Page 62 - Clinical Application of Mechanical Ventilation

28 Chapter 2

PULMONARY CONSIDERATIONS

This section compares the physiologic differences between spontaneous breathing
and positive pressure ventilation. Two of the major effects are the changes in airway
pressure and compliance.

Spontaneous Breathing
During negative pressure
ventilation, pressures in
the airways, alveoli, and During spontaneous ventilation, the diaphragm and other respiratory muscles cre-
pleura are decreased during ate gas flow by lowering the pleural, alveolar, and airway pressures. When alveolar
inspiration.
and airway pressures drop below atmospheric pressure, air flows into the lungs.
Negative pressure ventilation uses this principle by creating a negative pressure on
the chest wall. When negative pressure is used for ventilation, the pressures in the
positive pressure ventilation: airways, alveoli, and pleura are decreased during inspiration. Table 2-1 shows the
Mechancial ventilation in which relationship between barometric pressure (P ) and alveolar pressure (P ) during
the volume is delivered by a posi- B ALV
tive pressure gradient (i.e., airway spontaneous breathing.
pressure higher than alveolar The pressure readings in Table 2-1 are for illustration purposes only. The baromet-
pressure).
ric pressure is assigned 0 cm H O for easy comparison of pressure changes during
2
spontaneous breathing. (Wilkins et al., 2008).
pressure-controlled ventila-
tion: Mode of ventilation in which
a preset peak inspiratory pressure Positive Pressure Ventilation
is used to provide ventilation.
The delivered volume during this
mode of ventilation is affected During positive pressure ventilation, gas flow is delivered to the lungs under a
by the changing compliance and
resistance. positive pressure gradient (i.e., airway pressure is greater than alveolar pressure).
Under normal conditions, the tidal volume delivered to the lungs is directly related
to the positive pressure when pressure-controlled ventilation is used. In volume-
volume-controlled ventilation:
Mode of ventilation in which a pre- controlled ventilation, the level of positive pressure is dependent on the mechani-
set tidal volume is used to provide cal tidal volume, as well as the patient’s compliance and airflow resistance.
ventilation. The airway pressures
during this mode of ventilation are When positive pressure is used for ventilation, the pressures in the airways, al-
affected by the changing compli- veoli, and pleura are increased during inspiration. Table 2-2 shows the relation-
ance and resistance.
ship between inspiratory pressure (P ) and alveolar pressure (P ALV ) during positive
I

TABLE 2-1 Relationship of Barometric Pressure (P B ) and Alveolar Pressure (P ALV ) during Spontaneous Breathing

Spontaneous
Breathing P (cm H O) P ALV (cm H O) ∆P Flow
2
B
2
Inspiration 0 25 25 Into lungs
End-inspiration 0 0 0 None
Expiration 0 15 15 Out of lungs

End-expiration 0 0 0 None
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