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Objectives 

·         Define dynamic hyperinflation

·         Key Points for Vent Management

·         Discuss physiology of AutoPEEP

·         Discuss the prevalence

·         Discuss Management Strategies 

Dynamic Hyperinflation (DHI) 

·         Over distention of lung tissue due to trapped volume and pressure

·         Caused by obstructive lung pathology

·         Efforts should be made to minimize DHI  

Key Points in Vent Management

Mode: 

·         A/C - OK if used correctly, high inspiratory flows, reduce triggering effort, sedate if necessary

·         IMV - Monitor total rate as measure of comfort (should be < 30)

·         PSV - May be more comfortable for patients with an intact respiratory drive, adjust PSV levels to keep rate < 30

FiO2: 

·         Hypoxemia is usually easy to correct

·         Titrate to keep Sa0₂ > 90% during full support

·         Keep Pa0₂ 55-65 (Sa0₂ 89-90%) when attempting weaning 

Tidal Volume: 

·         Lower than normal (7 - 10 ml/kg) to avoid dynamic hyperinflation and alveolar rupture

·         Prevent the buildup of autoPEEP 

Inspiratory Flow Rates

o    Use high flows (70-100 lpm)

o    Maximize expiratory time

o    Decrease patient's air hunger

o    Decrease inspiratory work of breathing

Rate 

o    Sufficient to keep PaC0₂ 7.30 - 7.40 (avoid alkalemia)

o    Absolute PaC0₂ value less important

o    Monitor patient for autoPEEP 

Positioning 

o    Fowlers or Semi-Fowlers

o    Improve efficiency of ventilatory muscles

Weaning  

o    ASAP (1 - 2 days)

o    Avoid low IMV rates without added Pressure Support      

o    Use baseline pressure support during spontaneous trials prior to extubation

Vocabulary 

·         AutoPEEP - Undesirable positive end expiratory pressure that results from lung pathology, ventilator settings, or both.

·         Also known as inadvertent PEEP, occult PEEP, endogenous PEEP 

Intrinsic AutoPEEP 

·         Intrinsic autoPEEP - autoPEEP that results from obstructive pathology

·         Obstructive processes lead to incomplete emptying of some lung units

·         Retained volume results in increased intrapleural pressure at end expiration

Considerations 

·         May sometimes be corrected by treating the cause, (i.e. suctioning,

      bronchodilators, etc.)

·         Exists in the majority of ventilated patients with obstructive airways disease (OAD)

Mechanical AutoPEEP 

·         Mechanical autoPEEP - AutoPEEP that results from ventilator settings

·         These ventilator settings lead to incomplete emptying of some lung units

·         The result is increased intrapleural pressure at end expiration

Considerations 

·         In pressure ventilation, autoPEEP will lead to decreased Vt delivery due to an increase in FRC.

·         In volume ventilation, autoPEEP will lead to increased peak airway pressures due to an increase in FRC.

·         AutoPEEP often occurs as a result of mechanical and intrinsic problems. 

Prevalence 

·         50% Of all ventilated patients

·         100% Of all patients with Obstructive Airway Disease

·         Was 100% unrecognized without waveforms in one study

·         Post training, the overall incidence was reduced to 18%

·         Post training, mechanical autoPEEP was reduced to 4% 

Consequences 

·         Hemodynamic compromise

·         Erroneous hemodynamic measurements (wedge pressures)

·         Pulmonary barotrauma- Alveolar rupture

·         Increased inspiratory threshold load on respiratory muscles 

More About Work of Breathing 

·         Intrapleural pressure and autoPEEP must be equal for the patient to trigger the ventilator

·         AutoPEEP should be suspected with any triggering problems

·         AutoPEEP should be suspected with patient ventilator dysynchrony problems

Example: 

Measured autoPEEP level is 8 cm H₂0 Sensitivity is set at -2 cm H₂0. In this situation, the patient would have to pull -10 cm H₂0 to trigger the ventilator  

Measurement 

·         AutoPEEP is elusive and requires active investigation regardless of the cause

·         Levels can be greatly increased with no indication on the airway pressure manometer

·         Should always be suspected when ventilating any patient with Obstructive Airway Disease (OAD)

Connections 

·         Remember waveforms - Graphics analysis is one of the keys to the detection of autoPEEP 

Measuring AutoPEEP 

Two types: 

·         Direct Measurement

·         Indirect Measurement 

Direct Measurement 

·         Esophageal balloon catheters measure true auto PEEP levels

·         Esophageal pressure drops that do not result in upper airway pressure changes

Advantages to Esophageal Balloon 

·         Catheter enables numeric assessment of work of breathing

·         Allows for titration of optimal work load when weaning the patient

·         Also provides numeric assessment of strength and endurance 

Disadvantages to Direct Measurement 

·         Esophageal balloon catheters can be very cumbersome to place 

Indirect Measurement of AutoPEEP 

·         Use of an expiratory hold maneuver allows pressure in the respiratory system and vent circuit to become equal

·         Expiratory Hold Advantage

·         Expiratory hold maneuvers are easy to perform and are non-invasive 

Disadvantages 

·         Air trapped areas may not fully equilibrate with upper airway pressure

·         AutoPEEP levels may be underestimated due to circuit volume distensibility

·         Patient with spontaneous effort makes accurate measurement difficult 

Intervention Strategies 

·         Try to reduce intrinsic autoPEEP with aggressive pulmonary toilet

·         Larger diameter ET tube

·         Optimize ventilator settings to allow for as much expiratory time as possible:

o    Decrease rate

o    Increase inspiratory flow 

More Strategies: 

·         Equilibrate autoPEEP levels with applied peep

·         Use 75% of autoPEEP level as amount for applied PEEP

·         Why 75%?

·         To ensure FRC is not increased 

Effects of Applied PEEP 

·         Reduced inspiratory threshold load on inspiratory muscles

·         Decreased spontaneous drive

·         All pressure results in gas delivery improves apparent lung compliance Peak pressures will not increase

·         FRC will not increase

AutoPEEP 

·         Who has the necessary tools to make a difference?

·         Respiratory therapists!

Case Study 

·         73 Year old female with Hx of COPD, intubated due to respiratory failure

• Placed in pressure assist control mode

·         Noted to have inspiratory efforts that did not trigger the ventilator

·         Flow, airway pressure, and esophageal pressure tracings demonstrated 18 cm H₂0 intrinsic PEEP

·         Esophageal pressure dropped 18 cm H₂0 before changes in airway pressure occurred 

·         Varying levels of applied PEEP were added in order to equilibrate autoPEEP

·         Care was taken to avoid excessive peep which would increase FRC

·         Exhaled tidal volumes were monitored closely

Careful application of applied PEEP is beneficial to decrease work of breathing

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