FRCA Notes

Physiology of Prone Ventilation

Prone ventilating patients was à la mode during Covid-19.

It was the subject of a CRQ in March 2022 (59% pass rate) - questions on physiology were "answered poorly", hence the rationale for this page.

Resources

  • Prone ventilation is used in the patient with refractory hypoxaemia from ARDS
  • It is cheap, widely available and relatively safe
  • It carries a number of physiological benefits and is associated with reduced mortality (PROSEVA Trial)

Improved V/Q matching

  • In ARDS the lung bases receive the greatest blood flow but the smallest amount of oxygenated gas
  • This is because the lung bases are collapsed and pleural pressure exceeds opening pressure
  • In the prone position, the pleural pressure is less likely to exceed airway opening pressure

Increased homogeneity of ventilation

  • In the supine position, there is a significant pleural pressure gradient (and therefore compliance difference) between ventral and dorsal lung
  • Once prone:
    • This pressure gradient is reduced
    • Lung compliance is more homogenous
  • The more uniform distribution of pleural pressure and compliance leads to more uniform distribution of the plateau pressure
  • This reduces cyclical atelectasis, alveolar overdistension and ventilator-associated lung injury

  • In the supine position the orientation of the ribs causes the ventral chest wall to be more compliant than the dorsal side, which is supported by the spine and bed
  • With IPPV regional ventilation shifts towards non-dependent lung regions

  • When prone the ventral chest wall is now supported so regional chest wall compliance becomes more even
  • Overall chest wall compliance is reduced
  • There is an overall decrease in the pleural pressure gradient between ventral (dependent) and dorsal (non-dependent) lung regions

  • When prone, regional ventilation with IPPV becomes more even than when supine and so better matched to perfusion

Reduced lung compression

  • Less compression of the lungs by the heart and abdominal contents
  • This further improves compliance as do not require the pressure to push the heart/abdominal contents out of the way when ventilating

Increased FRC

  • In the normal individual, FRC increases 300 - 400ml when prone

Increased drainage of secretions

  • The large airways are orientated dorso-ventrally such that prone positioning enhances the drainage of respiratory secretions and aspirated material

Increased response to recruitment manoeuvres

  • Compared to supine patients, prone patients require less PEEP to sustain post-recruitment improvement in oxygenation

Improved mechanics of the chest wall in obesity

  • Obese patients in the supine position have even more ineffective V/Q matching

  • 50% of patients won't respond to prone positioning, and thus instigating it may delay other procedures e.g. ECMO

  • Some patients are unsuitable for prone positioning:
    • Those with open abdomens or ventral wounds/burns
    • Those with spinal or pelvic instability
    • Those with extreme abdominal distension

  • There is an airway risk, with:
    • Reduced control over ETT
    • Risk of airway dislodging
    • More difficulty intubating

  • General nursing care is impaired and workload may be increased

  • There is risk of pressure injuries, including:
    • Facial oedema and pressure sores on eyes, lips and bridge of nose
    • Shoulders and ulnar nerves at the elbow
    • Breasts | ASIS | penis/scrotum | knees

  • Raised intra-abdominal pressure
    • Impairs absorption of NG feed
    • May contribute to AKI

  • Cardiac arrest can occur whilst prone, if so:
    • Start CPR in prone position; two-hand technique over mid-thoracic spine between scapulae
    • Pad position bi-axillary or postero-lateral (right shoulder blade, left flank)