Modelling the mechanical effects of tracheal tubes in normal subjects.

The addition of tracheal tubes to the respiratory system contributes an extra mechanical burden in terms of pressure necessary to overcome its own resistance. On the basis of experimental data from the literature and on a previously reported mathematical model of the inspirogram, we wished to study predictions of pressures, volume, flow and work of breathing during the use of tracheal tubes. The present investigation indicates that: 1) the loss in volume is greater at the beginning of inspiration and with narrower tubes; 2) in order to preserve tidal volume the inspiratory drive must be increased at any time during inspiration with the use of diminishing internal diameter of the tubes; 3) alternatively, tidal volume can be maintained by increasing inspiratory duration; and 4) the addition of tubes with internal diameter of 9 mm (no.9) and 8 mm (no. 8) increases the total resistive work by 115 and 154%, respectively, whilst total elastic work decreases 9 and 16% in relation to the nonintubated patient. These findings are consequent to the turbulent flow pattern that normally occurs within the tracheal tubes and connectors themselves. We believe they are relevant to the physician confronted with a patient needing tracheal intubation.