In Reply:-We appreciate the opportunity to reply to the letter by Drs. Wiesel and Warm regarding the silicone ILM endotracheal tube (Euromedical, Sungai Petani, Kedah, Malaysia). The authors report successful tracheal intubation and ventilation in a 52-yr-old man for cervical spine stabilization using the intubating LMA-Fastrach (LMA North America, San Diego, CA) and a 7.5-mm silicone ILM endotracheal tube designed for use with it. The patient was ventilated for 3 days using this tube; however, when he subsequently required repeated intubation, the authors noted that a seal was not initially achieved. The problem was easily solved by inflating the cuff with more air, but the authors were concerned to note the pressure in the cuff was 100 cm water before a seal was achieved.
Wiesel and Warm make three points.
1. The product labeling does not state that the endotracheal tube is of the low-volume, high-pressure type, possibly resulting in anesthesiologists being unaware that they are not using a high-volume, low-pressure cuff.
2. Clinicians should test the cuff before use.
3. The product is unsuitable for patients who require postoperative mechanical ventilation.
We asked Dr. A. I. J. Brain, the inventor of the endotracheal tube in question, to comment on these points. His written reply from December 21, 1998 follows.
The silicone cuff of this tube is designed to form a close-fitting sleeve on the shaft to permit smooth passage through the intubating LMA. Technically, this makes the device a high-pressure, low-volume cuff that resembles the standard cuff of a plastic (nonsilicone) endotracheal tube, but the term is misleading when applied to this specialized tube. Unlike the standard plastic endotracheal tube cuff, the initial high pressure required to expand the silicone sleeve does not continue to increase when the cuff diameter reaches the tracheal diameter (Figure 1 and Figure 2). In other words, the pressure-volume curve reaches a plateau at a cuff diameter approximating the tracheal diameter. Consequently, at the "just seal" volume, the cuff becomes extremely efficient, because the volume can be adjusted precisely without further increases in intracuff pressure. For example, when samples of the three sizes of tube (7.0, 7.5, and 8.0 mm) were inflated within an artificial trachea with a 1.9-cm diameter, seals of 62.4, 37.7, and 42.9 cm water were obtained for calculated wall pressures of 6.5, 5.2, and 7.8 cm water, respectively. This means that at sealing volumes, mucosal pressures should be well within safe limits, even though measured intracuff pressures are likely to be in the range of 100–130 cm water. Figure 1 and Figure 2illustrate the difference between these silicone cuffed tubes and a standard low-volume cuffed tube made from plastic (polyvinyl chloride). I hope this explains why the term "high-pressure, low-volume" is best avoided when describing this special tube.
I do not necessarily agree that it is safer to avoid using the ILM silicone tube in favor of high-volume, low-pressure cuffed tubes if prolonged ventilation is planned. First, the intubating LMA was not designed for plastic tubes. Large plastic cuffs may be torn on passage through the mask aperture, whereas the harder, more laterally placed tube tip may damage the right vocal cord as it passes into the trachea. Second, using a high-volume, low-pressure cuffed endotracheal tube does not guarantee an effective seal against aspiration. Such cuffs have been shown in models  and clinically  to form longitudinal folds at their point of contact, with the mucosa resulting in leakage of fluids from above. Finally, recent data using an experimental tube with similar characteristics to the Euromedical tube indicate that by avoiding the longitudinal folding associated with the large-volume plastic cuff, an effective barrier to aspiration can be achieved.  In summary, no data exist to indicate that the Euromedical ILM tube is unsuitable for prolonged ventilation.
We do agree with Drs. Wiesel and Warm that all ILM endotracheal tubes should be tested before use, as stated on the labeling. With testing, it would be apparent to the clinician that the ILM endotracheal tube cuff is not the same as the high-volume, low-pressure cuffs found with polyvinyl chloride tubes.
Finally, I want to correct the impression that the newer ILM endotracheal tubes have high-volume, low-pressure cuffs. The cuff design has been improved to inflate more symmetrically, providing a better, consistent seal. Even though these newer cuffs are lower pressure than the original cuffs, they are not equivalent to the baggy, high-volume, low-pressure cuffs associated with polyvinyl chloride tubes.
Helen C. Young, M.D.
Director; Marketing and Medical Affairs; LMA North America, Inc.; San Diego, California
(Accepted for publication December 1, 1998.)