Page 41 - CASA Bulletin of Anesthesiology 2022; 9(2) (5)

Vol. 9, No 2, 2022

with the potential for aspiration . Devitt et al. assessed the leak fractions, measured from
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subtracting expiratory volume from inspiratory volume divided by inspiratory volume, and
gastric insufflation comparing classical LMAs versus standard endotracheal intubation at various
inspiratory pressures. The leak fraction increased with increasing positive pressure delivered
through the LMA and remained low and unchanged in the ETTs. At an inspired pressure of 15
cm H2O, the gastric insufflation with LMA use was 2.1%, while it was 35.4% with a pressure of
30 cm H2O . In a Cochrane review comparing the ProSeal™ LMA, a second-generation LMA
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with a gastric suction port and a posterior cuff for an improved seal, with Classical LMA with
PPV, Qamarul Hoda et al. concluded that there was no significant difference in rates of
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regurgitation . Both older and newer generations of the LMA have been successfully used
without clinical signs of aspiration if inspiratory pressures are limited to 15 cm H2O or lower 17, 18 .
Spontaneous ventilation vs. Mechanical ventilation

A benefit of LMA use is that it is less stimulating to a patient than an ETT; therefore, less
anesthesia is often required . Due to increasing comfort with use and the development of a new
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generation of devices, LMAs are routinely used safely with mechanical ventilation 18, 21–24 . Radke
et al. assessed the redistribution of ventilation by using electrical impedance tomography in
patients undergoing general anesthesia with an LMA . They observed no redistribution of
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ventilation with patients breathing spontaneously, and found ventral redistribution under both
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pressure-controlled ventilation (PCV) and pressure support ventilation (PSV) . Consequences
of ventral distribution of ventilation include increased dead space and atelectasis 21, 24 . The use of
volume control ventilation (VCV) with an LMA results in less compliance and higher peak
inspiratory pressures compared to PCV. PCV is a newer mode of ventilation that limits the
inspired pressure to maintain a set tidal volume . End-tidal carbon dioxide was higher, tidal
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volumes were smaller, and oxygen saturation was lower in patients undergoing spontaneous
breathing (SB) compared to PCV, VCV, and PSV modes 21, 23 . Brimacombe and Keller found
improved oxygenation and ventilation with the LMA by using PSV compared with continuous
positive airway pressure (CPAP) .
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There was no difference in gastric insufflation, airway or cardiovascular complications, or
problems ventilating patients in a study by Keller et al. comparing spontaneous ventilation to
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PPV . In a Cochrane review, the classic LMA was compared to the ProSeal™ LMA undergoing
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PPV . The ProSeal™ LMA had a better seal, suggesting that it may be more suitable for PPV;
however, overall the quality of evidence was low . In a randomized controlled trial, Capdevila
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et al. examined various modes of ventilation, VCV, PSV, and SB, on emergence time and
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intraoperative ventilation . Time to classic LMA removal was prolonged in patients undergoing
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VCV compared with PSV or SB .
Obesity and LMA

Another area of controversy is LMA use in obese patients. Physiological changes seen in
obese patients make them a challenging population, including a restrictive lung pattern due to
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abdominal contents limiting diaphragm motion and yielding less respiratory compliance.
Insufflation during laparoscopic procedures can further impair lung compliance and make
ventilation difficult . Cheong et al. found that in patients with a body mass index (BMI) over
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30, there was a 2.5 times increased risk of having ventilatory problems . Zoremba et al.
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assessed postoperative lung function and saturations in obese patients (BMI 30 to 35) undergoing
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