SGA placement

When supraglottic airways were first introduced into practice, the clinical scenarios in which they were used were more narrowly and clearly defined than is the case today. A 'typical' case for using a first generation SGA in the late 1990s could have been in a patient with normal BMI, undergoing a fairly short, peripheral/ limb surgerical procedure, breathing without ventilator assistance throughout the case.

Due to the fact that practitioners have become increasingly familiar and comfortable with using SGAs over the last 20 or so years, and the availability of second generation devices which reduce the aspiration risk and provide a better cuff seal, the boundaries of what is considered acceptable use have and continue to be pushed. These days, more than in the past, it can be difficult, particularly for the anesthesia novice, to get a clear understanding of what patients and which surgical procedures are appropriate for using an SGA as opposed to an ETT.

 

Areas of clinical practice in which the indications for using (second generation) SGAs are expanding include the following:

SGA use in obese patients

SGA use in obese patients can present problems mainly in two areas: These patient are

a) at higher risk of regurgitation and

b) invariably have more impared respiratory mechanics under anesthesia with higher airway resistance and reduced chest/ lung compliance.

Consequently, ventilator support and airway pressure requirements are higher than in non-obese patients, encrouching on the performance limits of first-generation SGAs in particular. If the decision is made to use an SGA in an obese patient a second generation device is therefore preferable.

SGA use in patients with aspiration risk

SGAs, even of the second generation, are still considered contraindicated by the vast majority of practitioners in patients with a 'full stomach'. This cut-off becomes more blurred in patients who fulfill npo time requirements before surgery but can be assumed to have delayed gastric emptying, which is a feature of diabetic autonomic neuropathies, some neurodegenerative diseases such as multiple sclerosis, Parkinson's disease, etc.

It can be equally difficult to draw the line between SGA and intubation for patients with gastro-esophageal reflux disease (GERD), which is very common. Clearly there is a spectrum of disease. At the extremes some patients have to sleep sitting up and have daily, or rather nightly symptoms, despite diet modifications and medications. At the other end of the spectrum, patients with very mild disease can have only occasional symptoms which are only ever related to certain triggering ffods. Symptoms in these patients never occur if these foods are avoided. It will be down to clinical judgment whether reflux is severe enough to require intubation.

PERSONAL OPINION: SGA use is probably safe in patients at the 'mild disease' end of the spectrum for GERD. As always: If in doubt, intubate.

SGA use with IPPV, PEEP, and/ or muscle relaxants

Different levels of ventilator support can be used in conjunction with SGAs in the operating room. By increasing 'invasiveness' or level of support these are:

• Spontaneous respiration without ventilator support: The cuff seal pressure of the SGA is not really tested when the ventilator is not switched on. With this mode the patient breathes without ventilator support spontaneously 'on the bag' only. Using SGAs for general anesthesia this way was more common clinical practice before ventilators with CPAP/ pressure support modes were available. The disadvantages of this with regards to FRC, atelectasis, and shunt are described below.
• Continuous positive airway pressure (CPAP): Modern OR ventilators usually allow for delivery of CPAP to patients breathing spontaneously, either through a dedicated CPAP setting or by choosing PEEP with pressure support of zero in a pressure support mode. While not augmenting the patient's inspiratory effort, CPAP will still somewhat prevent reduction in functional residual capacity (FRC) and atelectasis compared to breathing completely unaided (breathing 'on the bag' only). With using CPAP in an anesthetized, spotaneously breathing patient, peak airway pressures equal the CPAP and are therefore low. Consequently the risk of gastric insufflation should also be very low.
• Pressure support (PS) ventilation: The same considerations as for IPPV (see below) apply. Both PS and IPPV can and should be used together with PEEP for the reasons outlined above.
• Intermittent positive pressure ventilation (IPPV): IPPV is necessary when muscle relaxants are used or the patient is apneic for other reasons during general anesthesia. Generally the use of SGAs in combination with IPPV should be limited to patients (and surgeries) where 'moderate' airway pressures are sufficient for adequate ventilation. Pressure control-based ventilation modes are considered better since unexpectedly high airway pressures will per definition be avoided (compared to volume control-based modes). For pressure control ventilation with moderate peak pressures up to 20 cmH2O there seems to be no difference with regards to leaks between an ET tube and the i-gel SGA (Uppal et al., 2009).

