The effect of a gas leak round the endotracheal tube on the mean tracheal pressure during mechanical ventilation

Infants requiring mechanical ventilation are usually intubated with uncuffed endotracheal tubes, which permit gas to leak between the tube and the trachea. This gas leak may alter the mean pressure transmitted to the trachea by changing the pattern of airway flow and modifying the resistive behavior of the endotracheal tube. To test this hypothesis, we measured mean tracheal pressure, gas flow through the endotracheal tube, and resistance of the tube in rabbits ventilated with and without a leak. We also studied the effect of the tube size and the pattern of ventilation on these measurements. We found that a leak reduced the mean tracheal pressure by 15 to 21% with respect to the mean proximal airway pressure. This reduction was caused by an increased difference between inspiratory and expiratory flow through the endotracheal tube, and by the mean expiratory resistance of the tube being lower than its mean inspiratory resistance. The rabbits with smaller tubes had lower mean tracheal pressures. A ventilatory pattern of short inspiratory times and high peak pressures was associated with a proportionally greater decrease in mean tracheal pressure caused by the leak. These findings suggest that the mean proximal airway pressure, measured at the ventilator, may overestimate the mean tracheal pressure in the presence of a gas leak around the tube. Furthermore, the decrease in mean tracheal pressure caused by the leak may decrease oxygenation despite a constant mean proximal airway pressure.