TITLE: THE FLOW VS. TIME WAVEFORM DISTINGUISHES BETWEEN DECREASED COMPLIANCE AND INCREASED AIRWAY RESISTANCE IN PRESSURE CONTROL MODE

AUTHORS: Avery Tung, MD, S. Morgan, RT
AFFILIATION: University of Chicago, Chicago, IL.

INTRODUCTION: Mechanical ventilation in pressure control mode (PCM) is often used for patients with acute lung injury. Because tidal volumes are not fixed in PCM, changes in lung compliance or airway resistance may decrease delivered tidal volume. We hypothesized that the flow vs. time waveform would identify the cause of decreased tidal volumes in PCM.
METHODS: A Puritan-Bennett 7200 ventilator was connected via an 8.0 endotracheal tube (ETT) to a test lung (Michigan Test Systems 2600i). The ventilator was set to an inspiratory pressure of 35cm H₂O, a respiratory rate of 18 breaths/min, and a fixed I:E ratio of 1:2 in PCM. PEEP was 5cm H₂O. Test lung compliance was 0.03 L/cm H₂O. Using a C-clamp to progressively occlude the ETT and increase airway resistance, we monitored the flow vs. time waveform via the ventilator and tidal volumes with the Bicore CP-100. We repeated the above protocol varying lung compliance from 0.03 to 0.01 L/cm H₂O with airway resistance and all other ventilator settings kept constant.
RESULTS: Tidal volume was reduced 33% (900cc - 670cc) with 90% ETT occlusion or by decreasing test lung compliance from 0.03 to 0.02 L/cm H₂O. The flow vs. time waveform changed dramatically with progressive airway occlusion, developing a characteristic "spike" at breath initiation. The flow vs. time waveform changed minimally with reduced lung compliance.
Fig 1: Flow vs. time waveforms in PCM.
A. normal compliance & resistance, tidal volume=900cc
B. reduced compliance only, tidal volume=670cc
C. increased airway resistance only, tidal volume=670cc
DISCUSSION: In patients whose lungs are ventilated in PCM, detecting changes in lung mechanics can prevent ventilator catastrophes. The flow vs. time waveform distinguishes airway occlusion from changes in lung compliance in patients with decreased tidal volumes. Monitoring tidal volumes alone in PCM differentiates poorly between these two causes.