Bubble Solution Diagnoses ProSeal™ Insertion into the Glottis

To The Editor: The instruction manual describes three malpositions for the ProSeal™ laryngeal mask airway (PLMA) (1,2): 1) Insufficient insertion depth, 2) PLMA insertion into the glottis, and 3) PLMA tip folded backwards behind the bowl against the posterior pharyngeal wall (3). PLMA entry into the glottis is not uncommon during insertion attempts because of the bulky and rather flexible nature of the mask tip. When the PLMA enters the glottis, it is the drain tube (DT) and not the airway tube that acts as an extension of the tracheobronchial tree. Airway pressures are therefore transmitted through the DT and not the airway tube. We have found that PLMA entry into the glottis is usually easy to quickly diagnose. After insertion of the PLMA and inflation of the cuff to 60 cm H2O, we next place soap bubble solution (such as used by children to blow bubbles) on the DT port (4,5). We then observe this membrane before connecting the anesthesia circuit. Occasionally, we have noticed that a bubble is “spontaneously” blown from the DT port. Likewise, occasionally we have observed that the membrane moves upwards and downwards quite dramatically with the cardiac rhythm, “oscillating wildly.” If we observe either of these phenomena we diagnose PLMA insertion into the glottis. When the PLMA tip enters the glottis, the tracheobronchial tree is coupled directly to the DT. Unless it is somehow obstructed, the DT transmits airway pressures. When “spontaneous” bubble formation is observed following PLMA insertion, this is due to ongoing exhalation as the lung approaches functional residual capacity. Likewise, “wild oscillations” of the soap membrane demonstrate the rapid time variation of lung volume during the cardiac cycle. These particular oscillations are huge compared with the small, normal, membrane movements occasionally observed with regular PLMA positioning (because of rhythmic bulging of the esophageal mucosa abutting the DT tip) (4,5). Finally, gently pressing on the patient’s chest and observing soap bubble formation can help confirm the diagnosis of insertion into the glottis. When we have suspected glottic entry based on these observations, we have reinserted the PLMA. In every case (at least 5 or 6 to date), the PLMA was reinserted to a noticeably increased depth of insertion and all indications of glottic entry have disappeared. Location of the PLMA tip in the esophagus behind the cricoid cartilage has then been confirmed using the “suprasternal notch test (6). Table 1 summarizes how the soap membrane method can be used to quickly diagnose all three of the malpositions mentioned in the instruction manual.Table 1: diagnosis of 3 PLMA MalpositionsFinally, we would caution against interpretation of these observations when the patient is prone. We recently observed very large cardiac oscillations and bubble formation/bubble shrinking of the soap membrane during a prone lumbar laminectomy. Fiberoptically, it turned out that the esophagus was widely patent at the DT tip and that its expansive lumen extended well within the thoracic cavity. The large movements of the soap membrane were because of changes in this enormous esophageal volume resulting from the cardiac rhythm and positive pressure ventilation. In the supine position the esophagus was collapsed and the soap membrane was relatively motionless. Cornelius J. O’Connor Jr., MD Michael S. Stix, MD, PhD