Page 137 - Clinical Application of Mechanical Ventilation

Operating Modes of Mechanical Ventilation 103

patient’s inspiratory flow demand, (3) inspiration lasts only for as long as the patient
actively inspires, and (4) inspiration is terminated when the patient’s inspiratory
flow demand decreases to a preset minimal value. PSV can be used in conjunction
with spontaneous breathing in any ventilator mode.
A pressure-supported breath is therefore patient-triggered, pressure-limited, and
flow-cycled. It is pressure-limited because the maximum airway pressure cannot ex-
ceed the preset pressure support level. It is flow-cycled because a pressure-supported
breath cycles to expiration when the flow reaches a minimal level.
It is important to understand how the pressure plateau is created and maintained.
Essentially, when the pressure-supported breath is patient-triggered (either by pressure
or flow), the ventilator demand valve generates a flow high enough to rapidly increase
the airway pressure to the preset pressure limit and then maintain the pressure plateau
(via servo control and demand valve) for the duration of the patient’s spontaneous in-
spiratory effort. Typically, the flow pattern associated with pressure support is a steeply
descending tapered flow pattern. As previously described, the demand valve flow termi-
nates when it decreases to a preset lower flow limit. The point at which flow cycling oc-
curs varies with different ventilators but 5 L/min or 25% of peak flow are two examples.
Indications for PSV Mode

Pressure support is commonly applied in the SIMV mode when the patient takes spon-
taneous breaths. Pressure support is not active during the mandatory breaths. Pressure
support has been advocated as a stand-alone mode by some clinicians; however, this re-
quires close monitoring because as a stand-alone mode, every breath is patient-triggered.
Pressure support is typically used in the SIMV mode to facilitate weaning in a
Pressure support (1) ➞ difficult-to-wean patient. In this application, pressure support (1) increases the pa-
spontaneous tidal volume,
(2) spontaneous frequency, tient’s spontaneous tidal volume, (2) decreases the patient’s spontaneous frequency,
➞
and (3) work of breathing. and (3) decreases the work of breathing.
➞
These three effects have been used to titrate the proper level of pressure support.
For example, one physician may increase the pressure support level until a desired
spontaneous tidal volume is achieved (e.g., 10 to 15 mL/kg). Another physician
The level of pressure
support is titrated until may increase the pressure support level until the patient’s spontaneous frequency
(1) spontaneous tidal volume
5 10 to 15 mL/kg or decreases to a target value (usually 25/min or less) (MacIntyre, 1987).
(2) spontaneous frequency , The third endpoint for the pressure support level is to decrease the work of breath-
25/min.
ing (MacIntyre, 1986). This approach is probably less commonly used for the pa-
tient in immediate respiratory distress, but is more often used as a “routine” method
to decrease the work of breathing. Since an endotracheal tube increases the airway
resistance and the work of breathing, pressure support has been used successfully to
overcome this gas flow resistance. The airway resistance on most modern ventilators
may be obtained easily, but in ventilators not equipped with this function, the fol-
lowing equation may be used to estimate the airway resistance:
See Appendix 1 for
example. (Peak Inspiratory Pressure - Plateau Pressure)
Airway Resistance =
Mean Flow
Table 4-6 summarizes the major characteristics of the pressure support ventilation
mode.

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