A session blog from Day 2 of State of the Art, London December 2015. See the full list.
Blogger Jack Wong
From left to right – Fan, Gattinoni, MacSweeney
Stretching the lung is not harmful
- “VILI does not exist!”
- Physiologically speaking, as inferred from animal model, we can endure large lung volume without significantly harmful rise in lung pressure.
- The concept of continuous strain with high PEEP ventilation would equate no energy input hence little harm is incurred (ie you need energy to cause stress and strain)
- Lung injury is not determined by single factor such as pressure / volume / flow etc but rather a composite of energy caused but when it is managed in the lower threshold it is relatively safe.
- Quoting the history of the “Charge of the Light Brigade”, Rob quotes a handful of evidences which show protective lung ventilation strategy is preferred as compared to ventilation with larger tidal volume – Hinting that the idea of “VILI (VALI) doesn’t exist” by Gattinoni is akin to Lord Cardigan leading the crowd charging into “the VALI of death”
ECMO: You’re doing it wrong
- The lung rest concept using extracorpeal membrane lung was introduced by Kolobow in 1977 but is associated with high rate of complications such as blood loss and technical issues.
- However, while patient is on ECMO, the intended low volume ventilation (protective ventilation strategy) allows alveoli collapse which worsened patients’ condition due to atelectasis.
- The alveoli should instead be “stretched” to maintain opened alveoli.
- Therefore, it seems counter-intuitive to use low volume ventilation in ARDS (to allow atelectasis) instead of promoting some degree of “pressure” in the lung.
- Problems arise when lung pressure (Pl) is low
- Insufficient total ventilation
- pCO2 rise
- Worsening hypoxemia
- Increased pulmonary hypertension
- RV faliure.
- Hence, keep the lung “inflated” when ECMO is in use.
- Well documented uncertainty regarding what lung pressure / MV should be maintained at when ECMO was introduced.
- However, mortality data on various trials shows low volume is preferred – concept of “baby lung”:
- ARDSnet NEJM 2000 – 6ml/kg vs 12ml/kg (31.0% vs 39.8%)
- Needham BMJ 2012 – 6 ml has better 2 year mortality
- Terragni PP Anaesthesiology 2009 – TV lower than 6 ml / kg enhances lung protection
- Bein T Intensive Care Medicine 2013 – Xtravent-study =
- Pham T Am J Respir Crit Care Med 2013 – ECMO for Pandemic H1N1
- Various strategies to achieve “lung rest” have been described but effect of clinical outcomes is limited. Hence, more data is required – work in progress:
ARDS definition is killing advances in the field
- Comparing the rise and fall of Hindenburg to the introduction and limitation of ARDS definition – MacSweeney is setting a rather impressive presentation style!
- ARDS is described by Rene Laennec nearly two centuries ago, followed by recent definition attemps
- Current definition only correlates to 50% of actual pathology of ARDS – it is a flawed criteria and poor definition.
- Could this relate to the results in hundreds on negative trial?
- Perhaps we should be looking at tissue biomarker rather than syndrome definition and there is a need to move forward unto “post ARDS era
- Syndromes are commonly described in critical care medicine.
- Benefits of defining ARDS:
- Enable epidemiological studies
- Facilitate enrolment into clinical trials
- Allow comparison between studies
- Enhance linkage with basic science
- Ability to implement results of positive RCTs in clinical practise
- May be useful in prognostic discussion with patients and families
- Useful in planning resource allocation
- AECC definition
- CXR unreliable
- Poor inter-observer agreement on PAOP
- P/F ratio is treatment dependent
- Left atrium hypertension often co exists with ALI
- Berlin definition
- Limitation and concerns
- Berlin classification does not standardize mechanical ventilation at the time of oxygenation assessment
- Oxygenation after 24 hours of standardized MV clearly associated with mortality better than baseline oxygenation
- Many variables and measures of interest were not included (eg EVLW, biomarkers, CT, PEEP, compliance, dead space)