Model-based elastance and
optimal peep selection
Geoffrey M Shaw1
Yeong Shiong Chiew2
J Geoffrey Chase2
Ashwath Sundaresan2
Thomas Desaive3
1
2
3
Dept of Intensive Care, Christchurch Hospital
Dept of Mechanical Engineering, University of Canterbury
Thermodynamics of irreversible processes, Institute of Physics, University of Liege, Belgium
Department of Intensive Care
NZ ANZICS Dunedin March 15 2013
Declarations
Member, Medical Advisory Board, Baxter NSW
Director, Lifevent Medical
Hold world-wide patents on: Low flow CPAP device
Multi-modal high frequency ventilator
What do marriage and
ARDS have in
common?...
You only have a 50%
change of getting out
of either alive!
Introduction
Methods
Results
Discussion
Future Work
Acute lung injury /respiratory distress syndrome
10 to 80 cases per 100,000 persons per year
Mortality rate for ARDS: 30% to as much as
80%
In the US:
190,600 cases of ALI, including ARDS, per year
74,500 associated deaths per year
3.6 million hospital hours per year
Thus, ~12000 in the ANZ ICUs with ~4800
associated deaths
http://www.meddean.luc.edu/lumen/MedEd/medicine/pulmonar/diseases/pdis3.htm
Introduction
Methods
Results
Discussion
Future Work
Acute lung injury /respiratory distress syndrome
Inflammation
Capillary leak
Alveolar collapse
http://medicinembbs.blogspot.co.nz/2011/02/obstructive-vs-restrictive-lung.html
Introduction
Methods
Results
Discussion
Future Work
Superimposed pressure
Opening
Pressure
Inflated
0
Small Airway
Collapse
10-20cmH2O
Alveolar Collapse
(Reabsorption)
40-60cmH2O
Consolidation
(modified from Gattinoni)

Introduction
Methods
Results
Discussion
Future Work
Acute lung injury /respiratory distress syndrome
Introduction
Methods
Results
Discussion
Future Work
Current practice based on one size fits all...
Introduction
Methods
Results
Discussion
Future Work
Current practice based on one size fits all...
Introduction
Methods
Results
Discussion
The conundrum......
PEEP
Too high: Injure
healthy lung tissue
Too low: No lung
recruitment
Future Work
Introduction
Methods
Results
Discussion
Future Work
Aim...
To develop a model-based solution to guide PEEP
selection in mechanical ventilation that:
Predicts patient-specific response to treatment
Balances risks of overstretch vs. derecruitment
Monitors disease state
Optimises work of breathing
Introduction
Methods
Results
Discussion
Future Work
Study design
Ten patients with ALI/ARDS (PaO2/FiO2 = 150-300)
Protocolised recruitment manoeuvre; PEEP increased in 5 cmH2O
increments until Paw ≥ 45 cmH2O
PEEP
30
cmH2O
25
20
15
10
5
10 breaths
Time
Introduction
Methods
Results
Discussion
Study design
Participant is sedated and paralysed for duration
of RM
PB 840 ventilator; Vt 400-600 ml
Pnuemotachometer to measure pressure & flow
(Hamilton Medical, Switzerland flow sensor)
National Instruments USBB6009 and Labview
Signal Express to obtain measurements at
100Hz
(National Instruments, TX, USA)
Analysis performed using MATLAB
(The Mathworks, Natick, Mass, USA)
Laptop (Dell)
Future Work
Introduction
Methods
Results
Discussion
Future Work
Model-based analysis
Validated relevant recruitment model using single
compartment
Captures patient-specific fundamental lung mechanics
in real-time to identify :
Constant lung elastance (E lung)
Dynamic lung elastance (Edrs )
Compliance = 1/Elastance
Introduction
Methods
Results
Discussion
Future Work
Some scary maths on next side, look away if you wish
Introduction
Methods
Results
Discussion
Future Work
Model-based analysis
Lung Component
Airway Component
R
Paw
=
Paw (t) =
Elung V + Rlung Q + PEEP
Edrs (t)V (t) + Rlung Q (t) + PEEP
Introduction
Methods
Results
Discussion
Future Work
Model-based PEEP selection
During each breath Elung will fall if new lung volume is recruited
faster than the pressure builds up  RECRUITMENT
If little or no recruitment occurs Elung rises with PEEP, because at
that pressure level there is no further recruitment; recruited
lung is now beginning to stretch
Hence recruitment and potential lung injury can be balanced by
selecting PEEP at minimal Elung
Edrs allows this change to be seen within a breath providing a
more detailed view.
Introduction
Methods
Results
Discussion
Future Work
Three approaches to PEEP selection
Minimum Elung and Edrs
Over all peep levels (and pressure for Edrs)
Minimum Edrs Area.
