Dräger Instructional CD: Mechanical Ventilation - VentWorld

Leak
Pressure
Volume
Flow
A B
exhaled
volume less
than inhaled
volume
5. How To Read Ventilator Displays
constant leak in inspiratory direction
(A) Intermittent leak, as through a chest tube or around an uncuffed endotracheal tube.
Volume leaks out mostly during inspiration when pressure is elevated and the airways are
dilated. The volume waveform abruptly returns to zero because the integrator resets when
the software thinks expiration has ended (ie, flow crosses zero). However, it is clear that the
height of the inspiratory portion of the volume curve is bigger than the expiratory portion.
Flow returns to zero but the area under the flow curve (which is proportional to volume) is
smaller for expiration than inspiration. These are both signs that the exhaled volume is less
than the inhaled volume. (B) Constant leak. Notice the same signs of leak as in (A) but the
leak is present throughout the breath. This is indicated by the expiratory flow curve that
remains above zero, indicating a constant positive (inspiratory direction) flow. Large leaks in
the patient circuit or through a chest tube may do this. The CPAP level may be reduced by a
large leak if the ventilator cannot compensate.
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