Pressure Control review
·
Low lung compliance
·
When high flow rates are needed
·
Oxygenation difficulties (> 10 cm H2O peep)
·
Patient/ventilator dysynchrony
VCV vs. PCV
|
Volume Control |
Pressure Control |
|
·
Flow is preset
·
Flow waveform is preset
·
Volume is constant
·
Pressure is variable |
·
Flow is variable
·
Decelerating flow wave
·
Time cycled
·
Pressure is constant
·
Volume is variable |
·
Peak pressures are controlled
·
Tidal volumes are variable
·
Appropriate minute volume alarm settings are critical
·
Flow is variable and decelerating
·
Flow is not limited, leads to better synchrony
·
Rapid initial flow fills large airways first, smaller airways
then fill with a more laminar flow
Computer determined flow
·
Flow rate is variable
·
Determined by
o
Compliance
o
Resistance
o
Patient effort
·
Higher mean, with lower peak airway pressure
·
Increases oxygenation
·
Reduces ventilator induced lung injury by reducing shear forces
·
Decelerating flows are more physiologic
Improved gas distribution
·
Research documents benefits of high flows
·
Decelerating flow pattern causes less turbulence
·
Flow reaches small airways earlier
·
More time for distribution to low compliance areas
I Time
·
Inspiratory time is a primary control of oxygenation
·
Inspiratory time should be set so that flow returns to zero
before end expiration at a minimum
·
May be extended to increase paO2, sometimes inverse I:E
·
Be cautious with higher rates (>25)
·
With extended Ti, auto peep may build leading to reduced Vt
delivery
·
Watch ETCO2
·
Remember graphics analysis:
|
In pressure controlled ventilation, volume delivery
can be drastically different at any given level of
pressure control, depending on the patient's compliance
and resistance. |
 |
|
When ventilating fast spaces with pressure control,
lengthening inspiratory time will not increase tidal
volume if inspiratory flow returns to baseline as
indicated by the arrow. In this case the tidal volume
can only be increased with an increase in the level of
pressure control. |
 |
|
Since expiratory flow quickly returns to baseline in
fast space, much higher respiratory rates can be
tolerated without air-trapping. If air-trapping is
desired as in inverse ratio ventilation, inspiratory
time should be lengthened while observing flow
waveforms. It is better to fix the I:E ratio rather than
the inspiratory time when providing inverse ratio
ventilation. Otherwise, it is preferable to fix the
Inspiratory Time to minimize fluctuations in volume
delivery. |
 |
|
When ventilating slow spaces, inspiration typically
ends before inspiratory flow has returned to baseline.
Lengthening inspiratory time will increase volume
delivery. However, greater inspiratory volumes will
require more time for exhalation and lower respiratory
rates are necessary to prevent unwanted air-trapping. |
 |
PCV Disadvantages
·
Close monitoring is required
·
Adjustments may not be made at the most opportune time
·
May require increased staff education
·
Changes in compliance cause changes in volume delivery
·
To maintain pressure control and effective MAP, Ti is extended
·
Patient can breathe in during inspiration, but not breathe out
·
Long Ti = paralysis or deep sedation for patient compliance
Inherent Dangers of Paralytics
·
Immediate
o
Ventilator disconnect
·
Intermediate
o
Edema, hypostasis
o
Bedsores, venous thrombosis
·
Delayed
o
Prolonged paralysis
o
Muscle atrophy
Paralytics: The Two-Edged Sword
·
Patient is paralyzed to facilitate ventilation
·
Persistent muscle paralysis after paralytic drug withdrawn
o
Weaning from mechanical ventilation delayed
o
Weakness persists for 3 to 6 months
Effects of Prolonged Paralysis
·
Extended mechanical ventilation
o
Increased risk for nosocomial infection
o
Increased risk for ventilator induced injury
·
Extended ICU stay
·
Extended rehabilitation on general nursing floor
The Cost Factor
·
ICU operating cost for ventilated patient: $1500-$2500 per day
·
Post NMBA paralysis can add up to 7 days on ventilator
·
Considerable cost increase for additional rehab time on floor
Conclusion
·
Conventional PCV carries many advantages for the critically ill
patient
·
Is labor intensive
·
Often requires patient sedation for optimal ventilation
·
Newer “Hybrid” modes are now available
Supporting Literature
·
“Changing from VCV to PCV was associated with significant
improvement in PaO2, oxygen delivery and tissue oxygen
consumption. Peak inspiratory pressure fell. There were no
significant changes in cardio-respiratory values, such as BP,
not in ventilatory measurements, such as mean airway pressure,
associated with use of PCV.”
Edward Abraham, M.D.
Chest 1990; 98:1445-49
·
“Peak airway pressure was consistently lower in patients
randomized to pressure-limited ventilation. The use of
pressure-limited ventilation also was associated with a more
rapid increase in static compliance. There was a trend toward
more rapid normalization of CO2 minute excretion.
Pressure-limited treated patients who survived required fewer
days of mechanical ventilation”
Steven H. Rappaport, M.D.
Critical Care Medicine 1994, vol. 22, no. 1
Also see
Volume Control vs
Pressure Control Modes
