Based on the volume and flow waveforms depicted below, determine the cycle and limit variables

A. limit variables: pressure and volume; cycle variable: flow
B. limit variables: volume and flow; cycle variable: time
C. limit variables: volume and time; cycle variable: flow
D. limit variables: volume and flow; cycle variable: volume

?ANS: B?A. Incorrect response: The flow waveform cannot be the cycle variable because the flow is held constant throughout inspiration. Flow does not terminate when it reaches a preset limit. It maintains that preset limit throughout the inspiratory phase.
B. Correct response: Notice how both the volume and flow waveforms reach a limit and plateau at that level while inspiration continues. Based on that fact, the variable time serves as the cycle variable (i.e., the variable responsible for terminating inspiration).
C. Incorrect response: Limit variables include (1) flow, (2), volume, and (3) pressure. Time is not a limit variable, but can be a cycle variable (terminates inspiration).
D. Incorrect response: Both volume and flow are limit variables in this example because they both plateau during inspiration when they achieve their peak levels. Because volume is a limit variable, it cannot be a cycle variable. Because volume is held constant for a period during inspiration, it cannot cause inspiration to terminate.