Since its invention, the Laryngeal Mask Airway, or LMA, has become quite valuable as a surgical airway alternative to intubation. When I first started in anesthesia, the only way to avoid intubation during surgery was to manually assist ventilation with a bag-valve-mask attachment. Cases that went on for hours often resulted in cramped fingers, and sometimes progressively poorer ventilation over time as the hand holding the mask became overly tired. A poor mask seal could potentially cause the stomach to distend with air, pushing up the diaphragms, limiting tidal volume, and increasing the risk of aspiration. The LMA has changed anesthesia so much that residents now find it challenging to find cases to practice their masking skills.

However, the LMA is so commonly used, and so apparently safe, that it’s easy to become complacent. Research is showing that it’s apparently very common for us to over-inflate our LMA cuffs — to the potential harm of our patients.

What’s The Problem with Overinflation?

Part of the problem is that many people picture the LMA as “floating” in the posterior pharynx, overlying the larynx and allowing ventilation. They don’t picture the LMA as the potentially tight fitting “cork” that it is.

Once an LMA is positioned in the pharynx, and the cuff is inflated, the sides of the cuff seal against the pyriform sinuses. The upper cuff surface sits behind and against the base of the tongue. The epiglottis is inside pointing upwards. The tip of the cuff sits against the upper esophageal sphincter, blocking it — preventing air from entering the stomach and keeping gastric contents from spilling into the hypopharynx. The resulting peri-laryngeal seal allows effective positive pressure ventilation.

There’s a lot of surface contact with the inflated cuff and the mucosa. An effective seal depends on the cuff pressing firmly against all those surfaces. It’s really no different than having to press firmly to get a routine mask seal around the patient’s nose and mouth.

The oropharyngeal mucosa is fairly thin and easily damaged. Overinflation of LMA cuffs with pressures greater than 60 cm H2O (40 mmHg)is associated with a much higher incidence of sore throat. In fact, there are studies concluding that we should be keeping the cuff pressure less than 60 cm H2O to avoid this complication.

Randomized control trials have shown current sore throat rates around 40-50%. There are case reports of dysphagia, and nerve injury. I personally have seen a markedly swollen uvula after it was pinched against the wall of the posterior pharynx by an LMA for a few hours.

Even with ideal positioning, with the epiglottis upright within the rim of the device, the ability to seal the airway often worsens when a cuff becomes overinflated. Presumably this results from the distended and stiff cuff lifting up and away, breaking the seal rather than molding with the surrounding tissues. If positioning is not ideal, as is commonly the case, then more point pressure against structures is possible.

Why Are LMAs Routinely Overinflated?

Alternate Insertion Technique Can Lead To Malpositioning

When Dr. Archie Brain invented the LMA, he recommended that the device be inserted into the posterior pharynx by pressing the back of the deflated and flattened mask firmly against the palate — and then sliding it backward and downward until the tip seated itself against the upper esophageal sphincter. At that point he inflated the cuff until it just sealed, often with as little as 7 ml of air.

However insertion has “evolved”. Many providers use alternate techniques instead of Dr. Brain’s technique. Some insert the LMA upside down and rotate it into position. Some slide it in sideways around the tongue. Many insert the LMA with the cuff already inflated. See an earlier discussion of LMA insertion techniques here.

The potential problem with the alternate techniques is that the final position of the LMA can be less optimal. Studies have shown that the epiglottis is folded downward 50-90% of the time and the lateral aryepiglottic folds are folded inward 50% of the time.

Although the actual position of the LMA may vary from ideal, ventilation is rarely impaired and is judged without difficulty in 95-99% in most studies. This can lull us into a false sense of security. If the cuff is overinflated, this suboptimal positioning can cause problems, and as you might imagine, excessive point pressure.

Intructions On Inflation Have Been Misinterpreted

The instruction manual has always stated maximum volumes for each cuff size:

• 20 ml for size 3
  30 ml for size 4
  40 ml for size 5

While the instructions clearly state not to use pressures higher than 60 cm H2O, we typically haven’t had a manometer present to measure the pressure. We’ve had to go by “feel”.

Through no fault of the company, it’s been easy for providers to draw the erroneous conclusion, myself included, that as long as one kept the volumes below those maximums, that the pressures would stay below 60 cm H2O. This assumption is NOT true. These maximum volumes relate to volumes that avoid damaging the cuff, not damaging the patient.

Even putting as little as 20 ml in a size 5 can raise pressures above 60 cm H2O (40 mmHg) in some patients. Some studies have recorded pressures as high as 200 mmHg by fully inflating a cuff to the maximum allowable volume.

