A key development in our understanding of Acute Respiratory Distress Syndrome was Luciano Gattinoni’s paradigm-shifting Baby Lung idea. In the mid-1990s it was thought that ARDS was a homogeneous lung-stiffening process. The “proof” was the uniform bilateral infiltrate on AP chest X-ray. Gattinoni’s innovative cross-sectional imaging studies showed instead that ARDS lungs were inhomogeneous, and showed various functional areas: some consolidated alveoli, some collapsed alveoli, and some open alveoli. Efforts could be made to recruit and open the collapsed ones, but the consolidated ones were temporarily lost. The patients survived by gas exchange in the remaining open alveoli. The total volume of the open alveoli added together to make up a smaller lung – the “Baby Lung”. Nobody would dream of ventilating a neonate with a 1litre Tidal Volume – obviously the lung would be ripped apart (though it would also appear to be “stiff” while it was being so ventilated – because it was being overstretched). So if we realized that an adult with ARDS no longer had a 5litre lung capacity, but only (say) 1litre of functional lung, it would make sense to ventilate them with a smaller Tidal Volume than the 10-15ml/kg Tidal Volumes commonly in use in 1995.
So the 2000 ARDSnet study helped, by showing that fewer ARDS patients died by ventilating with 6ml/kg (of predicted mass) rather than 12ml/kg. Somehow 6ml/kg then became the “ideal volume” – something ARDSnet never claimed to show. It could still be too big for some patients.
Amato’s 2015 study suggested a new scalable limit. The “Driving Pressure” is the difference between PEEP and Pmax (in a Pressure Mode) and PEEP and Pplateau (in a Volume Mode with a pause). Driving Pressure greater than 15cmH2O correlated with increased mortality.
So perhaps Driving Pressure is our scaleable limit. Start ventilating your ARDS patient at 6ml/kg predicted mass. But if the Driving Pressure required to do that exceeds 15cmH2O, we are exceeding the capacity of the available lung. By all means try recruitment, try to expand the available lung. But once you have done that, and you still have too high a Driving Pressure, scale back on your Tidal Volumes.
Note that the above assumes no problem with the chest wall – it assumes the chest wall is soft and stretchy, and that all the Driving Pressure goes into stretching the lung. If the chest wall is stiff (eg: an obese patient, or a patient with abdominal hypertension), then we have to make allowances for the chest wall. The best way then is to get an estimation of pleural pressures by measuring oesophageal pressures. This enables one to work out the pressure over the chest wall and separate out the pressure over the lung only (the transpulmonary pressure). The aim must be then to keep the Transpulmonary Driving Pressure below 15cmH2O.
(This is also only relevant in alveolar lung disease – in patients with bronchospasm MUCH bigger Driving Pressures will be required to move air through the narrowed airways. Different pathology, different limits.)
So, although not everyone yet agrees, I think Driving Pressure limitation is the way to properly scale our Tidal Volumes to the size of the residual Baby Lung in ARDS.
vraiida 