Page 258 - Clinical Application of Mechanical Ventilation

224 Chapter 8

TABLE 8-8 Determination of Circuit Compressible Volume

1. With the circuit warmed to an operating temperature, set the frequency at 10 to 16/min
and the tidal volume between 100 and 200 mL with minimal flow rate and maximum high
pressure limit.
2. Completely occlude the patient Y-connection of the ventilator circuit.

3. Record the expired volume (mL) and the peak inspiratory pressure during
Y occlusion (cm H O).
2
4. Divide the expired volume (mL) by the peak inspiratory pressure during Y occlusion(cm H O);
2
this is the circuit compression factor.
5. Multiply the circuit compression factor (mL/cm H O) by the peak inspiratory pressure during
2
mechanical ventilation (cm H O), or (peak inspiratory pressure–PEEP) if PEEP is used.
2
Example:

Expired volume 5 150 mL; Peak inspiratory pressure (Y occlusion) 5 50 cm H O;
2
Peak inspiratory pressure (mechanical ventilation) 5 60 cm H O; PEEP 5 10 cm H O.
2
2
Circuit compression factor 5 150 mL/50 cm H O 5 3 mL/cm H O
2
2
Circuit compression volume 5 3 mL/cm H O 3 (60 2 10) cm H O 5 3 3 50 5 150 mL
2
2
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#
V vent : Inspiratory flow of ventilator, in L/min
#
V spon : Inspiratory flow during spontaneous breathing in L/min (obtained via
flow/time graphic or estimated to be 500 mL/sec or 30 L/min)

As shown in the equation, the level of pressure support needed is partly based on
the PIP and P . For this reason, the PS level must be adjusted on an as-needed
plat
basis depending on the changing conditions that alter the PIP and P .
plat
For weaning from mechanical ventilation with a spontaneous breathing trial, PS is
titrated until achieving a spontaneous frequency of 20 to 25/min or a spontaneous
tidal volume of 8 to 10 mL/kg predicted body weight (PBW). A PSV of greater than
30 cm H O is rarely needed since these patients are typically not ready for weaning.
2
For further weaning, the PS level is reduced by 2 to 4 cm H O increments as toler-
2
ated. Extubation can be considered when the PS level reaches 5 to 8 cm H O for
2
2 hours with no signs of respiratory distress.

F O 2
I

After stabilization of the For patients with severe hypoxemia or abnormal cardiopulmonary functions
patient, the F I O 2 is best kept (e.g., post-resuscitation, smoke inhalation, ARDS), the initial F O may be set at
below 50% to avoid oxygen- I 2
induced lung injuries. 100%. The F O should be evaluated by means of arterial blood gas analyses after
2
I
stabilization of the patient. It should be adjusted accordingly to maintain a PaO
2

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