If a rectangular (square) pressure waveform is generated during pressure-controlled ventilation, what type of waveforms will develop for volume and flow?

A. volume waveform: exponential rise; flow waveform: exponential decline
B. volume waveform: exponential decline; flow waveform: exponential rise
C. volume waveform: sinusoidal; flow waveform: ascending ramp
D. volume waveform: descending ramp; flow waveform: rectangular

?ANS: A?A. Correct response: A rectangular pressure waveform is accompanied by an exponentially rising volume waveform and an exponentially decaying (declining) flow waveform. The volume rises exponentially to a value equal to the product of the lung compliance and the difference between the PIP and the PEEP. That is,
volume = CL (PIP – PEEP)
L = L/cm H2O (cm H2O)
The inspiratory flow declines exponentially from its peak level, which is equal to the difference between change in airway pressure multiplied by the airway resistance. Hence,
flow = Raw × (PIP – PEEP)
L/sec = cm H2O/L/sec × (cm H2O)

B. Incorrect response: The choice has the waveforms opposite of what they actually are.
C. Incorrect response: When the pressure waveform is sinusoidal, the volume and flow waveforms are also sinusoidal.
D. Incorrect response: See explanation for choice A above.