Prone ventilation for ARDS

Prone ventilation,  a quintessentially ICUish technique of manipulating respiratory physiology,  has really entered the mainstream following the COVID pandemic. It's cheap, widely available, relatively safe, and it increases FRC, improves the drainage of secretions, decreases oxygen requirements and improves mortality (the PROSEVA trial demonstrated a mortality improvement from 32% to 16%). Thinking about the benefits, one comes to the conclusion that we should all be positioned prone, all the time.

Past CICM exam questions about prone ventilation have included:

• Question 10 from the first paper of 2022
• Question 15 from the first  paper of 2004
• Question 11 from the first paper of 2003

Unfortunately, the older of these questions had preceded the wildly awesome PROSEVA trial, which makes the college answers somewhat useless to the revising CICM exam candidate, as the contemporary studies which those college answers were based on were largely negative owing to weird patient selection. 

The best resource to revise this topic is the relevant LITFL CCC page. Of academic literature, a good article to read about the mechanisms of improved oxygenation from prone positioning was "Pragmatics of Prone Ventilation" by Messerole et al. (2002).

Rationale for prone ventilation

• Improved V/Q matching: This is probably the most important contribution. In the ARDS patient the bases of lungs both receive the greatest amount of blood flow and the smallest amount of oxygenated gas (they are usually all collapsed). According to Tobin and Kelly  (1999), this is all because in the prone position the pleural pressure is less likely to exceed airway opening pressure and cause airway closure.
• More homogeneous ventilation: Prone positioning reduces the difference between the dorsal and ventral pleural pressure, and the compliance of dorsal and ventral lung is therefore more homogeneous. As a consequence, there is no longer a situation where regions of lung have markedly different compliance, and this reduces ventilator-associated lung injury from alveolar overdistension. The benefits from this can be summarised as follows:
  • More uniform distribution of pleural pressure;
  • Thus, more uniform compliance;
  • Thus, more uniform distribution of plateau pressure;
  • Thus, less cyclical atelectasis and alveolar overdistension.
• Less lung deformation: There is less compression of the lungs by the heart (which sits on the sternum in the prone position) and by the abdominal content. In general, the lungs fit better into the chest cavity. This improves compliance, as one does not have to use their ventilation pressure to push these organs out of the way.
• Increased FRC: This ancient manuscript from 1977 reports that FRC in normal people increases by about 300-400ml when turned into the prone position.
• Improved drainage of secretions: dorsoventral orientation of large airways apparently enhances the drainage of respiratory secretions and aspirated material. This data is extrapolated from physiotherapy patients which were not completely prone and very much awake, with no ARDS, but the fact remains.
• Improved response to recruitment manoeuvres: Prone patients respond well to recruitment manoeuvres.  When compared to supine patients, prone patients seem to require less PEEP (8cm vs 14cm) to sustain the post-recruitment improvement in oxygenation.
• Improved mechanics of the chest wall in obesity - in fact, if the literature is to be believed, the entire population of obese non-ARDS patients should spend most of their lives in the prone position because of how disastrously ineffective their V/Q matching is in the supine position.

Limitations of prone ventilation

• Limitations and contraindications by patient factors
  • Open abdomen, wounds or burns over the ventral body surface
  • Spinal or pelvic instability
  • Massive abdominal distension, eg. pancreatitis
• Limitations of logistics:
  • difficulty of positioning and increased nursing workload for intubated patients
  • difficulty maintaining patient cooperation in awake patients
  • poor control of airway safety
    • In fact, poor control of all drains and tubes of any sort
    • There is a known risk of airway compromise
    • If the tube falls out, it is difficult to reintubate
  • poorer pressure area care
  • Difficult (impossible) central insertion while prone
  • Pressure prone areas include eyes, lips (frm the ETT), bridge of nose, shoulders, ulnar nerves at the elbow, breasts (particularly large ones and those that contain implants), pelvis (particularly interior superior iliac spines), penises and scrotums, and the knees.
• Limitations of physiology
  • Poor NG feed tolerance
  • Facial oedema
  • Raised intraabdominal and intracranial pressure
• Limitations of imagination
  • ECG electrode position will change, and so potentially "ECG changes" may appear
  • Up to 50% of ARDS patients do not benefit from prone position, and so this manoeuvre may be a time-wasting exercise, delaying the decision to start ECMO.

