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Using the Ventilator to Probe

Physiology: Monitoring Graphics and

Lung Mechanics During Mechanical

Ventilation

Dean Hess, PhD, RRT

Massachusetts General HospitalBoston, MA

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Respiratory Care, January and February, 2005

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Monitoring Respiratory Mechanics

• Pressure, flow, and volume in ventilator circuit• Esophageal (pleural) and gastric (abdominal) pressures

• Derived measures

– Compliance

– Resistance• Time-based graphics (waveforms)

– Pressure

– Flow

– Volume• Loops

– Pressure volume

– Flow volume

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Pres

sure

time

PIP:complianceresistancevolumeflowPEEP

PEEP

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PEEP

PIP

Pplat

resistanceflow

compliancetidal volume

No active breathing

Treats lung as single unit

end-inspiratory

alveolar pressure

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Nilsestuen, Respir Care 2005; 50:202-232

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PIP

Pressure(cm

H2

O)

Volume

(mL)

Flow

( L/min)

Lucangelo, Respir Care 2005; 50:55

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Pplat 30 cm H2O

transpulmonarypressure = 15 cm H2O

Pplat = Palv;

Pplat = Transpulmonary Pressure?

+15 cm H2O

Stiff chest wall

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PCV 20 cm H2O,PEEP 10 cm

H2O; Pplat 30 cm

H2O

-15 cm H2Otranspulmonarypressure = 45 cm H2O

Active inspiratory effort

Pplat = Palv;


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Pplat 30 cm H2O,

VCV

Pplat 30 cm H2O,PCV

Active inspiratory effort

Pplat 30 cm H2O,VCV

Pplat = Palv;


Risk of VILI may be different with the same Pplat

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normal 100 mL/cm H 2 O

C =Pplat - PEEP

tidal volume

Respiratory System Compliance

• mainstem intubation

• congestive heart failure

• ARDS

• atelectasis• consolidation

• fibrosis

• hyperinflation

• tension pneumothorax

• pleural effusion

• abdominal distension• chest wall edema

• thoracic deformity

Decreased with:

Correct for gas compression

Total PEEP

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ΔPeso ≈ ΔPpl

Benditt, Respir Care 2005; 50:68

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Full Ventilator Support

Ccw = VT /ΔPeso

= 350 mL/5 cm H2O

= 70 mL/cm H2O

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Inhalation Exhalation

Inhalation Exhalation

positive

pressureventilation

18

10

mm Hg

Chest wall compliance

spontaneousbreathing

18

10

mm Hg

Inspiratory muscle effort

Br J Anaesth 1976;48:474; Respir Physiol 1977;31:63; Crit Care Med

1983;11:271; Eur Respir J 1988;1:51; Chest 2002; 21:533-538

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Increased with:

Ri = PIP - Pplat

flow

Inspiratory Resistance

• Secretions

•Bronchospasm

• Small endotracheal tube

Normal: 5 - 10 cm H2O/L/s for intubated ventilated adults

measure with 60 L/min (1 L/s)

constant flow

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PIP

Palv(Pplat)

Ppl(Peso)

Crs = Pplat - PEEP

tidal volume

Ccw =∆ Peso

tidal volume

CL =(Pplat – PEEP) - ∆ Peso

tidal volume

Ri = PIP - Pplatflow

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Pressure-Controlled Ventilation

Increasing airways resistance

Decreasing lung compliance


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Pressure-Controlled Ventilation


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set PEEPauto PEEP

pressur

e

time

PIP PIP

No active exhalation or inspiratory effort

Treats lungs as single compartment

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Leatherman, Crit Care Med 1996; 24:541

auto-PEEP of

5 cm H2O byocclusiontechnique

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Problems with Auto-PEEP

• Increased Pplat and over-distention

– Increase work-of-breathing

– Hemodynamic effects

– Pneumothorax

• Difficulty triggering ventilator

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sensitivity

-1 cm H2Oauto-PEEP

10 cm H2O

trigger effort = 11 cm H2O

sensitivity

-1 cm H2Oauto-PEEP

3 cm H2

O

trigger effort = 4 cm H2O

PEEP

7 cm H2O

PEEP

10 cm H2O

PEEP

10 cm H2O

Auto-PEEP should be measured with set PEEP = 0

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time

flow inhalation

exhalation

0

auto-PEEP

Flow Waveform

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flow

(L/s)

volume

(L)

Paw

(cm H2O)

Peso

(cm H2O)

1

0.8

30

20

0

0

0

0estimation of

auto-PEEPmissed

trigger effort

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time

Volum

e

leak

Volume Waveform

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0 10 20 30 400

0.4

0

.8

1.2

1.6

normal

ARDS

airway pressure (cm H2O)

vo

lumeab

ov

eF

RC(liter

s)

lower inflection

point

upper inflection

point

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Issues with PV Curves• Requires sedation and often paralysis• Difficult to identify “inflection points”

Harris et al, AJRCCM 2000; 161:432

• May require esophageal pressure to separate lung

from chest wall effectsMergoni et al, AJRCCM 1997; 156:846

Ranieri et al, AJRCCM 1997; 156:1082

• Deflation limb may be more useful than inflation limbHolzapfel et al, Crit Care Med 1983; 11:561

Hickling, AJRCCM 2001; 163:69• Pressure-volume curves of individual lung units

unknownHickling, AJRCCM 1998; 158:194

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Dhand, Respir Care 2005; 50:246

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Summary

• Assessment of mechanics is useful inmechanically ventilated patients: PIP, Pplat,

auto-PEEP, Ccw, Pdi

• Assessment of mechanics provides insightsinto the pathophysiology of the lungs

Flow

Pre

ssure

time

time

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Who’s Watching the Patient?

Pierson, IN: Tobin, Principles and Practice of Critical Care Monitoring

03.01.05

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