In contrast, a person who depends on a mechanical ventilator for breathing must depend on the ventilator to deliver this pressure. Unfortunately, in most cases there is a long period in the ventilator's cycle that this minimum pressure differential does not exist. But, short phrases and long pauses are not the only speech problems experienced by ventilator users. There are also the large fluctuations in loudness and in voice quality. Speech loudness is linked to pressure.
Normal speech is generally quite even in loudness.
We adjust the amount of pressure by adjusting the amount of respiratory muscle activity we use. The fact that the ventilator-delivered tracheal pressure changes dramatically during inspiration and expiration means that loudness also can change dramatically. Often, the voice of the ventilator user can range from adequately loud to barely above a whisper. It is common for the voice to taper off substantially as the expiration ends and tracheal pressure falls below that required to continue vibrating the vocal folds. Loudness changes may be accompanied by changes in the quality of the voice over the course of the breathing cycle.