant intubate- cant ventilate scenario, where in desperation you shove a 14g cannula through your patients cricothyroid membrane. The flow rate in expiration will be remarkably low, because it the low pressure gradient cannot drive the gas sufficiently fast; in this sense the big cannula can provide oxygenation, but not ventilation.

The reason for selecting a sufficiently large tube is more to do with external diameter, which naturally increases. A large man has a large trachea, and to intubate him with a little girly tube will lead to a lot of leak around the cuff. And then you overinflate the cuff to compensate, exposing parts of his trachea to increased pressure. So: it is important to ensure that the tube matches the trachea. Typically, an 8.0 or 8.5 for adult men and 7.5 to 8.0 for adult women is an ideal choice.

Depth markers

These are essentially for record-keeping rather than for determination of position. However, if radiographic confirmation of position is not available, one may rely on depth markers to decide whether one has pushed the tube in too far. A small scale trial has suggested that 20-21cm for women and 21-22 for men is the ideal position with the neck in neutral position.

The correct place to measure the depth is for some reason held to be the corner of the mouth. However, if you think about it, this is a stupid place to put an ETT. Your ability to bite though something with your molars is much greater; the tube placed in the corners of the mouth will gnawed on and may occlude much more easily. In contrast, it is very hard to bite down hard enough to block the ETT using only your incisors.

Murphy's Eye

The Wood Library Museum of Anaesthesiology presents some nice old endotracheal tubes, as well as interesting tidbits from the history of anaesthetic equipment. Here, I discovered that the Murphy's Eye is named thus because there was indeed a man named Francis J. Murphy, who designed it that way. Not only that, but he was apparently a strong proponent of continuous oxygen supply during anaesthesia, which I suppose implies that may others during this time (first half of 20th century) were strongly opposed to intraoperative oxygen.

The whole point of Murphy's eye is to act as an additional air vent. Basically, even if the tip of the ETT is blocked with filthy secretions, there might still be some air entry through that eye.

Additionally, there is a (minor) side benefit.

As you intubate somebody, holding the bevel in an appropriate position, Murphy's eye looks at the right wall of the trachea. It is precisely here that, in about 0.5% of the population, the right upper lobe bronchus takes its origin anomalously from the trachea. It is a widely held belief that Murphy's eye protects these people from right upper lobe collapse, by allowing its ventilation.

Left-facing bevel tip

Many authors would have you believe that this is a means of ensuring that the tip of the tube is never occluded by coming in direct apposition with the tracheal wall. But even the bevel tip can be occluded by the tracheal wall; its only a matter of positioning it in just the right way. In actual fact, the left-facing bevel offers a certain sort of profile to the eye looking down the larynx. This profile allows the greatest amount of cord visibility. A perfectly cylindrical ETT would block most of the vocal cords at it goes though them, preventing the operator from seeing whether it has entered them correctly.

Interestingly, early versions of the endotracheal tube did not have bevels, mainly because they were inserted blind, into an awake patient, using instruments like this. As the history of laryngoscopy progressed though weird mirror-based periscopes and cruel-looking head suspension gallows, we eventually arrived at Janeway's first proper speculum for direct laryngoscopy, and Magill's first endotracheal tubes. Magill made his ETTs from commercial-grade rubber tubing, which he cut by hand; he recommended these tubes be cut obliquely, but probably because this made nasal insertion easier (and not because it was easier to see the cords). When the tubes began to be produced en masse, the bevel remained. Beautiful pictures of these early tubes can be found here, at the Virtual Museum of Equipment for Airway Management.

In short, the left-facing bevel improves the visibility of the cords, and has a convenient side benefit of being slightly harder to block with phlegm.

Radio-opaque blue line

This is just a convenient way of identifying the tube tip position on Xray. The radio-opaque blue line is usually just made of slightly "impure" PVC and is co-extruded along with the rest of the PVC tubing. Its a bit of a secret as to what the impurities are, but one can generally assume that it is nothing toxic. It has been pointed out to me that apart from acting as a convenient depth indicator this line also follows the curvature of the tube, which becomes important in the uncuffed world of paediatric airway management. Conceivably, one could end up with some sort of weirdly malrotated tube, positioned contrary to the normal curvature of the airway, rubbing against delicate tracheal tissues and causing mucosal damage. The adult cuff keeps this from happening by helping center the ETT in the trachea; moreover the ratio of tube wall thickness to diameter allows the PVC of the larger adult ETT to soften significantly during prolonged use, conforming to anatomy. 

