limitation of stress (transpulmonary pressure) is the safe strategy of mechanical ventilation to prevent VILI, rather than tidal volume (strain), as in viscoelastic lung stress is variable with severity of ARDS and respiratory rate.

1. MATHEMATICS
of
DRIVING PRESSURE
Dr. UBAIDUR RAHAMAN
M.D.
Internist and Critical Care Specialist

2. APPROACH
1. AIM OF MECHANICAL VENTILATION
2. CURRENT STRATEGY OF SAFE MECHANICAL VENTILATION
3. VILI AND CONCEPT OF STRESS- STRAIN
4. BABY LUNG ANS SAFE LIMIT OF STRESS-STRAIN
5. SAFE STRATEGY OF MECHANICAL VENTILATION: LIMITATION OF STRESS (TRANSPULMONARY PRESSURE), NOT STRAIN (TIDAL
VOLUME)
6. CONCEPT OF DRIVING PRESSURE: SURROGATE OF TRANSPULMONARY PRESSURE
7. MAKING DRIVING PRESSURE INDEPENDENT VARIABLE AND TIDAL VOLUME AS DEPENDENT VARIABLE
8. LIMITATION OF DRIVING PRESSURE
9. CONCLUSION

3. “Problems worthy of attack prove their worth by fighting back”
Piet Hein
Danish Mathematician and Poet

4. SPONTANEOUS VENTILATION MECHANICAL VENTILATION
INDEPENDENT
VARIABLE
PLEURAL PRESSURE AIRWAY PRESSURE/ FLOW
DEPENDENT
VARIABLE
AIRWAY PRESSURE/ FLOW PLEURAL PRESSURE

5. ARDS and MECHANICAL VENTILATION
AIM
MECHANICAL VENTILATION

6. ARDS and MECHANICAL VENTILATION
ARDS NET STRATEGY
MECHANICAL VENTILATION

7. MECHANICAL
FORCE
BIOTRAUMA
ATELECTO
TRAUMA
BARO
TRAUMA
VOLU
TRAUMA
RESPIRATOR
LUNG
VILI
STRESS-
STRAIN

8. VENTILATOR INDUCED LUNG INJURY
F/RR
∆P/
PEEP
VT
COLLPAS
E/OPENI
NG
INHOMOGE
NEITY/
RISESTRESS
RS
BABY
LUNG/
EDEMA
MECHANICAL VENTILATOR LUNG

9. MATHEMATICS OF STRESS AND STRAIN
Stress (PL)= K * strain (VT/ FRC)
K is specific lung elastance, proportionality constant equivalent in
pulmonary physiology
Assume VT = FRC
strain (VT/ FRC)= 1
Stress= K
Specific lung elastance is the PL, which doubles the lung
volume
K is animal species specific
In humans K= 13.5
In early ARDS, baby lung K is unaltered: not stiff but small
healthy lung

10. Human lung FRC= 35 ml/kg , TLC= 80 ml/kg
SAFE LIMIT OF STRESS-STRAIN AND VILI
One K (PL of 13.5 cmH2O) will increase lung
volume equal to FRC (35 ml/kg)
2.2 K will inflate lung to TLC (80/35)
PL of 30 cmH2O (13.5* 2.2) will increase
lung volume to TLC
TLC=complete unfolding of collagen
fibers=structural damage

11. SAFE LIMIT OF STRESS-STRAIN AND VILI
One K (PL of 13.5 cmH2O) will increase
lung volume equal to FRC (35 ml/kg)
1.3K will inflate lung to TLC from FRC
(45/35)
PL of 17 cmH2O (13.5* 1.3) will
increase lung volume to TLC from FRC
TLC=complete unfolding of collagen
fibers=structural damage

12. SAFE LIMIT OF STRESS-STRAIN AND BABY LUNG
PL (17 cmH2O)= K * strain (VT/ baby lung)
PL of 17 cmH2O will inflate baby lung to limit of structural damage,
irrespective of volume
Targeting VT normalized to IBW, as surrogate of baby lung is like
inflating tennis ball (baby lung) to the size of football (normal lung volume)

13. VISCOELASTIC LUNG
Stress = E * strain + ή * strain rate
DYNAMIC STRAIN
stress ᾱ strain and strain rate
STATIC STRAIN
stress relaxation

14. 30 healthy piglets, ventilated with same
strain and RR but varying I:E ratio
resulting in different strain rate.
Increasing strain rate resulted in 3 fold
increase in prevalence of pulmonary
edema and early death
Strain that were safe at lower rate,
became unsafe at higher strain
rate

15. TIDAL VOLUME PER KG IBW IS UNRELIABLE
SURROGATE OF STRAIN/VILI
BABY LUNG
Small not Stiff lung
Normal Compliance:
Specific lung elastance is
normal- 13.5
Baby lung volume is inversely
related to severity of ARDS

16. Similar VT produced different strain and stress,
Different tidal volume generated similar stress
and strain
depending upon difference in FRC
VT based on IBW is poor surrogate of lung strain
as it generates variable strain (safe to injurious)
depending upon the baby lung volume,
which varies with severity of ARDS.

