Providing a larger reservoir of gas within the cuff allows additional gas to be “milked” into the proximal end of the cuff without increasing baseline cuff inflation pressure (Fig 4). Thus, the selfsealing action of the cuff is improved while maintaining the lowest possible CT pressure and preserving tracheal mucosal blood flow.
Because cuff inflation is limited by the tracheal diameter when cuff diameter is greater than tracheal diameter, further increases in cuff diameter will not change operational cuff volume. In practice, the only means of increasing cuff volume without additional inflation is construction of a longer cuff. The improvement in performance gained by an increase in cuff length is exemplified in our experiment. The 8.0 MED cuff dimensions have been measured as 28.4 (diameter) x 33.5 mm (length), and the 8.0 HI cuff as 29.5 X 43.3 mm. The two cuffs have nearly identical diameters, both of which exceed the 19×22-mm internal diameter of the model trachea. The HI cuff is about 1 cm longer and provides a larger volume of gas that can be “milked” to fill the proximal end during inspiration. The result is an improvement in the 8.0 HI cuffs self-sealing action that is manifested by achieving tracheal occlusion with a 5 percent leak at a lower baseline cuff inflation pressure than the 8.0 MED cuff (Fig 2B).
Figure 4. Intracuff pressure equalizes with Paw as the latter rises. The gas within the cuff moves proximally toward the end of the cuff that is exposed to Patm. As a result, the distal end of the cuff collapses while the proximal end balloons proximally and laterally, changing the cuff shape from cylindrical to conical (A —* B). This redistribution of the gas within the cuff creates a self-sealing action that allows the cuff to maintain tracheal occlusion despite increases in airway pressure above the baseline cuff inflation pressure. The amount of air available for redistribution determines the degree to which airway pressure may exceed the baseline cuff inflation pressure before a leak will develop. Increases in cuff length provide additional gas that can be redistributed to maintain inflation of the proximal cuff and preserve tracheal occlusion (B —* C).