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Using Ventilator Graphics to Maximize Lung Protective Ventilation

Neil MacIntyre MD

Duke University Medical Center

Preventing VILI: Overdistention and Collapse Injury

“Lung Protective” Ventilation

V

O

L

U

M

E

Transpulmonary Pressure

Limit Distending Pressure

Add PEEP

Limit VT

Graphics and Lung Protection

•Maximal and Tidal Overstretch• Plateau vs TPP • Regional effects and VT• Intrinsic PEEP

•Collapse-Reopening injury• Balancing PO2 vs Pplat• TPP

•“Fine tuning”: PV relationships, the Stress Index

Overdistention

Transpulmonary Pressure

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TPP

30 cm H2O 30 cm H2O

5 cm H2O 20 cm H2O

30 cm H2O 30 cm H2O

Inspiration: Airway P (Pplat) is applied to lungs AND pleura

25 cm H2O 10 cm H2O

Most of Pplat dissipated across stiff lungs Most of Plat dissipated across stiff CW,Pplat approximates TPP Pplat >> TPP

High TPP, close to Pplat Low TPP, lower than Pplat

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30 cm H2O 30 cm H2O

-10 cm H2O 5 cm H2O

30 cm H2O 30 cm H2O

Inspiration: For Given Airway P, Effort Increases TPP

40 cm H2O 25cm H2O

Active Passive

High TPP, higher than Pplat Lower TPP, close to Pplat

Overdistention and VT Injury is RegionalVt diverted from injured units to overdistend healthier units

Evolving Clinical Goals: VT and Pplat

• In virtually all patients on MV:• Limit transpulm Pplat (corrected for Ccw) < 30

for sure and lower if possible• Limit VT to 6-8 ml/kg•Normal VT•Assumes lung size is normal when, in fact, it is

often a “baby lung”

Scaling VT: Ideal vs Actual Lung Size

6 ml/kg IBW

Distributes to many regions Distributes only to R ant lung

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Scaling VT: Ideal vs Actual Lung Size

6 ml/kg IBW

Distributes to many regions Distributes only to R ant lung

FRC : High Low

Cl : High Low

DP: Low High

Evolving Clinical Goals: VT and Pplat• In virtually all patients on MV:

• Limit transpulm Pplat (corrected for Ccw) < 30 for sure and lower if possible• Limit VT to 6-8 ml/kg

• Further adjust for FRC (Gattinoni)?• Vt/FRC: max VT <1-2 x FRC

• Further adjust for CL (Amato)?• Vt/Crs (=deltaP = Pplat-PEEP): <13-19 cm H2O

PEEPi affects VACV and PACV differently

VACV PACVPEEPi maintains VT, raises Pplat PEEPi reduces VT, maintains Pplat “Clamp” circuit

PEEPi = 10

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APRV: pressure target/spont breaths

Intrinsic vs Applied PEEP

Intrinsic gives longer I timebut distributes to compliantand obstructed units

Applied PEEP gives shorterI time but more evenlydistributed

? Which is best?

Graphics and Lung Protection

•Maximal and Tidal Overstretch• Plateau vs TPP • Regional effects and VT• Intrinsic PEEP

•Collapse-Reopening injury• Balancing PO2 vs Pplat• TPP

•“Fine tuning”: PV relationships, the Stress Index

Recruit Alveoli (actually prevent de-recruitment)

GOOD

BAD

PEEP is a “two edged sword”• By preventing collapse-reopening, it can reduce injury, improve

mechanics, and increase aerated lung regions – GOOD!

• If collapse-reopening not occurring, it can overdistend already aerated regions and increase injury – BAD!

• How do we do balance these?• Visual (CT, ultrasound, EIT)

• Gas exchange tables – balance the “good” (PO2) with the “bad” (Pplat and FiO2)

• Mechanical (PV curves, “best compliance”)

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PEEP-FiO2 TablesBalancing “adequate” PO2 against FiO2 and overdistention

0

4

8

12

16

20

24

PEEP

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

FIO2

Targets : PO2 55-80, Pplat < 30-35

15 cm H2O 15 cm H2O

5 cm H2O 20 cm H2O

15 cm H2O 15 cm H2O

Influence of chest wall stiffness - PEEP

10 cm H2O -5 cm H2O

Additional PEEP not needed Additional PEEP needed

Graphics and Lung Protection

•Maximal and Tidal Overstretch• Plateau vs TPP • Regional effects and VT• Intrinsic PEEP

•Collapse-Reopening injury• Balancing PO2 vs Pplat• TPP

•“Fine tuning”: PV relationships, the Stress Index

“Fine Tuning” the Vent: The Stress Index

• The stress index is the PV relationship during the tidal breath at a set PEEP

• It is driven by lung compliance changes during that breath

• A practical way to assess this:• Use a volume cycled breath with desired VT/PEEP

• Use a constant flow to minimize any flow related pressures

(ie resistance should change little over a constant flow

tidal breath)

• Measure the pressure waveform pattern over

the breath

V’

P

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Pressure

Vo

lum

e

Injury Zones

Pre

ssu

re

Volume (=Time if constant flow)

Injury Zones

Stress Index Stress Index

Possible Add PEEP Leave Alone Reduce VT or PEEPFix

Ranieri VM et al Anesthesiology 2000

A

PEEP = 5

B

PEEP = 15

C

PEEP = 25

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Summary – Using Graphics to Answer the Question:

Are the settings safe and effective?

• ABG targets:• PO2 55-80 mm Hg, pH >7.15

• Lung static strain minimal • End insp TPP (estimated by Pplat with normal Ccw

and no effort) < 30 cm H2O

• Lung dynamic strain minimal• VT < 8ml/kg IBW,

• Fine tune with stress index or DP

• PEEP-FiO2 balanced• PEEP-FiO2 Table

• Fine tune with stress index or DP

*ARDSnet targets

Goals: PO2 55-80, Pplat <30, FiO2 as low as possible*