17. BABY LUNG
VISCOELASTICITY
AND TIME
DEPENDENCY
Variable
Volume with
ARDS severity
Stress is
variable with
RR
TIDAL VOLUME PER KG IBW IS UNRELIABLE
SURROGATE OF STRAIN/VILI

18. CONSTANT
TIDAL
VOLUME
Stress is variable with severity
of ARDS/baby lung volume
Stress is
variable with RR
WHAT IS THE SAFE STRATEGY TO PREVENT VILI
LIMITATION OF TIDAL VOLUME OR TRANSULMONARY PRESSURE
TIDAL VOLUME per kg of IBW IS INACCURATE SURROGATE
OF SAFETY OF MV
As, for the same tidal volume, stress increases with
increasing severity of ARDS and RR

19. WHAT IS THE SAFE STRATEGY TO PREVENT VILI
SAFE TIDAL VOLUME IS DERIVATIVE OF BABY LUNG VOLUME
BABY LUNG VOLUME IS VARIABLE ACCORDING TO SEVERITY OF ARDS
STRESS IS VARIABLE WITH BABY LUNG VOLUME AND RR
LIMITATION OF PL WILL REDUCE STRESS AND STRAIN TO DANGEROUS
LEVEL
LIMIT STRESS
(TRANSPULMONARYPRESSURE)
but how
much?
IS IT
17 cmH2O?

20. STRESS RISERS
STRESSED
BUT NOT
STRAINED
INCREASED
STRESS AND
STRAIN
WHAT IS THE SAFE LIMIT OF STRESS (PL)

21. Stress =
4.5*PL
C1/10
Cnormal
Applied PL of 30 cmH2O
Generated strain of 132 cmH2O
WHAT IS THE SAFE LIMIT OF STRESS (PL)

22. Transpulmonary pressure of 17 would be multiplied
to more, by stress risers
Safe limit of PL is less than 17 cmH2O
SAFE LIMIT OF PL
LESS THAN 17 CMH2O
BUT HOW
MUCH
WHAT IS THE SAFE LIMIT OF STRESS (PL)

23. CONCEPT OF DRIVING PRESSURE
SURROGATE OF TRANSPULMONARY PRESSURE
PPLAT-PPL
PPLAT
CL= VT/ (PPLAT -PPL)
PL
CRS= VT/PPLAT

24. CONCEPT OF DRIVING PRESSURE
∆P
∆P is the distending pressure of the respiratory system, which is
plateau pressure above PEEP (PPLAT- PEEP)
It is considered as a surrogate of PL , as PL measurement requires
estimation of PPL which is invasive as well as complicating
Limiting ∆P, irrespective of severity of ARDS, would prevent dangerous
stress- strain and prevent VILI
DRVING
PRESSURE

25. CONCEPT OF DRIVING PRESSURE
∆P
INDEPENDENT VARIABLE: ∆P
DEPENDENT VARIABLE: VT
∆P is the targeted variable,
independent of mode of MV
Limiting ∆P is akin to limiting
dynamic strain at the cost of static
strain
DRVING
PRESSURE

26. CONCEPT OF DRIVING PRESSURE
∆P
DRIVING PRESSURE PRESUMES
Optima PEEP
P-V relationship on linear part
LIP
UIP
STRESSINDES≤1

27. Retrospective analysis of 3562 patients from 9 ARDS
RCTs
Mortality is associated with driving pressure, not PEEP
Increasing driving pressure with fixed PEEP leads to
higher mortality.
Increasing PEEP with fixed driving pressure has no
effect on mortality
When increasing PEEP leads to decrease in driving
pressure survival improves
Mortality difference associated with difference in
driving pressure
SIGNIFICANT INCREASE IN MORTALITY ONCE DRIVING
PRESSURE > 14

28. HOW TO VENTILATE ARDS
TILL FURTHER EVIDENCE
Ventilate as per ARDS net Protocol with VT 6 ml/kg IBW and target PPLAT
less than 30
Keep ∆P (PPLAT- PEEP) less than 14
Set PEEP to optimum- stress index zero,
Proning

29. DRIVING PRESSURE
LIMITATION
Driving pressure seems promising as it does not require invasive estimation
of pleural pressure making it easier to calculate
Its limitation lies in relationship to PL, which is the measure of stress.
Equation between end inspiratory plateau pressure (PPLAT) and PL is
governed by the influence of CCW on CRS.
For a given PPLAT, generated PL is variable depending upon the CCW.
Therefore, a patient with morbid obesity, chest wall deformity and raised
intra-abdominal pressure, which reduces CCW, higher driving pressure is
need to generate the same PL and VT, as in patient with normal CCW.
RELATIONSHIP BETWEEN PPLAT and PL
PL= PPLAT * CCW/ (CL+ CCW)

30. DRIVING PRESSURE
CONCLUSION
Driving pressure is an elegant concept that promises to simplify the
optimization of mechanical ventilation in patients with ARDS by
providing lung-protective ventilatory strategy that is adapted to the size
of the aerated lung
However the use of driving pressure is yet to be subjected to a high
quality randomized controlled trial confirming its clinical utility and safety