PERSONAL OPINION: While a first generation device is probably ok in many patients when CPAP or pressure support with low peak airway pressures is used, for more 'invasive' ventilation, i.e. pressure support and IPPV with peak pressures >15 -17 cmH2O, preferably use a second generation SGA.

SGA use for 'long' surgeries

There is no good published evidence available on surgical time limits for using an SGA as opposed to intubation. In the past, somewhat arbitrary time limits of 45 minutes up to 90 minutes or so were made for maximum duration of surgery where an SGA would be acceptable. This might have been due to the fact that back then most OR ventilators could not deliver CPAP/ pressure support. Patients would have to breathe spontaneously un-aided which often led to diminished tidal volumes and rising end-tidal CO2 with passing time under general anesthesia. Modern contemporary OR ventilators can prevent this so unless we assume that aspiration risk increases over time, there seems to be no logical reason why longer surgeries are not suitable for SGA use.

SGA use during pregnancy

Acid reflux/ GERD is more common in pregnancy, which is why these patients are at an increased risk from aspiration (see above) of gastric contents. The underlying mechanisms are a decreased lower esophageal sphincter tone and increased intra-abdominal pressure during pregnancy. These effects become gradually more marked as pregnancy progresses. Most providers would probably still not use an SGA beyond the 2nd trimester the latest and intubate instead. However, this is a topic of ongoing debate (Metodiev, 2020) and studies have even been published investigating the use of 2nd generation SGAs for cesarean section (Yao,2019)!

SGA use with unfavorable patient positions

SGAs are increasingly used for certain intra-operative patient positions which were previously considered a contraindication and mandated endotracheal intubation:

1. Patient position limits access to the airway: Limited acces to the airway can become a problem during surgery if some kind of troubleshoting of the SGA is required, such as repositioning, exchanging for a different type/ size SGA, or even converting to a tracheal tube. Clinical examples of limited airway access are the 'shared airway' during head & neck or maxillo-facial surgery, a patient lying on their side, semi-prone or prone, a patient on an OR table turned away from the anesthesia machine and provider, a patient in the MRI or CT scanner, a patient whose head is in a fixed position (stereotactic frame or scull clamps during neurosurgery, sitting/ beachchair position with head support), or a patient with a draped/ wrapped head or face.
2. Patient position carries higher risk of regurgiation/ aspiration: The commonly used Trendelenburg, i.e. patient head-down, and lithotomy positions significantly increase the risk of regurgitation.
3. Patient position likely to require increased levels of respiratory support, i.e higher airway pressures: These patient positions are the same as mentioned under '2.' above and affect respiratory mechanics, mostly through diaphragmatic splinting.

SGA use with laparoscopic surgery

Supraglottic airways have been studied for use in laparoscopic surgery, particularly in cholecystectomies, maybe because it is a very common and comparatively minor, fast laparoscopic procedure. In addition, a reverse Trendelenburg position is often employed for this type of operation which favors mechanical ventilation and generally reduces the regurgiation/ aspiration risk. There is a growing body of evidence that (second generation) SGAs are safe and maybe even preferable to ETTs for laparoscopic cholecystectomy (Park et al., 2016), however their use is not yet considered acceptable standard of care by a majority.

PERSONAL OPINION: With regards to SGA use in borderline suitable patients and surgeries it is helpful to remember that 'just because you can does not mean you should'. If in doubt, intubate!

When trying to decide whether to use an SGA rather than an ETT for a particular OR case, it might be helpful to think about SGA indications rather than contraindications! Ask yourself in what kind of surgeries and for what kind of patient you would use an SGA without hesitation and then decide if the patient in front of you for the planned operation fits that bill. With this approach you might find that your 'comfort bar' for using an SGA suddenly moved higher!

My personal (positive) indication checklist for using an SGA includes the following items:

• My patient does not have a significant aspiration risk
• My patient has a body mass that will allow me to use IPPV or PS if needed and achieve good tidal volumes without high airway pressures
• I can access the airway during surgery
• If I had to convert from SGA to ETT during surgery I could easily do so