A patient is receiving volume support (VS) ventilation, which is classified as dual control, breath-to-breath, pressure-limited, flow cycled ventilation. The therapist sets the VT to 600 ml, and the respiratory rate control knob to 10 breaths/min. How will the ventilator respond if the patient's minute ventilation falls below 6 L/min?

A. The ventilator will increase the flow over the next three breaths until the target minute ventilation is achieved.
B. Pressure will be added to the delivered VT over the next three breaths until the original minute ventilation is restored.
C. A backup respiratory rate will be activated to re-establish the target minute ventilation.
D. Inspiratory time will automatically be prolonged to increase the delivered VT, and restore the initial minute ventilation.

?ANS: B
A. Incorrect response: Volume support (VS) ventilation is pressure support ventilation employing volume as the feedback control for continuously adjusting the pressure support level. Like pressure support ventilation, VS is entirely a spontaneous breathing mode of ventilation.
B. Correct response: A volume support breath is a volume targeted breath that adjusts the pressure over several breaths to achieve a set VT. Therefore, if the volume exiting the ventilator does not equal the set VT, the pressure will increase to raise the VT. On the other hand, if the volume delivered is too large, the pressure will decrease.
If the target tidal volume (minute ventilation) is not met, these volume adjustments are accomplished by the ventilator over the course of three successive breaths. The volume added will not permit the pressure to increase by more than 3 cm H2O for each consecutive breath. The targeted VT will increase up to 150% of the set VT, thus maintaining a near constant minute ventilation.
C. Incorrect response: No backup respiratory rate exists for the VS mode. In fact, the respiratory rate is preset on the ventilator. So, when the minute ventilation decreases, the VT is adjusted to maintain a fairly constant minute ventilation.
D. Incorrect response: Inspiratory time will not be adjusted to achieve the targeted VT.