Vents 101
Ted Lee,MD
Objectives
Understand the basics of vent mechanics
Describe the various modes of ventilation
Learn how to initiate mech. ventilation
Learn how to troubleshoot problems
Vent demonstration workshop with
Hank Lockridge, RCP, RRT
Basic Vent Mechanics
Ideal gas law
PV=nRT
Volume is inversely related to pressure
You can’t control both vol. and pressure
Choose one as the constant and the
other one varies.
Basically, modes of ventilation are
broken down into pressure or volume.
Pressure as the Constant
Pressure Control



Constant pressure over a preset time
Tidal Volume will vary depending on compliance
and resistance
Modes can be AC or SIMV
Pressure Support



Constant pressure at a preset flow rate
Support breaths end when flow rate decreases
from initial rate to a preset level.
Pt controls RR, which is the most comfortable.
Volume as the Constant
Volume-cycled ventilation



Tidal volume is preset
The pressure then varies
Therefore we need to set pressure alarms
Basic Modes are AC or SIMV


AC – extra breaths are at full tidal volume
SIMV – set backup rate and extra breaths
are at variable volumes based on pt effort
and pressure support.
Pros and Cons of each mode
Mode
Pros
Cons
AC
volume
-reduced WOB
-VT is guaranteed unless
AC
pressure
-reduced WOB
-allows for PIP limits
SIMV
-maybe less hyperinflation -inc WOB compared to AC
-less interference with
-uncomfortable for pt,
Pressure
support
peak pressure exceeded
-potential hyperventilation
-hemodynamic effects
-excessive insp pressures
-hyper/hypoventilation as
resistance/compliance
changes
cardiovascular fxn?
since volume varies
-most comfortable mode
-improve synchrony
-reduced WOB
-VT is variable
-pt must have intact
respiratory drive
Initiation of Ventilation
Choose a mode that will:




Provide adequate oxygenation
Reduce work of breathing
Provide synchrony b/w pt and vent
Avoid high peak/plateau pressures
Start Fi02 at 100%, then titrate down
Initial Tidal Volume of 10cc/kg
Keep Plateau pressure less than 30
Initiation of Ventilation (cont.)
Consider PEEP, start at 5 then titrate up
Provide enough PS if using SIMV
Watch out for Auto-PEEP!
Provide adequate sedation and analgesia
If you make any changes, observe
closely to see how the changes affect
your patient.
Trouble Shooting
If BP or HR suddenly drops or pt codes:



Disconnect pt from vent and start bagging
Look for mucous plug, tension ptx, etc.
Bag slowly to allow enough expiratory time
If high pressure alarm goes off:



Look for pt/vent asynchrony
Is there a better mode?
Is pt adequately sedated?
What’s the problem with the ABG?
Oxygenation (PaO2) or Ventilation (PaCO2)?
Vent dynamics affecting PaO2
Fi02
Mean Airway Pressure
PEEP
Vent dynamics affecting PaCO2
Respiratory Rate
Tidal Volume
Dead Space
Compliance vs. Resistance
Peak pressure is Dynamic Compliance
which includes airway resistance
Plateau pressure is Static Compliance
Roughly speaking, resistance is the
difference between Peak and Plateau
pressures.
Peak and Plateau Pressures
www.ccmtutorials.com
Compliance Problem
Plateau pressure is elevated > 30
Peak pressure is only slightly higher
than plateau pressure
Decrease the tidal volume as tolerated
Treat the underlying problem



Pneumonia?
Pulmonary edema?
Tension Pneumothorax?
Resistance Problem
Peak pressure is elevated
Plateau pressure is usually normal
Allow for enough expiratory time
Watch for Auto-PEEP
Treat the underlying problem



Bronchoconstriction?
Mucous plugging?
Kinked endotracheal tube?
Auto-PEEP
What is it?
Air gets in, but not all of it comes out.
“Breath Stacking” which increases
intrathoracic pressure & dec. venous return
Decrease tidal volume as tolerated
Increase expiratory time by:


Decreasing respiratory rate
Increasing inspiratory flow rate
www.ccmtutorials.com
www.ccmtutorials.com
Example of Auto-PEEP
www.ccmtutorials.com
References and Resources
Fundamentals of Critical Care Support,
Society of Critical Care Medicine
www.ccmtutorials.com