Inserting With The Cuff Already Inflated Sets Us Up To Overpressurize

Many providers prefer to insert the LMA with the cuff fully inflated. The fact that LMAs come packaged for delivery with the cuff inflated can encourage this alternate technique.

Now, it’s completely logical to ship the LMAs with the cuff inflated. If the cuff were shipped deflated and flat, then any “ironed in” folds in the cuff might distort the cuff when it’s finally inflated. Folds might produce a seam that could cause point pressure. Folds maintained for a prolonged period might weaken the cuff wall. Seeing an inflated cuff also allows the provider to detect if the cuff is damaged and leaking.

However, again through no fault of the company, a provider without reading the instructions could erroneously assume that it arrives ready to insert: that one should, in other words, leave the air in during insertion.

Inserting the LMA already inflated can prevent the LMA from being inserted as deeply into the posterior pharynx as it needs to go in order to block the upper esophageal sphincter and seal the peri-laryngeal outlet. If the LMA seats too high, then the only way to obtain a good seal is to further inflate the cuff, potentially leading to over-pressure, and perhaps an even worse seal.

I see this all the time with trainees who insert an already inflated cuff and then automatically add air without even checking for a seal. Rather than being soft and compliant, the pilot balloon is then often tense and rigid like squeezing an olive.

In addition, if the tube keeps popping out of the mouth by a significant amount, most likely the tip of the tube is not resting low, behind the tongue. It’s most likely sitting high, partially on top of the back of the tongue. If it’s higher in the posterior pharynx you may not be able to obtain a seal and ventilate — and even if you can — the LMA will often slide out of position when you are least expecting it.

The best technique is to insert the LMA deflated, or only partially inflated, and then check the seal to see what if any air needs to be added.

Avoiding Overinflation

Just like with an endotracheal tube, go for minimum seal when inflating the cuff. If you insert the LMA with the cuff inflated, this may mean you actually have to remove air and then slowing reinflate.

How do you recognize minimum seal? Add additional air to the cuff using a syringe as needed to ensure a seal during positive pressure ventilation to a peak inflating pressure (peak ventilating pressure) of about 20-25 cmH2O.

The pilot balloon should be soft and flexible, not stiff and tense. If it’s stiff, take air out of the cuff.

If you have a manometer, use it. Studies have shown that you can initially use a manometer to train yourself to estimate the correct amount of pressure.

New LMA Product Line Has Built In Manometer

Recognizing the risks of overinflation, the vendor Teleflex has brought out a new line of LMA products with built in manometers located on the pilot tube assembly. The manometer is color coded.

• Yellow: less than 40 cm H2O
• Green: 40-60 cm H2O
• Clear: heading into the over pressure risk zone
• Red: greater than 60 cm H2O

This device provides easily seen, instantaneous visual feedback on cuff pressures. Accuracy of the color coding in clinical practice has been verified. Monitoring continuous pressure is also helpful over time if nitrous oxide is used as nitrous will diffuse into the cuff and raise the pressure further as the case progresses.

I got the opportunity to trial the device and I was startled at how much less air was often needed to obtain a good seal. I must admit, I appear to have been a (now reformed) over inflator.

Take Home Message

Just as we pay attention to endotracheal tube cuff pressures, we need to start paying attention to cuff pressures for laryngeal mask airways. Keep those pressures less than 40 cm H2O. The safety and postoperative comfort of our patients requires it.

May The Force Be With You

Christine Whitten MD, author: Anyone Can Intubate 5th Edition

 

Suggested Reading

1. Brimacombe, J: Insertion of the laryngeal mask airway – a prospective study of four techniques. Anaesth. Intensi. Care 1993; 21(1):89-92.
2. Editorial: Fewer Sore Throats and a Better Seal: why Routine Manometry for LMAs Must Become Standard Of Care. Anaesthesia 2014, 69: 1299-1313
3. Martin DP, et al: Clinical evaluation of a Novel LMA with a color-coded pressure gauge. Int J Pediatr Otorhinolaryngol. 2013 Jan;77(1):76-8. doi: 10.1016/j.ijporl.2012.09.036. Epub 2012 Oct 23.
4. Seet, E, et.al: Use of Manometry for Laryngeal Mask Airway Reduces Postoperative Adverse Events: A Propsective Randomized Trial. Anes 2010; 112: 652-7
5. Wong JG, et.al: Impact Of Laryngeal Mask Airway Cuff Pressures On incidence Of Sore throat In Children. Paediatr Anaesth 2009 May; 19 (5): 464-9