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.
Differences between High-Volume, Low-Pressure Designs
This study examines the performance of two different large-diameter cuff designs, both of which claim to be low pressure. While no differences are apparent at low PIPs, significant differences appear as the PIP increases (Fig 2). It is obvious from the difference in cuff inflation pressures shown in Figure 2 for the MED and HI cuff designs that all high-volume, low-pressure designs are not created equal. A reconsideration of the principles governing the self-sealing nature of these cuffs will expose their operational differences.
By increasing its intracuff (hence CT) pressure, the cuff is automatically compensating for the increases in airway pressure without additional inflation. This self-sealing action was described by Carroll et al who wrote.. we discovered in 1968 that any large diameter, large residual volume cuff can be inflated to a baseline pressure just adequate to prevent aspiration, and that this resting intracuff pressure then rises in synchrony with the airway pressure.”
The absolute intracuff pressures reported in this study are estimates and are subject to error; however, the fact that cuff inflation pressure must be increased in order to seal the trachea against high PIPs is clear. Nonetheless, it is not generally appreciated that current high-volume, low-pressure cuff designs cannot effectively seal the trachea with low pressures when the PIP is high. The problem does not revolve around improper cuff inflation techniques but is instead an inherent limitation of cuff design as outlined in the next two sections.
The 7.0 LO cuff has the same shape as the 8.0 LO cuff, but is slightly smaller in diameter than the tracheal model. Consequently, the 7.0 LO cuff had to be stretched in order to occlude the trachea, leading to high intracuff pressures and alteration of normal tracheal contour. The marked difference in 7.0 and 8.0 LO intracuff pressures results from slight differences in cuff vs tracheal diameter. This example shows that the relationship of CT diameter has been the key principle to the reduction of intracuff (hence CT) pressures in the change from low-volume, small-diameter cuffs to high-volume, large-diameter cuffs.
Although we designated 25 mm Hg as a reference point for safe cuff pressures, the limits of safe CT pressures have not been firmly established. In the early 1970s many investigators reported reductions in tracheal damage when CT pressure was limited to approximately 30 mm Hg, the estimated capillary perfusion pressure. Measurements by radioactive microsphere and micropuncture techniques have confirmed tracheal capillary pressure to lie between 20 and 30 mm Hg. Endoscopic studies have shown impaired tracheal blood flow at 22 mm Hg and total obstruction at 37 mm Hg, suggesting that CT pressures should not exceed a critical value of 20 mm Hg.
This in vitro experiment utilized a plastic tracheal model specifically recommended by the American National Standards Institute for the testing of ETT cuffs designed for prolonged intubation. Plastic tracheal models avoid the problems of rapid deterioration of freshly excised tracheas and poor correlation between animal species. While our mechanical model allowed strict control of test conditions, the absolute pressures reported may not accurately reflect in vi-Doresults.
When Cl decreased to 15 ml/cm H20 (Fig 2B), every cuff required higher baseline inflation pressures, and differences among the three cuff groups became evident. To achieve identical performance, especially under conditions of reduced Cl, the 7.0 LO required the highest pressures, the MED cuffs required intermediate pressures and the HI cuffs required the lowest pressures (p<0.05). Differences in performance also were discernible between the 7.0 and 8.0 HI cuffs.
Intracuff pressure measurements were divided into four groups according to leak volume and Cl, ie, 10 percent, high Cl; 5 percent, high Cl; 10 percent, decreased Cl; and 5 percent, decreased Cl. Within each group, the pressure readings for each ETT cuff type were compared using ANOVA and a Duncans multiple comparisons test. Alpha was set at 0.05 for statistical significance.