Airway releasing ventilation

SAMIR EL ANSARY

APRVAirway Pressure Release

Ventilation

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APRVAirway Pressure Release

Ventilation

SAMIR EL ANSARYICU PROFESSOR

AIN SHAMSCAIRO

Spontaneous breaths

CPAP Level

CPAP Level 2

CPAP Level 1

CPAP Released CPAP Restored

Time

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APRV(Airway Pressure Release Ventilation)

Is a form of Bi level ventilation that utilises a very short expiratory time

for pressure release

All spontaneous breathing is done at upper pressure level

to promote lung recruitment of collapsed and poorly ventilated alveoli

The CPAP is released periodically for brief period This short release along with spontaneous

breathing promotes CO2 elimination

The release time is short to prevent the peak expiratory flow

from returning to a zero baseline

Control Trigger Limit Cycle

Pressure Time Pressure Time

Time Triggered Time-cycled Ventilation

Airway Pressure Release Ventilation

The ventilator cycles from high CPAP to low CPAP

{high lung volume to lower lung volume}

Patient can breathspontaneously at either level

The presence of a dynamic expirtatory valve in these ventilators

Allows spontaneous breathing at high lung volumes

APRV should be seen as full tidal volume ventilation

The patient is ventilated on the expiratory limb of the volume

pressure curve

These breaths can beUnsupported, pressure supported

or supported by automatic tube compensation

To restore FRC through recruitment and to maintain FRC by creating intrinsic PEEP

APRVProvide lung protective ventilation

support while minimising alveolar distension

And avoidingrepeated alveoli collapse and re-expansion.

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Conventional ventilation spends most time here

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Problem with Conventional Ventilation Strategies

• Most time spent at baseline level (set PEEP)

• This level may not be sufficient to recruit lung units

• During tidal ventilation lung units are recruited

• MAP – a factor critical for good oxygenation remains low

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Conventional ventilation spends most time here

APRV spends most time here

Insp

Exp

Airway Pressure Release

Ventilation StrategyFundamental concept of

APRV• Maintain optimal V/Q

by optimising MAP• Ventilates from point

much higher on PV curve

• Maximizes the recruitable surface of

the lung

They become important In diseased states

APRV takes advantage of the collateral channels of ventilation that are barely used at the FRC level in normal,

healthy lungs

Kohn

Lambert

Martin

•The CPAP level drives oxygenation

• The timed

releases aid in CO2 clearance

ACUTE LUNG INJURIES AND ARDS

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Oxygenation and ventilation occur predominantly within the upper and

lower inflection points

How does APRV work ?

Airway pressure release ventilation begins on the pressure-volume curve between the lower and upper inflection points

And

Uses a release, not an increase, of pressure from its baseline.

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• Sustained plateau pressurepromotes alveolar recruitment

Rational for APRV

while being maintained at an acceptable level

• The number of respiratory cycles is minimized

prevents both the repetitive opening of alveoli and alveolar stretch, that may

result in lung injury

Rational for APRV

APRV can unload inspiratory muscles–Decrease the work of breathing associated with chronic obstructive

pulmonary disease

Rational for APRV

The end-inspiratory pressurewhich equates to P High or plateau pressure

should be kept

< 35 cmH20

Rational for APRV

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The preset pressure limit

prevents or limits, over-distension of alveoli and high-volume lung injury

APRV affects tidal ventilation by decreasing rather than increasing airway pressure

Rational for APRV

Decreasing lung volume for ventilation

Limits air space over-distension

Limits low-volume lung injuryby avoiding the repetitious opening of alveoli

Rational for APRV

Parameter DescriptionP High High pressure levelT High Time at high pressure levelP Low Low pressure (release) levelT Low Time at release level

Setting for APRV

 P High The upper CPAP level

Similar to MAP (mean airway pressure ) and thus

affects oxygenation.P Low

(Also referred to as PEEP)The lower pressure setting

Terminology

Terminology

T HighInspiratory time phase for the high CPAP

 T Low

Release time allowing CO2 elimination

(T high plus T low is the total time of one cycle) 

TerminologyI:E ratio in APRV becomes irrelevant

because APRV is really best thought of as CPAP with occasional release

Inverse ratio ventilation

Initial Setting

P highMean airway pressure

20-25cm H2Oor

MAP the patient is on conventional ventilation.

