1) basic respiratory mechanics relevant for mechanical ventilation

8/19/2019 1) Basic Respiratory Mechanics Relevant for Mechanical Ventilation

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Peter C. Rimensberger, MDProfessor of Pediatrics, Critical Care and Neonatology

Service of Neonatology and Pediatric Intensive CareDepartment of Pediatrics

University Hospital of GenevaGeneva, Switzerland

t

t

t

t

t

t

t

t

Volume Change

Time

Gas Flow

Pressure Difference

Basic respiratory mechanicsrelevant for mechanical ventilation

Volume

Pressure

V

P

C = V P

Compliance

CRS influences V T

PEEP PIP

V o

l u m e

Surfactant

for example with anintervention likesurfactant therapy:

The Feature of the Tube

Airway Resistance

Pressure Difference = Flow Rate x Resistance of the Tube

Resistance is the amount of pressure required to deliver a givenflow of gas and is expressed in terms of a change in pressuredivided by flow.

R = PFlow

P1 P2


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Mechanical response topositive pressure application

Active inspiration

Passive exhalation

R = ! P / V (cmH2O/L/s)

Equation of motion: P = (1 / C rs ) V + R rs V.

.

Mechanical response topositive pressure application

Compliance: pressure - volume

Resistance: pressure - flow

C = V / P (L/cmH2O)

Pressure – Flow – Time - Volume

Time constant: T = Crs x Rrs

Volume change requires time to take place. When a step change in pressure is applied, the instantaneous change involume follows an exponential curve, which means that, formerly faster, itslows down progressively while it approaches the new equilibrium.

P = ( 1 / Crs ) x V + Rrs x V.

Time constant: T = Crs x Rrs


Will pressure equilibrium

be reached in the lungs?

A: Crs = 1, R = 1B: Crs = 2

(T = R x 2C)

C: Crs = 0.5(T = R x 1/2C)(reduced inspiratorycapacity to 0.5)

D: R = 0.5(T = 1/2R x C)

E: R = 2(T = 2R x C)

Effect of varying time constants on the change

in lung volume over time(with constant inflating pressure at the airway opening)

A: Crs = 1, R = 1

C: Crs = 0.5(T = R x 1/2C)(reduced inspiratorycapacity to 0.5)

E: R = 2(T = 2R x C)

Ventilators deliver gas tothe lungs using positivepressure at a certain rate .

The amount of gasdelivered can be limited by time, pressure orvolume.

The duration can be cycled by time, pressure or flow.

Volume Change

Time

Gas Flow

Pressure Difference


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Nunn JF Applied respiratory physiology 1987:397

( )

Understanding Time Constants at the bedside

! All about the FLOW wave form

What would you do here?

TiTe

Optimizing tidal volume delivery at set pressure

Lucangelo U, Bernabe F, and Blanch L Respir Care 2005;50(1):55– 65

Flow termination and

auto-PEEP detection

PEEP i

Trappedvolume

Dhand, Respir Care 2005; 50:246

Correction for Auto-PEEPgive bronchodilators and/or increase expiratory time


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Appropriate inspiratory time

Vt 120 ml

Vt 160 ml Vt 137 ml


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Zero flow Zero flow Zero flow

Total insptime

Total insptime

Total insptime

Aim for zero flow condition being less than ! of total insp. time

How long should you set the Ti ?

Alveolar

Plateau

Brown, DiBlasi Respir Care 2011;56(9):1298 –1311.

How long should you set the Ti ?The patient will guide you !

Proximal Airway Pressure

Alveolar Pressure

Look at the flow – time curve !

P

V.

Cave! Ti settings in presence of anendotracheal tube leak

Flow

LeakTime

The only solution in presenceof an important leak:Look how the thorax moves

Time

Volume

Leak Vex < Vinsp

Who

s Watching the Patient?

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

Flow also influences Ti


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Flow alters rate of change in lung volume:Rheotrauma

• Sheer Stress is a stress parallel to thesurface of interest (strain)

• An important cause of VILI, especiallyin circumstances of atelectasis and/orhigh flows

Adapted from Jane Pillow

Progressive increase in inspiratory time


CAVE: you have to set Frequency and Ti (Control) or Ti and Te (TCPL)

time

f l o w

insufflation

exhalation

0

auto-PEEP

Premature flow termination duringexpiration = gas trapping =deadspace (Vd/Vt) will increase

Expiratory flow waveform:normal vs pathologic

Expiration is important too

Premature flow termination during expiration = gas trapping

Auto-PEEP Analysing time settings of the respiratory cycle

1) Too short Te " intrinsic PEEP 1) Correct Te

2) Unnecessary long Ti 2) Too short Ti


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V o

l u m e

PressurePressure

V o

l u m e

Pressure Ventilation Volume Ventilation

Decreased Tidal Volume Increased Pressure

ComplianceCompliance

Decreased PressureIncreased Tidal Volume

and do not forget, the time constant (T = Crs x Rrs) will change too!

