Chest wall and lung compliance changes

Welcome back to the tasty morsels of critical care podcast.

Today we look at something we do fairly frequently in ICU, especially in the post COVID era: prone positioning or to use its preferred technical term: adult tummy time. This has been around for a long time but was uncommonly done in the pre COVID days and was always a talking point when it did happen. But then 2020 came and you’d spend significant portions of the day proning and supinating patients in the unit. Fair to say it’s something we should have a keen understanding of.

Firstly we’ll talk about the physiology and potential mechanism of benefit behind proning. This comes from the proning chapter in Tobin’s mechanical ventilation textbook. Written by none other than the late, great Gattanoni. He argues that there are 3 mechanisms by which proning affects ventilation and oxygenation

1. changes in inflation
2. redistribution of ventilation
3. redistribution of perfusion

A lot of this comes from Gattanoni’s early work where they managed to do a whole bunch of CT scans on critically people with ARDS in both the supine and the prone position. Yes you heard that right they did the CT scan prone. The typical CT scan for many ARDS patients is a basal dorsal distribution of disease. One would think that flipping the patient might redistribute this atelectasis to the ventral surface. But what seems to happen is more of a homogenisation of the lung with an overall improved inflation of the lung tissue. No longer are you just hyperinflating the baby lung and doing nothing for the atelectatic lung. This should lead to better recruitment, better perfusion/ventilation matching, better oxygenation and in turn better clinical outcomes. There are some suggestions it may also aid secretion clearance which in a paralysed supine patient is obviously a problem.

Proning (as we shall we see) does seem to improve outcomes but the precise mechanism is unclear. Improved oxygenation seems plausible but it may also be a reduction in VILI by having a more homogenous lung that is less prone to injury of the baby lung.

Guerin (lead PROSEVA author) wrote a nice review article in 2020 highlighting that proning can make chest wall compliance worse. The anterior ventral wall is normally more mobile than the dorsal chest wall. When prone the ventral bit is now wedged and immobile against the bed hence the fall in chest wall compliance. However lung compliance is probably improved and now that the chest wall is moving less it’s probably increased diaphragmatic movement that recruits the bases. Overall compliance should improve.

We turn now to the evidence base for proning our patients. This, like many critical care interventions, this  has a little bit of a narrative to it with some early trials lacking benefit followed by the paradigmatic trial that shapes practice. What follows is a brief summary of some of the important studies and is neither intended nor considered to be comprehensive.

Back in the early noughties there were a flurry of RCTs looking at prone positioning in ARDS. The late and great Gattanoi was of course involved in some. The “dose” of proning was variable with sometimes only short periods like 6 hours being used. Results were variable and a 2011 meta analysis of 7 RCTs did not show a definitive mortality benefit but did suggest that those with the sicker lungs had a benefit

Enter PROSEVA. A name, that if you’re going into an ICU viva, is probably something that you should keep in your head. This was 26 centres in France who were already experienced with proning. They took people with severe ARDS and randomised them to 16 hrs a day of proning vs no proning. They used mortality at 28 days as a primary outcome and they were looking for a 15% absolute reduction in mortality (which is pretty huge). It was, for obvious reasons, an open label trial. They enrolled 450 patients and found a 32% vs a 16% mortality favoring proning. It’s possible this trial found a benefit due to the dose – they proned for much longer than many of the other trials.

It’s worth having some problems related to proning in your back pocket to pull out. The list of potential contraindications was initially quite long pre-COVID but it turns out that when your back is up against the wall we all became a little bolder with our proning. While you can prone the vast majority of patients it’s not going to be possible in those with unstable spinal injuries. One would think that abdominal surgery or advanced pregnancy might be a problem but you can usually work round this with some discussion with your surgeons.

The main downsides (beyond the hassle factor) are related to safety. The facial oedema and skin injuries are not insignificant and no matter how careful you are some people just aren’t a great shape for proning. There is a chance that the ET tube can kink or dislodge either on the proning or on the head turns so you need to have a good plan in your head how to confirm this and get someone flipped back if they need it.

Reading

Tobin Chapter 49

Deranged Physiology 

Prone Position and Mechanical Ventilation