MATHEMATICSof

DRIVING PRESSURE

Dr. UBAIDUR RAHAMAN

M.D.

Internist and Critical Care Specialist

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

“Problems worthy of attack prove their worth by fighting back”

Piet Hein

Danish Mathematician and Poet

SPONTANEOUS VENTILATION MECHANICAL VENTILATION

INDEPENDENT VARIABLE

PLEURAL PRESSURE AIRWAY PRESSURE/ FLOW

DEPENDENTVARIABLE

AIRWAY PRESSURE/ FLOW PLEURAL PRESSURE

ARDS and MECHANICAL VENTILATIONAIM

MECHANICAL VENTILATION

ARDS and MECHANICAL VENTILATIONARDS NET STRATEGY

MECHANICAL VENTILATION

MECHANICAL FORCE

BIOTRAUMAATELECTOTRAUMA

BAROTRAUMA

VOLUTRAUMA

RESPIRATOR LUNG

VILISTRESS-STRAIN

VENTILATOR INDUCED LUNG INJURY

F/RR

∆P/

PEEP

VT

COLLPASE/OPENI

NG

INHOMOGENEITY/

RISESTRESSRS

BABY LUNG/ EDEMA

MECHANICAL VENTILATOR LUNG

MATHEMATICS OF STRESS AND STRAIN

Stress (PL)= K * strain (VT/ FRC)K is specific lung elastance, proportionality constant equivalent in

pulmonary physiology

Assume VT = FRCstrain (VT/ FRC)= 1

Stress= K

Specific lung elastance is the PL, which doubles the lung volume

K is animal species specificIn humans K= 13.5

In early ARDS, baby lung K is unaltered: not stiff but small healthy lung

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

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

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)

VISCOELASTIC LUNGStress = E * strain + ή * strain rate

DYNAMIC STRAIN stress ᾱ strain and strain rate

STATIC STRAINstress relaxation

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

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

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.

BABY LUNGVISCOELASTICITY

AND TIME DEPENDENCY

Variable Volume with

ARDS severity

Stress is variable with

RR

TIDAL VOLUME PER KG IBW IS UNRELIABLE SURROGATE OF STRAIN/VILI

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 VILILIMITATION 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

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?

STRESS RISERS

STRESSED BUT NOT STRAINED

INCREASED STRESS AND

STRAIN

WHAT IS THE SAFE LIMIT OF STRESS (PL)

Stress = 4.5*PL

C1/10

Cnormal

Applied PL of 30 cmH2OGenerated strain of 132 cmH2O

WHAT IS THE SAFE LIMIT OF STRESS (PL)

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)

CONCEPT OF DRIVING PRESSURESURROGATE OF TRANSPULMONARY PRESSURE

PPLAT-PPL

PPLAT

CL= VT/ (PPLAT -PPL)

PL

CRS= VT/PPLAT

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

DR

VIN

G

PR

ESSU

RE

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

DR

VIN

G

PR

ESSU

RE

CONCEPT OF DRIVING PRESSURE∆P

DRIVING PRESSURE PRESUMES

Optima PEEP

P-V relationship on linear part

LIP

UIP

STR

ESS

IND

ES ≤

1

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

HOW TO VENTILATE ARDSTILL 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

DRIVING PRESSURELIMITATION

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)

DRIVING PRESSURECONCLUSION

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