Compliance decrease


0.4 (s) 0.35 (s) 0.65 (s) 0.5 (s)

52 cmH2O 33 cmH2O

Change in compliance after surfactant =change in time constant after surfactant

PEEP PIP PEEP PIP

pre post

V o

l u m e

Adapted from Kelly E Pediatr Pulmonol 1993;15:225-30

C dyn did not change, C stat improved with surfactant

C stat

C stat

Cdyn

Cdyn

PEEP PIP

post

C stat

Cdyn

Effect of high airway resistance on flow

Time (ms)

0 100 200 300 400 500 T i d a

l V o

l u m e

( m L / k g

)

0

2

4

6

Time (ms)

0 100 200 300 400 500 T i d a

l V o

l u m e

( m L / k g

)

0

2

4

6

Time (ms)

0 100 200 300 400 500 T i d a

l V o

l u m e

( m L / k g

)

0

2

4

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+,-,#, ./0+ 123%&.),$,$4#%), 5.6,%6,7! !"#&%- ()*+(, - ." !"# &%$ +(, - .*()*"/+

82#").9 &()* 5.6,%6,! !"#&%0 ()*+(, - ." !"# &%-1 +(, - .*2*()*"/+

Model: Ventilation with a pressure of 12 cmH2O overPEEP in various conditions (respiratory pathologies anddisease stages) in a neonate

courtesy (modified) from Jane Pillow, Perth

3 – 5 time constants

2


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Volume Guarantee: New Approaches in Volume Controlled Ventilation for Neonates. Ahluwalia J, Morley C, Wahle G. Dräger Medizintechnik GmbH. ISBN 3-926762-42-X

Too short Te will not allowto deliver max. possibleVt at given P

" will induce PEEPi" increases the risk for

hemodynamic instability

Too short Ti willreduce delivered Vt

" unneccessary highPIP will be applied" unnecessary highintrathoracic pressures

The patient respiratory mechanics dictate the maximal respiratory frequency

Time constant: = Crs x Rrs

To measure compliance andresistance - is it in the clinicalsetting important ?

Use the flow curve for decision makingabout the settings for respiratory rate andduty cycle in the mechanical ventilator

The saying we ventilate at 40/min or with a Ti of 0.3 is a testimony of no understanding !

AlveolarPressure

AirwayPressure

Residual Flows

Avoid ventilation out of control !

t

t

t

t

t

t

t

t

Modes of Ventilation

• Controlled – not synchronized – IMV

• Controlled - synchronized – AC / PC – SIPPV / SIMV

• Assisted - synchronized – Pressure Support / NAVA

• Pressure versus Volume Ventilation – PC / VC / VG / TTV

• High Frequency Ventilation – HFOV / HFJV / HFPPV


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Pressure Control- A/C or PTV (VAC)

Assist/Control

Inspiration: Flow or pressuretriggered

Breath Initiation (Inspiratory Triggering)

IMVControlled

– not synchronized

IMV

IMV versus SIMV

SIMV

SIMV versus IMV in neonatal ventilation

Conclusions: We found that SIMV wasat least as efficacious as conventionalIMV, and may have improved certainoutcomes in BW-specific groups.

Bernstein G et al. J PEDIATR 1996;128:453 -63 Greenough A et al. Cochrane Database of Systematic Reviews 2004, Issue 3. CD000456.

" Better entrainement of respiratory muscles ?

Observational study conducted in 349 ICUs from 23 countries

Propensity score stratified analysis to compare 350 patientsventilated with SIMV-PS with 1,228 patients ventilated with A/Cventilation.

Primary outcome was in-hospital mortality.

No significant differences in in-hospital mortality After adjustment for propensity score, overall effect of SIMV-PS versus A/C on in-hospital mortality was not significant(OR, 1.04; 95% CI, 0.77-1.42; P=0.78).

Ortiz et al. CHEST 2010; 137(6):1265–1277

Outcomes of Patients Ventilated With SIMV + PSA Comparative Propensity Score Study


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Effects of NIMV, synchronized NIMV (S-NIMV), and NCPAPon ventilation, gas exchange, and infant-ventilator interaction

Chang H-Y, Claure N et al. Pediatr Res 69: 84–89, 2011

Non-synchronized NIMV

Synchr onizedNIMV

Synchronized intermittent NIMV

Effects of NIMV and synchronized NIMV (S-NIMV) onventilation, gas exchange, and infant-ventilator interaction

Chang H-Y, Claure N et al. Pediatr Res 69: 84–89, 2011

Patient-Ventilator Asynchrony with SIMV(in a Traumatically Injured Population)

SIMV with set breathing frequencies of > 10 breaths/minwas associated with increased asynchrony rates (85.7% vs14.3%, p= 0.02).

