Episode #77 Keeping Patients Safe During Anesthesia Care with an LMA

Hello and welcome back to the Anesthesia Patient Safety Podcast.  My name is Alli Bechtel and I am your host. Thank you for joining us for another show. Last week, we kicked off a two-part series about the laryngeal mask airway or LMA, but we have more to talk about today. Have you ever placed an LMA for a patient with obesity or during laparoscopic surgery or with the use of mechanical ventilation or muscle relaxants? Stay tuned as we talk about considerations for LMA use in clinical practice.

Before we dive into the episode today, we’d like to recognize Masimo, a major corporate supporter of APSF. Masimo has generously provided unrestricted support to further our vision that “no one shall be harmed by anesthesia care”. Thank you, Masimo – we wouldn’t be able to do all that we do without you!”

Our featured article today is once again, “The Laryngeal Mask Airway: Expanding Use Beyond Routine Spontaneous Ventilation for Surgery” by Schwartz and Peng. To follow along with us, head over to APSF.org and click on the Newsletter heading. First one down is the current October 2021 Issue. Then scroll down until you get to our featured article today. I will include a link in the show notes as well.

Last week, we reviewed the advancements and modifications of the LMA device over time as well as the absolute and relative contraindications. We also dove into the literature for considerations about LMA size, placement, and timing for removal. Let’s keep the conversation going.

Another important considerations is the risk of aspiration with LMA use. Have you seen this during anesthesia care with LMA use? Many of the common contraindications to LMA placement that we reviewed last week are patients who are at risk of aspiration including non-fasted patients or during pregnancy or emergency surgery following a trauma or with known bowel obstruction or gastroparesis. In patients who are appropriately NPO, the risk of aspiration during LMA use is quite low. Let’s look at the literature for this as well. Brimacombe and colleagues studied the risk of aspiration with LMA use compared to endotracheal tube and facemask use and found an incidence of 2 per 10,000 in the LMA cohort compared to 1.7 per 10,000 in the endotracheal tube and facemask cohort. Another study by Bernardini and Natalini evaluated the risk of aspiration in almost 66,000 surgical procedures including over 2,500 laparoscopic and major abdominal surgeries and reported no significant difference in the rate of aspiration with the use of the Classic LMA compared to an endotracheal tube while using positive pressure ventilation. We also have evidence from a meta-analysis by Park and colleagues that compared second-generation LMA use with endotracheal tube use in about 1,400 patients for laparoscopic surgery. There was no difference in oropharyngeal leak pressure, gastric insufflation or aspiration. An important finding of this study is that there was no difference in oropharyngeal leak pressure between the LMA and the endotracheal tube during laparoscopic surgery. Thus, the second generation LMAs may be able to provide some airway protection as well as adequate mechanical ventilation even with abdominal insufflation. We still need to be careful with the use of LMAs for patients undergoing laparoscopic procedures and consideration for second generation devices may be beneficial with the higher oropharyngeal seal pressure and the gastric suction port.

So, what are the advantages to using a second generation LMA with the gastric suction port? The evidence we have suggests that placement of an orogastric tube through this port can help to prevent aspiration. The authors discuss the large observational study by Yao and colleagues of 700 fasted patients who required general anesthesia for cesarean section with an LMA supreme. There were no aspiration events in the patients with an LMA supreme and an orogastric tube placed through the gastric port.

Another  concern for increased aspiration risk is the use of higher peak inspiratory pressures of greater than 15cm H2O which may lead to incompetence of the lower esophageal sphincter and gastric insufflation. There is an interesting study by Devitt and colleagues that evaluated leak fractions, defined as the inspiratory volume minus the expiratory volume and then divided by the inspiratory volume, and gastric insufflation with the use of classic LMAs compared to endotracheal tubes at different inspiratory pressures for ventilation. The investigators found increased leak fraction with increasing positive pressure ventilation through the LMA with a low leak fraction that did not change with endotracheal tube use. When the inspired pressure was 15 cm H20, the gastric insufflation was only 2.1%, but when the inspired pressure doubled to 30 cm H20, the gastric insufflation increased dramatically to over 35%. Studies have compared first generation classic LMAs with second generation LMAs and found that there are no significant differences in rates of regurgitation. It appears that first and second generation LMAs may be used safely without leading to aspiration, but it is vital to maintain inspiratory pressures at 15cm H2O or lower.

