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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