Question 11

College Answer

The effects of prone positioning on oxygenation are best studied in ARDS patients.  Short lived improvements in oxygenation are common (eg. 70%) and sometimes dramatic.  Some patients have no effect, and others have a long lasting effect (persisting well after rolling supine again).  Potential mechanisms for improving oxygenation during proning include: an increase in end-expiratory lung volume (with better response to applied PEEP and tidal recruitment), better ventilation–perfusion matching (with more homogeneous distribution of ventilation, and less shunting), and regional changes in ventilation associated with alterations in chest-wall mechanics (allowing more of applied pressure to inflate the lungs).  Prolonged benefits may be seen if inflation of recruitable lung has resulted in more lung units being held open when returned to the baseline ventilatory settings (Vt and PEEP).

Discussion

Mechanisms for improved oxygenation during 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.

Lai-Fook, STEPHEN J., and JOSEPH R. Rodarte. "Pleural pressure distribution and its relationship to lung volume and interstitial pressure." Journal of Applied Physiology 70.3 (1991): 967-978.

Tobin, A., and W. Kelly. "Prone ventilation-it's time." Anaesthesia and intensive care 27 (1999): 194-201.

Douglas, William W., et al. "Improved Oxygenation in Patients with Acute Respiratory Failure: The Prone Position 1–3." American Review of Respiratory Disease 115.4 (1977): 559-566.

Oczenski, Wolfgang, et al. "Recruitment maneuvers during prone positioning in patients with acute respiratory distress syndrome." Critical care medicine 33.1 (2005): 54-61.

Takahashi, Naoaki, et al. "Anatomic evaluation of postural bronchial drainage of the lung with special reference to patients with tracheal intubation: Which combination of postures provides the best simplification?." CHEST Journal 125.3 (2004): 935-944.

Mackenzie, Colin F. "Anatomy, physiology, and pathology of the prone position and postural drainage." Critical care medicine 29.5 (2001): 1084-1085.

Lamm, W. J., Michael M. Graham, and Richard K. Albert. "Mechanism by which the prone position improves oxygenation in acute lung injury." American journal of respiratory and critical care medicine 150.1 (1994): 184-193.

Lamm, W. J., Michael M. Graham, and Richard K. Albert. "Mechanism by which the prone position improves oxygenation in acute lung injury." American journal of respiratory and critical care medicine 150.1 (1994): 184-193.

Pelosi, Paolo, et al. "Effects of the prone position on respiratory mechanics and gas exchange during acute lung injury." American journal of respiratory and critical care medicine 157.2 (1998): 387-393.