To the Editor:—
The Laryngeal Mask Airway™ (LMA ™) is a well-known method of clinical airway management now in worldwide use. 1This letter describes the design, construction, and preliminary clinical evaluation of a new medical instrument for respiratory monitoring that attaches to the LMA ™ cuff inflation line (LMA ™ Audio Monitor). The device is essentially a new form of electronic stethoscope and is intended for qualitative respiratory monitoring during general anesthesia when the LMA ™ is in use.
A special, leakfree microphone assembly was designed, fabricated, and pressure-tested to 300 mmHg using a miniature electric microphone (Radio Shack 30-3013, Radio Shack, Fort Worth, TX) and a shortened 3-ml plastic syringe with a Luer lock end. Epoxy glue was used to secure the microphone into the barrel of the syringe. A high-gain monaural audio amplifier was used to amplify the microphone signal for use with a headset or for computer analysis.
Following institutional approval, the system was evaluated clinically in 10 patients undergoing general anesthesia using the classic LMA with spontaneous ventilation. In all cases, attaching the device to the pilot line of the LMA was straightforward. Clearly identifiable breath sounds were heard in all cases; these were thought to be suitable for qualitative respiratory monitoring. It was observed that with normal breathing, the sounds were regular and smooth. In one patient who developed partial airway obstruction with the LMA in situ , the sounds become chaotic, irregular, and intense. A sample normal recording is available as a Web Enhancement at http://www.anesthesiology.org.
Additional material related to this article can be found on the Anesthesiology Web site. Go to the following address, click on Enhancements Index, and then scroll down to find the appropriate article and link. http://www.anesthesiology.org
Esophageal and precordial stethoscopes are often used during surgery for monitoring breath sounds. Some of these devices are electronic in design. 2,3
Potential drawbacks of these methods include relatively poor acoustic properties, awkward placement, and, in the case of esophageal stethoscopes, the remote possibility of injury to the patient. The LMA Audio Monitor allows anesthesia providers to hear clear respiratory sounds, and it works with all forms of the LMA ™. Because the unit is battery-operated, and because there is no direct connection of the device to the patient, the device poses no risk to the patient. The expected value of the device is that, with experience, anesthesia providers will be able to interpret the obtained sounds to detect various normal and pathologic states. These include normal breathing, tachypnea, phonation, partial airway obstruction, wheezing, and ventilation leaks with positive pressure ventilation.