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Mathematics of driving pressure pdf

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.

Mathematics of driving pressure pdf

  1. 1. MATHEMATICS of DRIVING PRESSURE Dr. UBAIDUR RAHAMAN M.D. Internist and Critical Care Specialist
  2. 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. 3. “Problems worthy of attack prove their worth by fighting back” Piet Hein Danish Mathematician and Poet
  4. 4. SPONTANEOUS VENTILATION MECHANICAL VENTILATION INDEPENDENT VARIABLE PLEURAL PRESSURE AIRWAY PRESSURE/ FLOW DEPENDENT VARIABLE AIRWAY PRESSURE/ FLOW PLEURAL PRESSURE
  5. 5. ARDS and MECHANICAL VENTILATION AIM MECHANICAL VENTILATION
  6. 6. ARDS and MECHANICAL VENTILATION ARDS NET STRATEGY MECHANICAL VENTILATION
  7. 7. MECHANICAL FORCE BIOTRAUMA ATELECTO TRAUMA BARO TRAUMA VOLU TRAUMA RESPIRATOR LUNG VILI STRESS- STRAIN
  8. 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. 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. 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. 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. 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. 13. VISCOELASTIC LUNG Stress = E * strain + ή * strain rate DYNAMIC STRAIN stress ᾱ strain and strain rate STATIC STRAIN stress relaxation
  14. 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. 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. 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. 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. 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. 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. 20. STRESS RISERS STRESSED BUT NOT STRAINED INCREASED STRESS AND STRAIN WHAT IS THE SAFE LIMIT OF STRESS (PL)
  21. 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. 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. 23. CONCEPT OF DRIVING PRESSURE SURROGATE OF TRANSPULMONARY PRESSURE PPLAT-PPL PPLAT CL= VT/ (PPLAT -PPL) PL CRS= VT/PPLAT
  24. 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. 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. 26. CONCEPT OF DRIVING PRESSURE ∆P DRIVING PRESSURE PRESUMES Optima PEEP P-V relationship on linear part LIP UIP STRESSINDES≤1
  27. 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. 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. 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. 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

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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.

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