F10i: Descrbe the physiological consequences of IPPV

IPPV Definition

IPPV is artificial ventilation whereby mouth pressure is elevated above atmospheric pressure to inflate the lungs

Respiratory Effects

Inspiration

  • Mouth P > Atmospheric P
  • Lung inflates according to its compliance & resistance
  • Slow insp. → gas flows in accordance with C
  • Fast insp. → flow distributes to parts of lung with short time constants
  • ↑Alveolar & Intrapleural pressures
  • Alveolar P → P required to overcome elastic recoil of lung
  • Intrapleural P → P required to overcome elastic recoil of lung

For INSP to occur must overcome:

  • Elastic recoil of lung
  • Resistance to airflow

Expiration

  • Passive
  • Differs from SPV because no diaphragmatic tone
  • Mouth pressure just falls back to atmospheric P

Cardiovascular Effects

Inspiration

  • ↑LV preload
    • ↑Intrathoracic P squeezes pulm. vessels
    • ↑VR to L) heart
  • ↑CO
    • F – S mechanism; ↑preload = ↑CO
  • ↓LV afterload
Wall Tension

Transmural P = Intraventricular P – Intrapleural P

  • IPPV = ↑intrapleural P
  • ↓Transmural P
  • ↓Wall tension
  • ∴↓afterload

(Sustained ↑Intrathoracic P)

  • ↓RV preload
    • ↑intrathoracic P
    • ↓VR
    • ↓RV preload
  • ↑PVR
    • ↑alveolar P = ↑PVR
  • ↑RV Afterload
    • Due to ↑PVR
  • ↓RV output → ↓LV output

Expiration

  • ↓CO
    • ↓intrathoracic P
    • ↑VR
    • Blood pools in pulm. vessels
    • Sustained ↓LV preload → ↓CO
    • Eventually restored as blood returns to LV

Renal (↓Urine Output)

  • ↓LA stretch receptors (vol)
  • ↑ADH
  • ↑H2O reabsorption