Proning refers to the positioning of the patient on their front, rather than on their back (supine), which would be the normal position for care. The reasoning for this is because of the evidence of improved oxygenation in this position, making it an effective approach for patients with severe acute lung disease, primarily ARDS. However, it is not without its risks, thus is often used only for the more extreme forms of respiratory disease.
The theory of the benefit of the prone position is based on some of the favourable physiological changes that result from this position:
More homogenous ventilation
Less lung compression (by heart and abdominal contents)
Better lung perfusion
Improved V/Q matching
Improved secretion drainage
Part of the benefit derives from the improved ventilation. From understanding lung physiology, we know that the superior components of the lung are normally better ventilated. This is because the transpulmonary pressure is less here: consider the lungs as being squashed by their own weight, whilst this weight is hanging on the upper lungs, essentially opening them up here. In the supine position, this means that the dorsal aspect, which makes up a significant amount of lung tissue, will ventilate less well. This is further aggravated by the pathology of ARDS. There is a vicious cycle of poorer ventilation, dependent lung atelectasis and unequal ventilation. In the prone position, this previously dependent lung is now superior, with a greater transpulmonary pressure, and therefore better ventilation. This, coupled with appropriate use of PEEP to maintain alveolar patency, allows more homogenous ventilation of the lungs, and improved oxygenation.
As well as the effect of the lungs itself on ventilation, the supine position imposes some external compromise on ventilation. The heart will, under the influence of gravity, tend to compress part of the left lung, and the abdominal contents, through pressure on the diaphragm, will impose on the lower lung segments. In the prone position, the effects of gravity will direct their weight away from the lungs, reducing the compressing effect.
The improvement in ventilation and thus oxygenation is also thought to improve pulmonary blood flow. This could be through reduced hypoxic vasoconstriction.
These effects combine to improve the matching of ventilation and perfusion in the lungs (V/Q matching), the factor essential for effective oxygenation. In the supine position, the gravitationally affected perfusion is primarily to the poorly ventilated dependent lung regions. When prone, the ventilatory improvements, as well as some perfusion ones, results in better matching, and thus better oxygenation.
These changes to the physiology appear to persist following turning supine, suggesting some reversal of the vicious circle of pathophysiology seen in these patients.
Moderate-severe ARDS with significant hypoxia
Posterior wounds/burns (uncommon)
Prone position is generally recommended after initiation of other strategies for ARDS and challenging oxygenation. Most patients who will respond will demonstrate a response in the first hour, although some can still improve after this. There is a recommendation from FICM/ICS about early consideration of proning (<48h of the disease).
Some relatively absolute contraindications include:
Raised intracranial pressure
Raised intra abdominal pressure
Some additional factors that require consideration include:
There are different techniques described to achieve this. The focus is to allow effective turning of the patient whilst maintaining both patient and staff safety. There are a number of complications that can occur from turning prone (see below) and so these approaches aim to minimise these.
Potential emergency procedures should be discussed prior to intervention.
Care in the proning position is important to avoid complications. A key part of this is regular repositioning to avoid pressure/traction injuries. When in the “swimmers position”, this side should be alternated every 2-4 hours.
The evidence suggests a high rate of response to this maneuver (up to 70% of patients). This appears to persevere across a range of causes of ARDS.
Key studies include:
Cochrane review (2015)
Gattinoni et al. (2001)
The PROSEVA study is probably one of the most notable studies on the benefit of prone ventilation. This RCT demonstrated a notable improvement in all cause mortality in those patients with severe ARDS who were proned early (16% vs 32.8%). This suggested a NNT of 6.