The two sets of pressure, flow, and volume waveforms depicted below indicate pressure support ventilation (PSV). Set A represents normal PSV, and set B shows PSV with a shortened (fast) rise time. What effect does the shortened rise time have on the patient's tidal volume and inspiratory flow?

Question

A. tidal volume: increases; inspiratory flow: decreases
B. tidal volume: decreases; inspiratory flow: increases
C. tidal volume: decreases; inspiratory flow: decreases
D. tidal volume: increases; inspiratory flow: increases

Answer 1

?ANS: D
A. Incorrect response: See explanation D.
B. Incorrect response: See explanation D.
C. Incorrect response: See explanation D.
D. Correct response: With PSV, the patient is assisted with a constant pressure from the ventilator during inspiration. The constant pressure supplements the patient's spontaneous inspiratory efforts. The delivery of an appropriate inspiratory flow at the onset of inspiration is important during PSV. The speed at which the breath reaches the set pressure is called the inspiratory rise time. If the pressure is delivered too quickly to the patient, the set pressure may be exceeded, leading to the likelihood of premature cycling. On the other hand, if the rise time is too slow, the patient's work of breathing increases.
The pressure tracing in set B reflects a more rapid rise time than that in set A. Consequently, the inspiratory flow in set B is greater than the flow in set A, and the volume associated with the faster rise time is larger than the volume in set B.