Smooth atraumatic edge

The first endotracheal tubes (Bouchut's tubes, for instance) were straight, made of metal, and probably very uncomfortable. The consequence of wearing such a tube in one's trachea was frequently an ulcerated trachea. The next incarnation of ETTs produced by Joseph P O'Dwyer in 1885 was made of rubber, and this time much better tolerated (with rounded edges).

Details of function

The endotracheal tube allows endotracheal intubation and mechanical ventilation, as well as tracheal toilet.

Indications for intubation

There are some separate indications for intubation and ventilation, even though we usually see the two together. We can separate them to some extent. One might say that the need for mechanical ventilation is one of the indications for intubation.

Indications for intubation:

• To overcome an airway obstruction and to protect the airway
• To allow access to the lower airway for suctioning of secretions
• To allow mechanical ventilation in a patient in whom non-invasive ventilation is contraindicated.

Indications for mechanical ventilation:

• To manipulate PaO₂ and PaCO₂
• To decrease the work of breathing (whether to reduce respiratory distress or to decrease total body oxygen demand)
• To increase the functional residual capacity (FRC)
• To stabilize the chest wall in serious chest injuries

Contraindications for intubation

Actually there are few real hard contraindications to intubation.

• Absence of upper airway (eg. radical laryngectomy)
• Laryngeal trauma which would be exacerbated by ETT insertion (eg. fractured larynx)
• Transection of the airway which could be exacerbated by ETT insertion

Relative contraindications are many, but can be summarised as all situations where the intubation will be difficult (for whatever reason) and you don't feel you could accomplish it. In those cases, its just not a good idea to proceed with intubation. Not only will you botch your attempt, you could do some real damage, and to make the process more difficult for the next person.

Methods of use

Much is already written about intubation. Let us not add to the cacophony.

Rather, let us focus on the specifics of ETT positioning.

Appropriate ETT tip position

The appropriateness of ETT position is really a factor of its security.

The ETT needs to do two things. It needs to stay with the cuff below the cords, and it needs to not block any of the main bronchi. Provided both of those conditions are satisfied, everybody is happy.

Thus, there is some length of the trachea in which the ETT needs to sit, in order to satisfy these conditions.

We know that as the patient flexes and extends their neck, the endotracheal tube moves up and down by about 2 cm in either direction.

Thus, it is recommended that the endotracheal tube tip be fixed at 5cm above the carina, with the head in a neutral position. That way, it can move 2cm up and 2cm down, without the risk of exiting the cords or going down the right main bronchus.

Of course, this precise "5cm rule" relies on a person knowing precisely where the carina is. This, in the setting of poor quality ICU bedside films, is totally unrealistic. Half the time we don't know where the hell the carina is.

So: the abovelinked study has established radiographically that the ETT tip should be around the T3-T4 level, going by fixed bony landmarks. This corresponds to a carina position at around T5-6.

You generally never get the mandible in these mobile ICU shots, but if you did, you would know that a "neutral" head position corresponds to a mandible at the level of C5-C6. This level is also where the vocal cords are located .

Safety features

In summary:

• Single use item, no risk of cross-infection
• Standardised 15mm connector to fit all airway devices
• Low-allergen PVC construction, free of latex
• Transparent body,to see blood or vomit
• Markings to indicate depth of insertion
• Black line to guide insertion to appropriate depth
• High volume low pressure cuff to seal the trachea
• Size labelling on pilot balloon
• Pilot cuff to gauge cuff pressure
• Rounded atraumatic edges
• Murphy's eye to protect against occlusion
• Bevelled tip to assist insertion
• Radio-opaque line to help gauge position on chest X-rays

Complications of intubation

• Failure of intubation
• Oesophageal intubation
• Obstruction of the tube (be it kinked by teeth or clogged with phlegm)
• Dislodgement above the glottis (tube falling out)
• Endobronchial intubation (tube falling in)
• Cuff rupture, pressure loss
• Trauma due to intubation (eg. tracheobronchial injury, even perforation)
• Mucosal ulceration and necrosis from prolonged intubation