Thinking of the lung-ventilator unit in terms of this
simple model is also useful in aiding an understanding of the use of
monitoring end-inspiratory pause pressure. In volume and flow preset
modes pressure becomes a dependent variable. It is important to
monitor pressure in order to minimize the risk of barotrauma.
However, in this context it is alveolar pressure not airway pressure
that is important. By measuring the airway pressure during an
end-inspiratory pause it is possible to eliminate the component due
to resistance because during an end-inspiratory pause there is no
flow and thus PAW=PALV. In most circumstances the contribution of the
resistance component to airway presssure is relatively small and
constant so it is reasonable to monitor airway pressure, however in
patients with high resistance (eg patients with obstructive lung
disease) it is important to monitor end-inspiratory pressure.
Measurement of end-inspiratory pressure may also help determine the
cause of a sudden rise in airway pressure. If both are high then the
problem is due to a fall in compliance (eg endobronchial intubation,
pneumothorax) while if only the airway pressure is high then the
problem is due to increased resistance (eg partially blocked ETT,
bronchospasm).