Evidence in support of prone ventilation

LITFL have an excellent literature summary section on their prone ventilation page, which has been pillaged for references. In brief, three contemporary studies (Gattinoni, Beuret and Guerin)  were available to the trainees at the time of writing Question 15 from the first  paper of 2004 and Question 11 from the first paper of 2003, in addition to early pioneering work by champions of "extreme positioning".

Piehl et al (1976) were the first to play with prone position: their study had 5 patients in it.

• All 5 patients had at least moderate to severe ARDS (60% FiO₂)
• Proned for 4-8 hours
• An improvement of oxygenation by up to 40-50mmHg PaO₂ was noted
• It was easier to clear secretions
• The authors used this study to trial a new Stryker CircOlectric bed, which resembled a small Ferris wheel.

Gattinoni et al (2001): multicentre RCT - 304 patients with  ARDS

• Proned for only 7 hours a day, starting late in the course of ARDS
• Oxygenation improved in the majority of prone-ventilated patients, but there was no improvement of survival (keeping in mind that the study was not powered to detect a significant change in mortality)

Beuret (2002): single centre RCT - 54 patients with coma (not ARDS)

• Proned for only 4 hours a day
• Reduced incidence of VAP was observed; lung injury score was much better in the prone group even though the period of prone positioning was laughably brief, and more resembled postural drainage  physiotherapy.

Guerin (2004): multicentre RCT - 791 patients with acute respiratory failure

• All patients had P/F ratios worse than 300, fitting into the modern "mild ARDS" definition
• Proned for only 8 hours a day
• 90 day mortality was no different, and on top of that the prone group ended up with more proneness-related complications, raising "some safety concerns".

On the basis of the available evidence, the college answer to the 2004 question mentioned the 2001 Gattinoni paper, lamenting the negative data. The college answer was fairly optimistic; they quoted an opinion piece by Alain Broccard from 2003, who also  felt that there were "good reasons not to regard the recent negative prone positioning study as indicating that the prone position is of no interest". Broccard pointed out that it took five RCTs to finally arrive at the correct conclusion that low tidal volume ventilation improved survival in ARDS. This was prescient. Many trials followed.

Sud et al (2010) collected the data into a meta-analysis (n=1,867).

• Heterogeneous group of studies, including adults and children
• All forms of ARDS were included, from mild to severe
• Broad range of therapeutic strategies and prone protocols
• A survival benefit was demonstrated in the severe group, with NNT =11, but not in the other groups (i.e. you had to have a P/F ratios worse than 100 to benefit)
• Oxygenation improved by 27-39%
• Risk of your tube falling out also increased.

PROSEVA (2013); multicentre RCT - 466 patients with severe ARDS, dissected by The Bottom Line

• Population/intervention
  • All were FiO2 60% or more, failing lung-protective ventilation
  • Proned for at least 16 hours a day for 4-5 days on average (i.e. for ~ 70-75% of the time)
• Important findings:
  • Significant improvement in 28-day and 90-day mortality: 16% in prone vs. 32.8% in supine group
  • No increase in adverse events in prone group
• Strengths:
  • Standardised ventilation and weaning strategy
  • Appropriate power calculation
  • Intention to treat analysis
• Weaknesses:
  • 2015 patients not screened for inclusion, raising suspicion of selection bias
  • Some differences in baseline characteristics (eg. SOFA score)
  • All centres were very experienced in prone ventilation (reduces generalizability)