18. While mechanically ventilating a patient with VC-SIMV with PSV of 10 cm H2O, the therapist seeks to increase the patient's spontaneous VT from 150 ml to 300 ml. How can the therapist accomplish this task?

A. Switch to PC-SIMV with PSV.
B. Increase the minute ventilation control.
C. Increase the inspiratory rise time.
D. Adjust the flow termination criterion from 40% to 25%.

?ANS: D
A. Incorrect response: Switching to PC-SIMV with PSV would have no influence on the magnitude of the patient's spontaneous tidal volume. All that would be accomplished by making this switch is substituting volume-control ventilation for pressure-control ventilation. This change would affect only the mandatory breaths.
B. Incorrect response: MMV enables the patient to breath spontaneously, and ensures that the patient receives a minimum minute ventilation initially set by the therapist. However, MMV is a stand-alone mode and is not used with PC-SIMV with PSV.
C. Incorrect response: Increasing the inspiratory rise time has the effect of lengthening the time required for the pressure to reach its target value. Increasing the inspiratory rise time inappropriately can produce an increased work of breathing for the patient. Conversely, decreasing the inspiratory rise time too much can cause pressure overshoot, which contributes to premature cycling.
D. Correct response: Changing the flow-cycle criterion from 40% to 25% during PSV lengthens the inspiratory time and increases the tidal volume. Conversely, if the flow-cycle criterion is changed from 5% to 50%, for example, the inspiratory time shortens and the tidal volume decreases. For ventilators lacking the flow-cycle criterion function, the therapist will increase the PSV level.