Initial Setting

P lowZero cm H2O

This provides

Rapid drop in pressureMaximum delta p

It also avoids collapse during T low

Initial Setting

T HighThe inspiratory time

4.0 seconds It is progressively increased as the patient

is weaned 

Initial Setting 

T Low0.2 to 0.6 seconds

Too long a release time would interfere with oxygenation and allow alveolar collapse and

atelectasis can develop quickly

Control Settings for CO2

{ PHigh - P Low} Determines the volume exchange

therefore affecting PCO2 clearance as TV would

Optimising spontaneous ventilation will also help clear CO2

To decrease PCO2

 Decrease T-high

Increase P High

Increase T Low to allow more time for “exhalation.”

  To increase PCO2  Increase T High

Decrease P High to lower the MAP (Monitor oxygenation and avoid derecruitment)

It may be better to accept hypercapnia than to reduce P High so much that

oxygenation decreases

  To increase PaO2

Increase FiO2 Increase MAP

by increasing P High slowly(by increments of 2cm H2o at a time)

Increase T High slowlyUse recruitment manoeuvres

  Weaning From APRV

FiO2 should be weaned first Reduce P high

Increase T High gradually

The patient essentially transits to CPAP with very few releases

  Weaning From APRV

Patient should increasing his spontaneous rate to compensate

During weaning closely monitorMAP - O2 Sat

Exhaled Minute VolumeEnd Tidal CO2

  Weaning From APRV

Always reduce FiO2 before P High!

If PHigh > or = 30cmH2OReduce FiO2 to 50% before

decreasing P High

  Weaning From APRV

“Drop and Stretch”PHigh is dropped and the Thigh is stretched

out sequentially

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  Weaning From APRV

The “drop and stretch technique” is continued until

PHigh is at 10 – 15 cmH2O and the T High is at 10 -15 seconds.

At this point the patient is effectively on CPAP

“Mandatory rules“ in APRV

The expiratory timeshould be short enough to prevent

derecruitment

And long enough to obtain a suitable tidal volume

“Mandatory rules“ in APRV

The expiratory time 0.4 to 0.6 seconds

The tidal volume is your target(between 4 and 6ml/kg)

“Mandatory rules“ in APRV

If the tidal volume is inadequate

the expiratory time is lengthened

If it is too high ( > 6ml/kg) the expiratory time is shortened

“Mandatory rules“ in APRV

If you are starting off with APRV then start high (28cmH2O of less) and work your way

down

Higher transalveolar pressures recruit the lungs

“Mandatory rules“ in APRV

Low PEEP is set at 0 cmH2O The large pressure ramp allows for tidal ventilation in very short expiratory times

The inspiratory time is set at 4-6 seconds

(the respiratory rate should be 8 to 12 breaths per minute - never more)

“Mandatory rules“ in APRV

There are two different ways to wean patients from APRV

If lung mechanics rapidly return to normal the patient should be weaned to

Pressure support

“Mandatory rules“ in APRV

If ARDs is prolonged, then the high CPAP level is gradually weaned

down to 10 cmH2Oand then the patient is converted to a

standard vent wean

The pressure support {High CPAP- Low CPAP}

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“Mandatory rules“ in APRV

BiLevel Ventilation

PEEPH

PEEPL

Pressure SupportPEEPHigh + PS

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GOOD LUCK

SAMIR EL ANSARYICU PROFESSOR

AIN SHAMSCAIRO

[email protected]

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