Three sets of pressure, flow, and volume waveforms are depicted below from a patient who is receiving continuous mandatory ventilation. Identify the control variable, and which set of waveforms reflects a decreased lung compliance

A. control variable: pressure; decreased lung compliance: set C
B. control variable: pressure; decreased lung compliance: set A
C. control variable: volume; decreased lung compliance: set B
D. control variable: volume; decreased lung compliance: set C

?ANS: D
A. Incorrect response: Pressure is not the control variable because its value is different in each set of waveforms. Set C is the correct set of waveforms reflecting decreased lung compliance (stiffer lungs).
B. Incorrect response: Pressure is not the control variable because its value is different in each set of waveforms; therefore it is not constant.
C. Incorrect response: Volume is the control variable because it remains constant in each of the three sets of waveforms. The pressure tracing in set B represents the lowest pressure value among the three presented. Therefore the pressure waveform in set B does not reflect decreased pulmonary compliance.
D. Correct response: Volume is the control variable because it remains constant in each of the three sets of waveforms. The expression volume control means that volume is the control variable, and does not change. Set C signifies the response to decreased lung compliance. Especially, note that the pressure tracing in set C indicates the highest pressure among the three tracings. This finding is consistent with decreased pulmonary compliance.