Resting cuff volumes (the performed just inflated volume) were measured using a calibrated syringe. A mechanical lung (Vent Aid TI L, Michigan Instruments, Inc), which was connected to a lubricated (lightweight household oil, WD-40) 19 x 22-mm model trachea (Imatrach, Mallinckrodt) was used to simulate changes in Cl from 100 to 15 mL/cm HaO. A volume-cycled ventilator (Bear 2) delivered a tidal volume (Vt) of 1,000 ml; inspiratory and expiratory volumes were measured with two rotating vane-type spirometers (Wright) connected in series just proximal to the ETT.
The reasons for the excessive intracuff pressure requirements were not clear.
Recently we had occasion to review two cases in which patients with ARDS sustained ischemic tracheal complications following a prolonged period of mechanical ventilation with high PIP above 50 cm H20. One of these patients developed a tracheoesophageal fistula requiring tracheal reconstruction and prolonged hospitalization. The other patient suffered massive tracheal dilation before succumbing to multiple organ failure. In both cases, ETTs with high-volume, low-pressure cuffs had been continuously employed. Proper cuff inflation (minimal leak) techniques had been utilized, together with appropriate humidification, tidal volume and minute ventilation. As in the report of Stauffer et al, we were unable to explain the extensive tracheal damage associated with the use of high-volume, low-pressure ETT cuffs and created a laboratory model to examine intracuff pressure and volume characteristics.
When used properly, these large cuffs sealed the trachea by draping themselves freely along the contours of the tracheal wall without altering the normal “C” shape of the trachea. As a result, these cuffs did not need to be inflated beyond their resting diameter and required lower inflation pressures than the earlier low-volume, high-pressure design. Furthermore, cuff inflation pressures approximately equalled CT pressure because no pressure was generated by stretching the cuff to fill the trachea. Thus, the high-volume, low-pressure design operated with less CT pressure and reduced the overall incidence of ischemic tracheal complications.
In the 1960s, ischemic tracheal complications such as tracheal stenosis and tracheoesophageal fistula occurred in up to 20 percent of intubated and mechanically ventilated patients. Endotracheal tubes were typically constructed with low-volume cuffs which had little or no resting volume and a diameter smaller than tracheal diameter. Such cuffs frequently required inflation pressures of 160 to 300 mm Hg to seal the trachea and deformed the trachea from its normal “C” shape into an expanded circular shape. The pressure exerted upon the tracheal mucosa (CT pressure) by inflating a low-volume, high-pressure cuff was impossible to estimate; however, directly measured CT pressures ranged up to 200 mm Hg or more. Furthermore, small increments (1 to 3 ml) in cuff volume beyond the MOV produced increases in the CT pressure of up to 100 mm Hg or more. Such high CT pressures severely restricted tracheal wall blood flow and caused mucosal ulceration, tracheomalacia with tracheal dilation, perforation and tracheal stenosis as scar formation occurred with healing.