The supraglottic airway has proven its versatility in the operating room emergency room, and field resuscitation. However, the risk of pulmonary aspiration associated with supraglottic airways still exists. The fundamental design of the supraglottic airway has not been significantly changed since its inauguration in the 1980s. Second-generation supraglottic airways were introduced to minimize the occurrence of aspiration by creating a channel for insertion of a gastric tube to suction gastric content. The efficacy of this additional channel remains unknown in human subjects. Modern anesthesia ventilators are equipped with a gas-sampling line that samples at rates up to 250 ml/min. This line can be used to identify the occurrence of regurgitation and subsequently remove regurgitated material, minimizing aspiration. The conventional configuration cannot achieve this goal, as the insertion point of a gas-sampling line is at the Y connection of the breathing circuit. To reduce the risk of aspiration, we created a new configuration in which the gas-sampling line originates from the bowl of a supraglottic airway and conducted an experiment to determine its efficacy in removing regurgitated material.
We used a customized artificial trachea and lung model that mimics the interaction of a laryngeal mask airway (LMA) and glottis. A #3 LMA (LMA Unique, USA) served as the glottis and trachea, which was then attached to a #5 LMA as shown in figure 1. The two inflated cuffs were placed facing each other, with the cuff of the #5 LMA on the bottom and facing upward. An adequate seal was achieved by inflating the cuffs after fixation with adhesive tape. A total of 130 ml of saline was used as regurgitated material and delivered at rates of 0 to 120 ml/min via a feeding catheter that was inserted below the aperture bars of the #5 LMA. The distal end of the artificial trachea was connected to a single lung model (3-l ventilation balloon; AMBU Inc., USA) with dynamic compliance of 23 ml/cm H2O. An anesthesia ventilator (model Apollo; Drager, Germany) was connected to the proximal end of the #5 LMA. Ventilation was set at a tidal volume of 560 ml, a rate of 10 breaths/min, an inspiratory-to-expiratory time ratio of 1:2, and positive end-expiratory pressure of 2 cm H2O. A reservoir was installed in the middle of the sampling line to collect the regurgitated material.
The study resulted in four findings. First, moving the gas-sampling line point of insertion from the Y connection to the pharynx efficiently removes regurgitated material since as little as 1 ml of regurgitated material can be suctioned out. Second, it allows for early detection of regurgitation. Within 6 s, the regurgitated material appeared in the reservoir after a single bolus of 1 ml of regurgitated material was injected in approximately 1 s. Third, the maximum rate of regurgitation in which this system can completely prevent aspiration is approximately 25 ml/min. The gas-sampling line is efficient enough to completely remove regurgitated material and prevent aspiration at or below this rate. As the rate of regurgitation was further increased, regurgitated material began to reach the breathing circuit. The relationship between the rate of regurgitation and the fraction of aspiration is illustrated in figure 2. Fourth, at the maximum tested rate (120 ml/min), more than 50% (volume) of the regurgitated material was suctioned out into the reservoir. It is important to note that the actual suctioning does not occur throughout the entire respiratory cycle. A future design may improve the suction efficiency and allow suctioning at its full capacity of gas sampling.
Our study demonstrates that moving the insertion point of a gas-sampling line from the conventional Y connection to the pharyngeal cavity allows for earlier diagnosis of regurgitation, complete prevention of aspiration at low rates of regurgitation, and a significant reduction in the fraction of aspiration at high rates of regurgitation. We recognize there are certain limitations with this study. The use of saline simplifies the real-life variability in viscosity and particulate material of gastric contents. Therefore, it may overestimate the efficacy of removing regurgitated material via the gas-sampling line. It is also worth noting that the sampling line may become occluded during a regurgitation event or nonregurgitation event that would register a notable change on capnography. This study was conducted under positive pressure ventilation. Supraglottic airway ventilation is also commonly carried out using supportive modes and spontaneous breathing modes, which this study does not address. This bench study is intended as an early proof of concept. A further sophisticated experimental design with a larger diameter of the suction channel and material more akin to actual regurgitant material is needed to validate our observation before a large animal study or clinical study is conducted.