1. Vents 101
Ted Lee,MD

2. 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

3. 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.

4. 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.

5. 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.

6. Pros and Cons of each mode
Mode Pros Cons
AC volume
reduced WOB
VT is guaranteed unless peak pressure exceeded
potential hyperventilation
hemodynamic effects
excessive insp pressures
AC pressure
allows for PIP limits
hyper/hypoventilation as resistance/compliance changes
SIMV
maybe less hyperinflation
less interference with cardiovascular fxn?
inc WOB compared to AC
uncomfortable for pt, since volume varies
Pressure support
most comfortable mode
improve synchrony
VT is variable
pt must have intact respiratory drive

7. 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

8. 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.

9. 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)?

10. Vent dynamics affecting PaO2
Fi02
Mean Airway Pressure
PEEP

11. Vent dynamics affecting PaCO2
Respiratory Rate
Tidal Volume
Dead Space

12. 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.

13. Peak and Plateau Pressures
                                                                                                                                                     

14. 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?

15. 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?

16. 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

19. Example of Auto-PEEP

20. References and Resources
Fundamentals of Critical Care Support, Society of Critical Care Medicine