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.
Resting cuff volumes ranged from 5.2 ml for the 7.0-mm ID LO, to 20.6 mL for the 8.0-mm ID HI ETT cuff (Table 1). When Cl was 100 ml/cm H20, a PIP of approximately 15 cm HaO (11 mm Hg) was generated; reducing Cl to 15 ml/cm H20 resulted in a PIP of approximately 80 cm H20 (60 mm Hg). After cuff inflation to the predetermined leak volumes we noticed that the airway pressure was transmitted to the cuff, causing intracuff pressure to rise and fall with inspiration and exhalation (Fig 3). The intracuff pressure measurements presented in Figure 2 were recorded at end-exhalation their lowest point (referred to as the baseline cuff inflation pressure). Figure 2A shows that in the highly compliant lung (Cl =100 mL/cm HaO) there is little difference between the baseline cuff inflation pressures of the MED and HI cuff groups with either a 10 or 5 percent leak. The 7.0 LO cuffed ETT required markedly high baseline cuff inflation pressures, whereas the 8.0 LO which had a resting diameter slightly larger than that of the trachea required baseline inflation pressures closer to those of the MED and HI groups.
Figure 2. Baseline cuff inflation pressures for each cuff type under conditions of (A) high lung compliance (100 ml/cm HaO) and (B) reduced lung compliance (15 ml/cm HaO), with a 10 and 5 percent leak in delivered Vt. A reference line is drawn at 25 mm Hg, indicating the maximum safe cuff pressure, ie, one that will not cause significant interference with tracheal wall blood flow.
Figure 3. Typical airway and intracuff pressure tracings for a 7.0 HI cuff during two cycles of ventilation, under conditions of high compliance (Cl =100) and reduced compliance (Cl =15). Above each pressure tracing is a photograph of the model trachea containing the 7.0 HI cuff at the PIE Note that under conditions of reduced compliance and high PIP, the cuff is deformed into a cone shape as the airway pressure compresses the distal end of the cuff, “milking” the gas within the cuff to fill and distend the proximal end. Under conditions of high compliance and low PIP, the cuff retains a cylindrical shape.
Table 1—Resting Intra-cuff Volumes for Three Types of Endotracheal Tubes
|7.0 mm||5.2 ±0.27||10.2 ±0.26||15.5 ±0.73|
|8.0 mm||9.4 ±0.31||12.9 ±0.32||20.6 ±0.92|