19A20: Exam Report

Describe the cardiovascular effects of positive pressure ventilation on a patient who has received a long acting muscle relaxant.

33% of candidates passed this question.

Structured answers separating effects of positive pressure on right and left ventricle, on preload and on afterload were expected. Overall there was a lack of depth and many candidates referred to pathological states such as the failing heart. Simply stating that positive pressure ventilation reduced right ventricular venous return and/or left ventricular afterload, without some additional explanation was not sufficient to achieve a pass level.

G5ii / 19A20: Describe the cardiovascular effects of positive pressure ventilation on a patient who has received a long acting muscle relaxant

Definitions

  • IPPV = application of positive pressure to the airway to inflate the lungs
    • Results in ↑pleural pressure during INSP
    • During EXPR intrapleural & airway pressures return toward atmospheric, unless PEEP is added

Cardiovascular Changes

Inspiration

  • Initial ↑CO
    • +ve pressure
    • Compression of pulm. vessels
    • ↑LV preL
    • ↑LV SV

THEN ↓CO…

  • ↓ Venous return
    • ↓RV preL
    • ↓ RV SV
    • ↓ pulm. blood flow
    • ↓LV preL
    • ↓LV output
  • ↑ PVR
    • ↑airway pressure compresses pulm. vessels → ↑PAP
    • ↑PVR
    • ↑RV afterL
    • Bulging of IV septum into LV
    • ↓LV compliance & ↓LV filling
    • Exacerbates ↓ LV output
  • ↓ LV Afterload↑ITP
    • ↓ transmural P of LV
    • ↑ wall tension by Law of LaPlace
    • ↓ LV afterL
    • But, ↓LV filling dominates ∴even though ↓LV afterL there is ↓LV CO

(Transmural P = Intraventricular P – Intrathoracic P)

Expiration

      • ↑PreL
        • ↓Intrathoracic P
        • ↓Vessels compression
        • ↑VR to RV
        • ↑VR to LV
        • ↑CO
      • ↑AfterL
      • ↓ Intrathoracic P
      • ↑ Transmural P
      • ↑ AfterL
      • ↓ CO

OVERALL EFFECT: depends on cardiac function & volume status

Myocardial O2 Consumption

  • Myocardial O2 Consumption = Stroke work + potential E in ventricle at End Systole
  • IPPV ↓ preL and afterL
  • ∴ Shifts PV loop LEFT + DOWN
  • ∴↓ elastance defined potential work (Pot E)
  • ∴↓ myocardial O2 consumption