Positive Pressure Ventilation

Describe the physiological consequences of intermittent positive pressure ventilation and positive end-expiratory pressure.

Physiological effects of positive pressure ventilation are mostly related to the increased mean airway pressure. This is a function of:

• Ventilation mode
• Tidal volume and peak (and plateau) airway pressure
• Respiratory rate
• I:E ratio
• PEEP
  PEEP has a much larger effect than the other factors.
  • PEEP is defined as a positive airway pressure at the end of expiration
  • PEEP is distinct from positive airway pressure (which is not confined to a phase of the respiratory cycle) and CPAP (which is a mode of ventilation)
  • iPEEP refers to intrinsic PEEP, auto PEEP or dynamic hyperinflation
    iPEEP is PEEP generated by the patient, and occurs when expiration stops before the lung volume reaches FRC.
    • Application of external PEEP may limit the generation of iPEEP by maintaining airway patency in late expiration

Respiratory Effects

• Decreased work of breathing
  • Decreased VO₂
    More important when work of breathing is high.
• Alteration in anatomical/apparatus dead space
  • Intubation typically reduces dead space, as the additional apparatus dead space is of smaller volume than the anatomical dead space it replaces
  • Non-invasive ventilation masks cause a large increase in dead space
• Increases lung volume (and FRC, for PEEP) by an amount proportional to the compliance of the system
  • Improves oxygenation via alveolar recruitment
  • Improves lung compliance via alveolar recruitment, reducing work of breathing
  • Elevated airway pressures may increase the proportion of West Zone 1 physiology and alveolar dead space
    In healthy lungs an increase in the $V_D \over V_T$ ratio is seen when PEEP exceeds 10-15cmH₂O.
• Reduces airway resistance
  Airway resistance decreases as lung volume increases.

Cardiovascular Effects

• Alteration in cardiac output
  • PEEP and IPPV generally decrease CO via decreasing VR due to the increase in intrathoracic pressure.
    Leads to reduction in RV filling pressure, LV filling, and CO.
    • This is the predominant reason why CO falls with the application of PEEP
      In a well patient, CO falls by:
      • 10% with IPPV and ZEEP
      • 18% with IPPV and 9cmH₂O of PEEP
      • 36% with IPPV and 16cmH₂O of PEEP
    • These changes are:
      • More marked in hypovolaemia
        Changes are reversed with volume expansion.
      • Less severe with poor lung compliance
        Reduced compliance greatly reduces the effect of PEEP and IPPV on the vasculature, as the change in intrapleural pressure is reduced.
  • LV preload may also be reduced due to increased RV afterload
    • Increased RV afterload may increase RV EDV, displacing the interventricular septum into the LV
    • The bulging septum decreases LVEDV, causing LV diastolic function and reduced LV filling
      This is an example of ventricular interdependence.
  • Reduced LV afterload due to reduced LV transmural pressure
    In some cases, IPPV augments circulatory function by reducing LV afterload to a greater extent than preload.
    • Effects in a well patient are minimal, as PEEP is relatively small in magnitude compared to systemic arterial pressures
    • In patients generating highly negative intrathoracic pressures, the LV transmural pressure can increase markedly, increasing LV afterload and reducing cardiac output

• Reduction in MAP
  MAP decreases as PEEP increases.

• Changes to oxygen flux
  PEEP will tend to improve PO₂ whilst reducing CO.

• Changes to pulmonary vascular resistance and RV afterload
  • If lung volume is lower than FRC, then PVR will reduce as PEEP stretches open extra-alveolar vessels
    • Alveolar recruitment will reduce hypoxic-pulmonary vasoconstriction, further reducing PVR
  • If lung volume is higher than FRC, then PVR will increase as PEEP compresses alveolar vessels
  • Therefore, PEEP has variable effects on RV afterload depending on how it changes lung volume with respect to FRC

End-Organ Effects

• Reduced urine output due to:
  • Reduced CO and renal blood flow
  • ADH release as a consequence of reduced atrial stretch and ANP release
    May worsen oedema in patients with prolonged periods of ventilation.

• Reduced hepatic blood flow due to:
  • Increased CVP and decreased CO lowering the pressure gradient for hepatic flow
    • May result in circulation only intermittently throughout the cardiac cycle

References

1. Lumb A. Nunn's Applied Respiratory Physiology. 7th Edition. Elsevier. 2010.
2. Luecke T, Pelosi P. Clinical review: Positive end-expiratory pressure and cardiac output. Critical Care. 2005;9(6):607-621. doi:10.1186/cc3877.
3. Yartsev, A. Positive End-Expiratory Pressure and it's consequences. Deranged Physiology.
4. Yartsev, A. Positive Pressure and PEEP. Deranged Physiology.
5. Yartsev, A. Indications and Contraindications for PEEP. Deranged Physiology.
6. Yartsev, A. Effects of Positive Pressure and PEEP on Alveolar Volume. Deranged Physiology.
7. Yartsev, A. [PEEP and Intrinsic PEEP}(http://www.derangedphysiology.com/main/core-topics-intensive-care/mechanical-ventilation-0/Chapter%202.1.6/peep-and-intrinsic-peep). Deranged Physiology.