The effective tracheal diameter that causes air trapping during jet ventilation

Jet ventilation consists of injection of gas at high flow rates through a small-diameter tracheal catheter. Air trapping (increase in end-expiratory lung volume) can occur during jet ventilation if the diameter of the trachea proximal to the tracheal catheter tip is too small (at least at one point in the trachea) to permit complete exhalation of the tidal volume around the tracheal catheter (ie, through the effective tracheal diameter). A mechanical lung model was used to determine the critical effective tracheal diameter at which air trapping starts to occur during jet ventilation. The experiment allowed derivation of a multivariable equation, namely: [formula: see text] to express the tidal volume produced by jet ventilation (y) as a function of gas flow rate (C), jet injection time (D), lung compliance (B), upper airway resistance (A), and effective tracheal diameter (E). As A to D increased and E decreased, y increased. More importantly, exhalation time was measured over the full range of values for A to E, and it was found that for every possible combination of values for A to D, there was always a unique critical effective tracheal diameter, 4.0 to 4.5 mm, that began to cause a very large increase in expiratory time (and with a sufficiently rapid respiratory rate [greater than 12 beats/min in this experiment], air trapping). Thus, when lung/jet ventilation factors tend to promote entry of jetted gas into the lungs (increased A to D, decreased E), even a small tidal volume has difficulty exiting the lung, if E is smaller than 4.5 mm.(ABSTRACT TRUNCATED AT 250 WORDS)