" No difference in ventilator days, ICU or hospital stay,

percent discharged home, or mortality between theasynchronous and non-asynchronous subjects.

Robinson BR et al. Respir Care 2013;58(11):1847–1855

Pressure Control- A/C or PTV

Assist/Control

FSV

PSV



Pressure Support

Breath Initiation (Inspiratory Triggering)

Pressure Control- A/C or PTV

Assist/Control

FSVPSV

Inspiration: Flow triggeredExpiration: Flow cycled

Inspiration: Flow triggeredExpiration: Time cycled

Pressure Support

Breath termination (Cycling off)

Pressure Ventilation - Waveforms

Flow

Pressure

Tinsp.PIP

PeakFlow

25%

Pressure Control Pressure Support

Inspiratory time or cycle off criteria

Ti given by thecycle off criteria

Pressure Control versus Pressure Support

Ti set


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Flow Termination Sensitivity

Nilsestuen J Respir Care 2005; 50:202-232

; : .

. . ; :

:

I

I .

I

. ; ;

Early flow termination will reduce delivered Vt

The non synchronized patient during Pressure-Support(inappropriate end-inspiratory ow termination criteria)

Nilsestuen J Respir Care 2005;50:202–232.

Pressure-Support and ow termination criteria

Pressure-Support and ow termination criteria

Increase in RR, reduction in VT, increase in WOB Nilsestuen JRespir Care 2005

Termination Sensitivity = Cycle-off CriteriaHow to set in presence of a leak ?

Flow

Peak Flow (100%)

TS 5%

Tinsp. (eff.)

Leak

Set (max)Tinsp .

Time

TS 30%


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F l o w

% of MAX Flow

PROS

• Adapts to Patientsphysiology as itchanges

• Adapts to Ti• Set max Ti• Delivers better V T

CONS

•

Leak• Asynchrony- Termination Sensitivity too

high = breath starving- Termination Sensitivity too

low = breath stack• Caution using PSV in devices with

a FIXED Termination Sensitivity• COMPLICATED

PSV: Termination Sensitivity

Leak Cycle off with changes in mechanics

10%

Flow

Time

T1

T2

T3

Compliance• Describes the ability of the

respiratory system to moveduring an applied pressure

• CRS = ! V/ ! P• Slope of the P-V curve

CRS

P-V loops and C RS

Improved C RS

WorseCRS

GoodCRS =Zoneof VT

Overdistension =Bad C RS

Atelectasis =Bad C RS


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Why isnt the P-V relationship linear?

Ventilator pressures

Alveolar volumeOpening and closing

pressures

Superimposed pressure

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c m H

2 O

InflationLimb

DeflationLimb

Crotti AJRCCM 2001

Salazar & Knowles JAP 1964

VOLUME HISTORY Oxyg

Lung Volume+ Diffusion andperfusion

Key points: Basic respiratory mechanicsduring mechanical ventilation

• An understanding of time constants, complianceand resistance will help you individualise treatmentirrespective of modality

• General principles of relationships between

inspiratory/expiratory times and V T are: – Absolute max V T is determined by C RS and DP – Ti relative to t determines what max V T is delivered

• Flow and pressure waveforms assist at thebedside unlocking the mechanics of the lung

What do airway pressures mean?

P alv = P pl + P tp

Where Ptp is transpulmonary pressure, Palv is alveolar pressure, and Ppl is pleural pressure

P tp = P alv - P pl

Pplat 25 cm H 2O,PCV

Pplat = Palv;Pplat " Transpulmonary Pressure (Ptp)

0 cm H 2O

P tp = 25




+10 cm H 2O0 cm H 2O

P tp = 25 P tp = 15


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P tp = P aw x [E cw / (E tot )]

when airway resistance is nil (static condition)P aw = P Pl + P tp

E tot = E L + E cw

Gattinoni LCritical Care 2004,8: 350-355

soft stiff


Pplat 25 cm H 2O,PCV (PSV)

Active inspiratory effort



+10 cm H 2O0 cm H 2O -10 cm H 2O

P tp = 25 P tp = 15 P tp = 35

Agostini E J Appl Physiol 14:909-913, 1959

Neonates require in general lower pressurethan children and adults because of respiratory

system behaviour and not because they are small


. V o r V

P or V

Loops

V /

P / V

. tCurves

but we still have to decide about respiratory rate, tidalvolumes, pressure settings …

… that have to be adapted to the patients respiratorymechanics

The ventilator display can help us….

1) basic respiratory mechanics relevant for mechanical ventilation

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