G5ii: Describe the cardiovascular consequences of PEEP

Definitions

PEEP = Pressure above atmospheric, maintained at the airways at the end of expiration

Cardiovascular Effects

Intrathoracic Pressure

  • On expiration, ITP returns to zero
  • Positive pressure applied at the end of expiration will ↑ITP through expiration

HR

    • No ∆
    • ∴CV ∆ are due to ↓SV

RV Preload

  • Venous return is maximal when RAP = 0mmHg
  • ↑ITP = ↑RAP = ↓VR
  • By F – S: ↓PreL = ↓RV output
  • RA is highly compliant & ∴RAP will vary with ITP

RV Afterload

    • To eject blood, RV must generate enough P to overcome PEEP & PAP
    • ↑RV afterL with PEEP = ↓RV CO
    • ↑RV End Systolic Volume
    • Further ↑RV wall stress

Pulmonary Vascular Resistance

@HIGH LUNG VOLUMES & ↑PEEP

  • Overdistention of alveoli
  • Compression of pulm. vessels
  • ↑PVR
  • ↓pulm. blood flow

However, if PEEP recruits collapsed alveoli

  • ↑Regional PaO2
  • ↓HPVC
  • ↓PVR

IV Septum

    • 2 ventricles share common septum
    • ↑RV end systolic volume
    • Deviation of IV septum
    • Impairs LV filling

LV Preload/Compliance

  • ↓RV inflow
  • ↓RV outflow
  • ↓LV preload → due to ventricles being in a series
  • Dilation of RV & IV septum deviation → ↓LV diastolic compliance

LV Output

    • Depends on LV preL, afterL, contractility
LV Output
    • PEEP ↓LV afterL: Transmural P = Intraventricular P – Intrathoracic P

 

      • ↑ITP
      • ↓Transmural P of LV
      • ↓Wall tension of Law of LaPlace
      • ↓LV afterL

→ But ↓LV filling deominates, so even with ↓LV afterL there is ↓LV CO

ANP

  • ↑PEEP = ↑RAP = ↑ANP = ↑natriuresis (Na + H2O excretion)