Integrating Edrs over each breath for each PEEP level is more
clinically relevant and is proportional to WOB
Inflection Method
The PEEP that corresponds to the point where Edrs or E lung is
105-110% of the minimum as the point of inflection where
there are diminishing returns
Introduction
Methods
Results
Discussion
Future Work
Patients
Patients
Sex
Age (year)
1
F
61
2
M
3
Clinical Diagnostic
P/F Ratio
(mmHg)
FiO2
Peritonitis, COPD
209
0.35
22
Trauma
170
0.50
M
55
Aspiration
223
0.35
4
M
88
Pneumonia, COPD
165
0.40
5
M
59
Pneumonia, COPD, CHF
285
0.40
6
M
69
Intra-abdominal sepsis, MOF
280
0.35
7
M
56
Legionnaires
265
0.55
8
F
54
Aspiration
303
0.40
9
M
37
H1N1, COPD*
193
0.40
10
M
56
Legionnaires, COPD*
237
0.35
Introduction
Methods
Results
Discussion
Edrs
*APE = Absolute Percent Fitting error
Similar results for Elung (APE = 5.9 %) and Edrs Area
Future Work
Introduction
Methods
Results
Discussion
Future Work
Dynamic lung elastance Edrs vs. PEEP
Pt 2: (Trauma)
Pt 6:
(Abdominal
sepsis, CHF)
Pt 8: (Aspiration)
Pt 10:
(Legionnaires,
COPD0
Introduction
Methods
Results
Discussion
Future Work
Constant lung elastance Elung vs. PEEP
Pt 2:
(Trauma)
Pt 6:
(Abdominal sepsis, CHF)
Minimal Elastance PEEP
= 15cmH2O
Minimal Elastance PEEP
= 15cmH2O
Inflection PEEP
= 6~9cmH2O
Inflection PEEP
= 7.5~10cmH2O
Pt 8:
(Aspiration)
Pt 10:
(Legionnaires, COPD)
Minimal Elastance PEEP
= 25cmH2O
Minimal Elastance PEEP
= 20cmH2O
Inflection PEEP
= 12~18cmH2O
Inflection PEEP
= 12~15cmH2O
Similar results for Edrs Area
Introduction
Methods
Results
Discussion
Response to PEEP in H1N1
Future Work
Introduction
Methods
Results
Discussion
Future Work
Low heterogeneity = recruitment
PEEP (cmH2O)
5
10
15
Edrs (cmH2O/L)
Median [IQR]
40.5
[36.4-52.8]
39.9
[35.8-48.7]
31.2
[30.2-33.6]
E drsArea (cmH2Os/L)
55.2
51.3
38.3
Elung (cmH2O/L)
39.1
38.2
31.1
•
Reduced Edrs range with increased
PEEP shows improved heterogeneity
Introduction
Methods
Results
Discussion
Future Work
Clinical vs. modelled Edrs Area (inflection) PEEP
Modelled PEEP
10
Clinical PEEP
9
Patient No.
8
7
6
5
4
3
2
1
0
5
10
15
PEEP (cmH2O)
20
25
In all but one patient (pt 2), clinically-selected PEEP was significantly
less than a model-based estimate using minimal elastance
Introduction
Methods
Results
Discussion
Future Work
Limitations
Uses a single compartment model, so results are for the whole
lung. Thus regional differences are not detected
Patients required sedation/paralysis. Model assumes pleural
pressure = 0, which may not always be valid
Elung , Edrs are effective respiratory system elastance values; within
these metrics are “unmeasurables” such as regional
resistances within the lung
Requires validation in prospective clinical trials
Introduction
Methods
Results
Discussion
Future Work
Summary (1)
This model-based approach provides patient-specific insight not
easily directly measurable
Method can be easily implemented with minimal PEEP titrations.
Currently PEEP is selected on descending limb of RM
Obviates the need to use a “one-size-fits-all “maximal recruitment
technique, typically with pressures up to 55cmH2O
Modest changes in PEEP will detect Edrs changes
Can be used to trend changes in lung condition
Introduction
Methods
Results
Discussion
Future Work
Summary (2)
Current methods of PEEP selection poorly agree with modelbased results. This suggests nearly all patients are suboptimally ventilated, which may significantly impact on
outcomes
Results from large RCTs to date have not optimised PEEP to
minimal elastance and therefore we may need to re-think the
interpretation of these findings- 6 ml/kg cannot be optimal for
all patients
Introduction
Methods
Results
Discussion
Future Work
Where are we going? What does the future hold?
Introduction
Methods
Results
Discussion
Future Work
Elastance changes following recruitment
Successful recruitment manoeuvre
Two unsuccessful recruitment manoeuvres
PEEP (cmH2O)
Elastance (cmH2O/L)
Little change
16% elastance decrease
Time (mins)
Time (mins)
Introduction
Methods
Results
Discussion
Future Work
Elastance trending
Worsening condition
Elastance increase
Time (mins)
Time (mins)
Decreases in elastance over time means patient condition is improving:
Good PEEP selection
Increases in elastance over time indicates patient is derecruiting:
PEEP change or recruitment maneuver may be needed
PEEP (cmH2O)
Elastance decrease
Elastance
(cmH(cmH
2O/L)
Elastance
2O/L)
Improving condition
Introduction
Methods
Results
Discussion
Future Work
dFRC Model : Monitoring patient lung volume
1600
1400
1200
1000
800
600
Recruited lung
“Derecruited” lung
dFRC
400
200
0
Picture (modified) sourced from UCSMT
0
5
10
15
20
25
Airway pressure (cm H2O)
30
Introduction
Methods
Results
Discussion
Future Work
dFRC Model : Monitoring patient lung volume
Volume (L)
Pressure cmH2O
Patient condition
constant, average
dFRC constant
Patient condition
improving with more
lung availability
Time (h)
Introduction
Methods
Results
Discussion
Future Work
Clinical interface
Patient TOP and TCP
Edrs following RM
0.36
31
Acknowledgements
Yeong Shiong Chiew
J Geoffrey Chase
Ashwath Sundaresan
Thomas Desaive
Richard Fernando
Laura Badcock
Sarah Poole
James Williams
Acknowledgements
Intensive Care Staff Christchurch Hospital