The pressure, flow, and volume waveforms illustrated below were obtained from a patient with COPD being mechanically ventilated with pressure-controlled ventilation (PC-CMV). Where along any of these tracings can the influence of volume compressed in the patient's breathing circuit be noticed?

A. the slight pressure elevation situated at the front end of the pressure plateau
B. the gap along the baseline between the two upward deflections on the volume tracing
C. the initial downward spike located along the flow waveform
D. the abrupt drop-off down toward the baseline on the flow tracing

?ANS: C
A. Incorrect response: See explanation C.
B. Incorrect response: See explanation C.
C. Correct response: The initial downward deflection on the flow-time tracing indicates the volume of gas compressed in the ventilator circuit at the end of inspiration. When inspiration ends, the compressed volume exits the ventilator breathing circuit rapidly. In fact, this volume is expelled more rapidly from the circuit than the patient's own exhaled volume. This sudden flow of gas leaving the breathing circuit causes an initial steep decline in along the flow tracing, followed by a gradual tapering of flow to zero.
D. Incorrect response: See explanation C.