Driving pressure

Reviewed and revised 22 December 2015; co-authored by  Jack Iwashyna (@iwashyna) who provided the analysis of the Amato et al, 2015 trial

OVERVIEW

• Driving pressure has been suggested by Amato and colleagues to be the key variable for optimisation when performing mechanical ventilation in patients with acute respiratory distress syndrome (ARDS)
• Driving pressure (ΔP) is the ratio of tidal volume to (static) respiratory system compliance ; i.e. ΔP = V_T/C_RS
• Driving pressure (ΔP) can be calculated at the bedside as plateau pressure minus positive end-expiratory pressure (P_plat – PEEP)

RATIONALE

Clinical need

• Protective lung ventilation strategies and ‘open lung’ approaches are associated with less ventilator-induced lung injury (VILI), improved oxygenation and improved outcomes
• Important components of these strategies all decrease stress on the lung:
  • lower tidal volumes
  • lower plateau pressure
  • higher PEEP
• However, clinical trials have found conflicting responses to the manipulation of these separate components of lung protection and when the optimization of one component negatively affects another it is often unclear at the bedside which is preferred

Physiology

• Normalised target tidal volumes to predicted body weight (PBW), as per the ARDSNet ventilation strategy, does not take into account the varying proportion of lung that is not available for ventilation in ARDS (‘baby lung’ concept)
• The decrease in available lung for ventilation manifests as a decrease in respiratory system compliance (C_RS)
• If two lungs are the same size, but the first lung has lower C_RS, a delivered tidal volume calculated according to PBW will cause more mechanical stress in the first lung than the more compliant second lung
• Thus normalizing V_T to C_RS and using the ratio as an index to indicate the “functional” size of the lung may provide a better predictor of outcomes in patients with ARDS than V_T alone
• This ratio is termed the driving pressure (ΔP = V_T/C_RS) and can be routinely calculated for patients who are not making inspiratory efforts as the plateau pressure minus positive end-expiratory pressure (ΔP = P_plat – PEEP)

EVIDENCE

Amato et al, NEJM 2015

• Retrospective analysis of previously prospectively collected patient-level RCT data
• n=3,562 patients from 9 major previously reported ARDS trials
• Methods
  • Driving pressure was calculated as ΔP =P_plat – PEEP (if no inspiratory effort) and was averaged over the first 24h post-randomization
  • A customized risk-adjustor was used to control for differences in risk-of-death to see which variable best explained the mortality pattern
  • the first analysis used a standard risk analysis with multivariate adjustments and multilevel mediation analysis to ask: “can changes in driving pressure explain who received the mortality benefits of alternative ventilation strategies?”
  • In a second analysis, patients were divided into quintiles differing in combination of PEEP, P_plat and ΔP, testing to see which was more consistently associated with trends in mortality
• Outcomes
  • In the first analysis, driving pressure (ΔP) was a better predictor of mortality than Compliance or V_T: “ΔP mediated 75% of the benefits due to treatment-group assignment in the V_T trials (P=0.004 for the average causal mediation effect) and 45% of these benefits in the PEEP trials (P = 0.001).”
  • In the second analysis, across quintiles:

PPlat	PEEP	ΔP	Mortality
rising	same	rising	rising
rising	rising	same	same
same	rising	falling	falling

• Commentary and criticisms:
  • Although an elegant epidemiologic examination of physiology, a secondary re-analysis of RCTs does not equal an RCT!
  • Calculated driving pressure (ΔP) is a post-randomization variable, so if making inspiratory efforts or P_plat contains information about response to therapy, then ΔP is confounded
  • If the findings are valid, does this mean we should set P_plat as high as we want as long my ΔP is low? Conversely, does this mean a low P_plat but with quite low PEEP harms patients?
  • It is unclear if ‘improving’ driving pressure using an open lung approach (high PEEP and lung recruitment manoeuvres) would improve patient outcomes

CONCLUSION

• Driving pressure is elegant concept that promises to simplify the optimisation 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 randomised controlled trial confirming its clinical utility and safety

References and Links

LITFL

• CCC — Protective lung ventilation
• CCC — Open lung approach to ventilation
• CCC — Positive end-expiratory pressure (PEEP)
• CCC — High airway and alveolar pressures
• CCC — Transpulmonary pressure and oesophageal manometry

Journal articles

• Amato MB, Meade MO, Slutsky AS. Driving pressure and survival in the acute respiratory distress syndrome. The New England journal of medicine. 372(8):747-55. 2015. [pubmed] [free full text] [letters]
• Borges JB, Hedenstierna G, Larsson A, Suarez-Sipmann F. Altering the mechanical scenario to decrease the driving pressure. Critical care (London, England). 19:342. 2015. [pubmed] [free full text]
• Loring SH, Malhotra A. Driving pressure and respiratory mechanics in ARDS. The New England journal of medicine. 372(8):776-7. 2015. [pubmed] [free full text]

FOAM and web resources

• Maryland CC Project — Brower: Driving pressure in ARDS, does it matter?!? (2016)