Resp – PEEP

Master answer

Positive Pressure Ventilation

• PEEP = Positive End Expiratory Pressure.
  • Equivalent to a constant pressure applied throughout the respiratory cycle.
• Intrinsic PEEP = unintentional or un-measured end-expiratory hyperinflation
• Physiological effects of Positive Pressure Ventilation mostly related to increased mean airway pressure

Cardiovascular Effects

• Causes constant ↑ intrathoracic pressure (ITP) throughout respiratory cycle
• Left Heart
  • On Preload
    • Initially: → ↑ LV PL → ↑ CO
    • Secondarily: → ↓RV output → ↓ LV PL → ↓ CO
  • On Afterload
    • → ↓myocardial transmural pressure → ↓LV AL → ↑CO
    • PEEP > Ao diastolic pressure
      • → Collapse of intrathoracic aorta → Starling resistor mechanism → ↑LV AL → ↓ CO
    • PEEP < Ao diastolic pressure
      • ↑ pressure gradient for flow to systemic circulation
      • ↓ AL → ↓ myocardial work
  • On Compliance
    • diastolic buldging of septum → ↓ LV compliance → ↓LV CO
• Right Heart
  • → ↑ Pulmonary vascular resistance → ↑ RV AL → ↓ RV output
  • → ↓ Venous return → ↓ RV PL → ↓ RV output
• In the failing LV → ↑CO
  • ↓ LV afterload
    • ↑ pressure gradient thorax to abdomen
    • ↓ transmural pressure → ↓ LV wall tension = ↓afterload
  • ↓ LV preload (ie more favourable position on compliance curve)

Respiratory Effects

Beneficial Effects:

• ↓ atelectasis and gas trapping
  • ↑’s FRC > “closing capacity”
  • Shifts position on P-V curve right, above the “closing point”
• ↑ lung compliance → shifts back to steep/compliant part of P-V curve
• ↓ intrapulmonary shunt thus improved V/Q match and ↑ PaO2
  • ↓ atelectasis and gas trapping
  • ↓ extravascular lung water → ↓interstitial/alveolar oedema
• ↓ AWR
  • ↑ lung volume → ↑ radial traction by parenchyma → ↑ airway calibre
• ↓ work of breathing
  • ↑ lung compliance = ↓ elastic work
  • ↓ AWR = ↓resistance work

Negative Effects:

• ↑ V/Q mismatch
  • ↑ west zone 1
• ↓ lung compliance
  • shift to flat part of P-V curve
• ↑ Work of breathing (due to compliance change)
• ↑ PVR and RV afterload
  • extrinsic compression of pulmonary vessels
• Barotrauma

End-Organ Effects

• Renal:
  • ↓CO & ↑renal venous pressure
    • → Reduced renal blood flow → Reduced GFR → Reduced urine output
    • → Reduced atrial stretch and ANP release  → Increased ADH → Fluid retention → Oedema
• Hepatic:
  • 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
  • Hepatocyte dysfunction
• Haematological:
  • Neutrophil sequestration in the compressed pulmonary vasculature
• CNS:
  • ↓VR ⇒ ↑CVP ⇒ ↑ICP

Gladwin / Mooney / JC 2020

Answer by Gladwin, JC, Mooney