Question 15

College Answer

The prone position has a number of obvious potential advantages to critically ill patients. These include enhancing the ability to rest and dress soft tissue injuries (including burns, skin grafts, plastic surgical flaps etc). The majority of ICU interest however relates to the potential ventilatory benefits associated with prone positioning: increased homogeneity of ventilation, improved ventilation:perfusion matching, increased Functional residual capacity, reduced atelectasis, and facilitation of drainage of secretions. Improved gas exchange is seen in approximately 2/3 of patients, and these improvements are persistent in some. A prospective randomised study confirmed these improvements in oxygenation in patients with Acute Lung Injury/ARDS, but was unable to demonstrate any short or long term mortality benefits. Many details are still under much discussion (e.g. which groups should be “proned”, when in their course, duration of time left prone, and for how many days to persist with prone positioning). Unfortunately, positioning patients prone is not without problems: expertise, manpower and time required for turning; potential for dislodgement of lines/tubes; problems with airway access; increased number of new pressure sores; increased new pressure sores in prone-related areas; increased intracranial pressure and decreased tolerance of enteral feeding. (Gattinoni L et al. N Engl J Med 2001; 345:568-73; Broccard AF. Chest 2003;
123:1334-6; Beuret P et al. Intensive Care Med 2002; 28 :564-69).

Discussion

Prone positioning is seldom seen during one's ICU practice as a junior, but one takes notice of it when it happens.

Rationale for prone ventilation

• Improved V/Q matching
• More homogeneous ventilation
  • 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 compression of the lungs by the heart and by the abdominal content.
• Increased FRC by about 300-400ml
• Improved drainage of secretions
• Improved response to recruitment manoeuvres: 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

Limitations of prone ventilation

• Limitations and contraindications by patient factors
  • Open abdomen, sternotomy, 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
  • 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
  • There is concern that some proportion of ARDS patients may not benefit from prone position, and so this manoeuvre may be a time-wasting exercise, delaying the decision to start ECMO. How large that proportion, remains debatable - in the literature one sees the figure of up to 50%, though these data are from Chatte et al (1997), pre-dating both PROSEVA and sane tidal volume ventilation. In the PROSEVA trial, only 0.8% of the proned patients transitioned to ECMO. The concern remains, i.e. it is possible that the clinician proning the patient has completely misread the situation and ECMO is inevitable.

Evidence in support of prone ventilation

Three 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:

• Gattinoni et al (2001): 304 patients, proned for only 7 hours a day, starting late in the course of ARDS; oxygenation improved but not survival.
• Beuret (2002):  54 patients with coma (not ARDS), proned for only 4 hours a day- reduced incidence of VAP was observed, but the study wasn't looking at survival.
• Guerin (2004): 791 patients proned for only 8 hours a day: no change in mortality

The college answer also quotes an optimistic review paper by Alain Broccard from 2003. Its overall tone was "don't write proning off just yet, the jury is still out".

More modern data in support of prone ventilation:

• Sud et al (2010) meta-analysis (n=1,867): yes there was a survival benefit, but you had to have a P/F ratios worse than 100 to benefit. NNT for this group was 11.
• PROSEVA (2013) multicentre RCT - 466 patients with severe ARDS, proned for at least 16 hours a day for 4-5 days on average (i.e. for ~ 70-75% of the time) - showed a significant improvement in 28-day and 90-day mortality (16% vs. 32% in the supine group).

Mancebo, Jordi, et al. "A multicenter trial of prolonged prone ventilation in severe acute respiratory distress syndrome." American journal of respiratory and critical care medicine 173.11 (2006): 1233-1239.

Gattinoni, Luciano, et al. "Effect of prone positioning on the survival of patients with acute respiratory failure." New England Journal of Medicine 345.8 (2001): 568-573.

Sud, Sachin, et al. "Prone ventilation reduces mortality in patients with acute respiratory failure and severe hypoxemia: systematic review and meta-analysis." Intensive care medicine 36.4 (2010): 585-599.

Beuret, Pascal, et al. "Prone position as prevention of lung injury in comatose patients: a prospective, randomized, controlled study." Intensive care medicine 28.5 (2002): 564-569.

Guerin, Claude, et al. "Effects of systematic prone positioning in hypoxemic acute respiratory failure: a randomized controlled trial." Jama 292.19 (2004): 2379-2387.

Broccard, Alain F. "Prone position in ARDS: are we looking at a half-empty or half-full glass?." CHEST Journal 123.5 (2003): 1334-1336.

Cho, Young-Jae, et al. "411: The Efficacy and Safety of Prone Positional Ventilation in Acute Respiratory Distress Syndrome." Critical Care Medicine41.12 (2013): A99.

Messerole, Erica, et al. "The pragmatics of prone positioning." American journal of respiratory and critical care medicine 165.10 (2002): 1359-1363.

Chatte, Gerard, et al. "Prone position in mechanically ventilated patients with severe acute respiratory failure." American journal of respiratory and critical care medicine155.2 (1997): 473-478.