Page 63 - Clinical Application of Mechanical Ventilation

Effects of Positive Pressure Ventilation 29

TABLE 2-2 Relationship of Inspiratory Pressure (P I ) and Alveolar Pressure (P ALV ) during Positive Pressure
Ventilation

Positive
Pressure
Ventilation P (cm H O) P ALV (cm H O) ∆P FLOW
2
2
I
Inspiration 20 0 120 Into lungs
End-inspiration 20 20 0 No flow

Expiration 0 20 220 Out of lungs

End-expiration 0 0 0 No flow
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pressure ventilation. The pressure readings in Table 2-2 are for illustration purposes
only. The barometric pressure is assigned 0 cm H O for easy comparison of pressure
2
changes during positive pressure ventilation.
Under normal conditions, the pressure gradient and tidal volume are directly
related. During pressure-controlled ventilation, a higher peak inspiratory pressure
typically results in a larger tidal volume. However, there are some exceptions to this
relationship. (Table 2-3).

Airway Pressures

During pressure-controlled ventilation, the peak inspiratory pressure (PIP) is
peak inspiratory pressure
(PIP): Maximum pressure mea- preset according to the estimated tidal volume requirement of a patient. The inspi-
sured during one respiratory cycle, ratory phase terminates once the preset pressure is reached. For this reason, the pa-
usually at end-inspiration.
tient may receive a smaller volume when the preset pressure is reached prematurely,
as would happen under conditions of low compliance or high airway resistance.
During volume-controlled ventilation, the tidal volume is preset and the pres-
sure used by the ventilator to deliver this preset volume is variable. The PIP at

TABLE 2-3 Conditions That Limit the Volume Delivered by Positive Pressure Ventilation

Conditions Examples
Peak inspiratory pressure reached Airway obstruction
too soon Kinking of ET tube
Bronchospasm
Low lung compliance
Pressure limit set too low
Unable to reach peak inspiratory ET tube cuff leak
pressure Ventilator circuit leak
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