Vents 101 Ted Lee,MD
Dec 30, 2015
Vents 101
Ted Lee,MD
Objectives
Understand the basics of vent mechanicsDescribe the various modes of ventilationLearn how to initiate mech. ventilationLearn how to troubleshoot problemsVent demonstration workshop with Hank Lockridge, RCP, RRT
Basic Vent Mechanics
Ideal gas law PV=nRTVolume is inversely related to pressureYou can’t control both vol. and pressureChoose one as the constant and the other one varies.Basically, modes of ventilation are broken down into pressure or volume.
Pressure as the ConstantPressure 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
AC volume
-reduced WOB
-VT is guaranteed unless peak pressure exceeded
-potential hyperventilation-hemodynamic effects-excessive insp pressures
AC pressure
-reduced WOB-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-reduced WOB
-VT is variable
-pt must have intact respiratory drive
Mode Pros Cons
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 downInitial Tidal Volume of 10cc/kgKeep Plateau pressure less than 30
Initiation of Ventilation (cont.)
Consider PEEP, start at 5 then titrate upProvide enough PS if using SIMVWatch out for Auto-PEEP!Provide adequate sedation and analgesiaIf you make any changes, observe closely to see how the changes affect your patient.
Trouble ShootingIf 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
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Compliance Problem
Plateau pressure is elevated > 30Peak pressure is only slightly higher than plateau pressureDecrease the tidal volume as toleratedTreat the underlying problem Pneumonia? Pulmonary edema? Tension Pneumothorax?
Resistance Problem
Peak pressure is elevatedPlateau pressure is usually normalAllow for enough expiratory timeWatch for Auto-PEEPTreat 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 returnDecrease tidal volume as toleratedIncrease expiratory time by: Decreasing respiratory rate Increasing inspiratory flow rate
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Example of Auto-PEEP
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References and Resources
Fundamentals of Critical Care Support, Society of Critical Care Medicine
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