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 study demonstrates several different properties of ETT cuff design: (1) the influence of cuff diameter to tracheal diameter on intracuff pressure, (2) the dependence of cuff inflation pressure on airway pressure, (3) the self-sealing action of large diameter cuffs and (4) the differences in performance between different high-volume, low-pressure designs.
Cuff vs Tracheal Diameter
Although designed to operate in similar fashion to the 7.0-mm LO cuff (a high pressure design), the 8.0mm LO cuff actually possessed pressure characteristics more like the MED and HI cuffs. This unexpected low pressure performance is attributable to the fact that the 8.0 LO cuff resting diameter was slightly larger than that of the tracheal model and did not have to be stretched to fill the trachea, instead passively assuming tracheal contour as it was inflated.