Another non-standard clinical practice with LMA use that we mentioned last week as the use of mechanical ventilation. The standard practice is to maintain spontaneous ventilation during LMA use, but more recently with the second generation devices, mechanical ventilation is often used as well. What is the difference in the distribution of ventilation with spontaneous versus mechanical ventilation with LMA use? Let’s look at the study, “Spontaneous breathing during general anesthesia prevents the ventral redistribution of ventilation as detected by electrical impedance tomography: a randomized trial” that was published in Anesthesiology in 2012. The patients underwent general anesthesia with an LMA. The investigators found no redistribution of ventilation with spontaneous ventilation and ventral redistribution with pressure controlled ventilation and pressure-support ventilation. Ventral redistribution of ventilation may lead to increased dead space and atelectasis.

What about the use of volume control ventilation with an LMA? This is associated with decreased compliance and higher peak inspiratory pressures compared to pressure controlled ventilation. Additional differences in respiratory parameters with LMA use and spontaneous ventilation include higher end-tidal CO2, smaller tidal volumes, and lower oxygen saturation compared to pressure controlled, volume control, and pressure support ventilation. There is evidence for improved oxygenation and ventilation when pressure support ventilation is used with an LMA compared to spontaneous ventilation with continuous positive airway pressure. In addition, a study by Keller and colleagues reported no differences in gastric insufflation, airway complications, hemodynamic changes, or ventilation with the use of an LMA with positive pressure ventilation compared to spontaneous ventilation. What is the impact of the type of LMA? There is a 2017 Cochrane review of the classic LMA compared to the ProSeal LMA with positive pressure ventilation. The quality of evidence was low, but the ProSeal LMA demonstrated a better seal. Another study evaluated the effect of ventilation mode with timing for LMA removal and reported an increased time for removal of the classic LMA when volume-control ventilation was used compared with pressure support or spontaneous ventilation.

We have one more non-standard LMA use setting to discuss today and that is the use of the LMA with obese patients. The concerns in this setting include the physiological changes of decreased respiratory compliance due to decreased chest wall compliance and abdominal contents limiting diaphragm motion. Obese patients undergoing laparoscopic surgery are at risk for additional reductions in their respiratory compliance. Ventilation may be challenging in this setting with a LMA or even with an endotracheal tube. If we look at the literature, patients with a BMI greater than 30 have a 2.5 time increased with of having difficulty with ventilation. The 2009 study by Zoremba and colleagues looked at obese patients undergoing minor peripheral surgery with a ProSeal LMA or an endotracheal tube and reported adequate ventilation in both groups as well as decreased postoperative pulmonary complications in the LMA group. Another use for an LMA with obese patients that has been studied is temporary ventilation prior to intubation on patients with a BMI greater than 35. Keller and colleagues reported that the ProSeal LMA provided effective temporary ventilation in this setting.

The authors highlight important considerations for LMA use including the following:

• LMAs use with mechanical ventilation is likely safe in fasted patients while minimizing the inspiratory pressures.
• Second generation LMAs may minimize leak around the device and decrease gastric insufflation compared to first generation LMAs.
• Administration of muscle relaxants with LMA use may aid in LMA placement as well as mechanical ventilation.
• LMA use in obese patients remains controversial and there is a risk for inadequate ventilation.
• The LMA may be used as a rescue device as part of the difficult airway algorithm in patients with difficult ventilation or intubation.

Now, before we wrap up for today, we are going to hear from Shauna Schwartz again. I asked her what do you hope to see going forward.

[Schwartz] After we completed this review article, it has certainly given us a different perspective about the utilization of the LMA in variable clinical settings. Multiple studies have demonstrated that using the LMA with muscle relaxants and mechanical ventilation is both safe and effective in selective clinical settings and patient populations. However, as we are in the midst of an obesity epidemic in this country, we would like to see more studies evaluating the safety of the LMA with mechanical ventilation in patients with BMIs greater than 30. We would also like to see the development of a clinical algorithm to help guide clinicians in determining the risk and benefit profile of the use of the LMA in various clinical scenarios.”

[Bechtel] Thank you so much to Schwartz for contributing to the show today. To conclude, the authors leave us with this call to action that I will read now:

“It is important to recognize the potential complications and relative contraindications to the LMA and adjust a clinical algorithm, which would optimize the use of the LMA in airway management.” This is so important to help keep patient safe during anesthesia care with LMA use and we are looking forward to future studies that continue to evaluate the use of LMA in clinical practice as well as further device modifications and improvements.

If you have any questions or comments from today’s show, please email us at [email protected]. Please keep in mind that the information in this show is provided for informational purposes only and does not constitute medical or legal advice. We hope that you will visit APSF.org for detailed information and check out the show notes for links to all the topics we discussed today.

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Until next time, stay vigilant so that no one shall be harmed by anesthesia care.

© 2021, The Anesthesia Patient Safety Foundation