G6i: The cardiovascular response to central neural blockade

Definitions

  • CNB achieved by subarachnoid or epidural anaesthesia
  • Injection of local anaesthetic (Ropi, Bupi) +/- opioid (fentanyl, morphine)
  • pre-ganglionic fibres lie in thoraco-lumbar spinal cord → ∴blocking output will alter vasomotor tone & CV function normally strictly regulated by SNS

CVS Response = ↓MAP

  • Anaesthetics block β-preganglionic sympathetic fibres
  • Inhibition:
    • α1 ( vaso & venoconstriction)
    • β1 (vaso/veno-constriction)
    • β2 (VD liver, skeletal m.)
  • Resulting:
    • Vasodilation → ↓afterL
    • Venodilation → venous pooling → ↓preL → F-S = ↓CO

Level of Block & CVS Effect

  • Symp. block

→ From RVLM (medulla)

→ Preganglionic symp fibres run in columns in SC from T1 – L3

  • T1 – T4
      • Vasomotor to aorta
      • Symp. outflow to H
      • Unable to ↑HR/FoC with SNS stimulation
      • Profound ↓MAP
  • T5 – L3
      • Vasomotor to abdominal organs, peripheral muscles, renal plexus, LL
    • Sacral: no symp. chain blockade → minimal effect on tone/BP
    • Lower thoracic/renal: ↓SVR → RAS still able to activate → BP preserved
    • Mid thoracic: ↓SVR → no compensatory RAS activation. But preserved BaroR → mild ↓MAP
    • Brain stem/High thoracic: blocks vasomotor tone
      • Inhibits cardiac symp. compensation → profound ↓MAP
      • Unopposed vagal tone → may cause cardiac arrest

Compensatory Mechanisms

High Pressure Afferents

    • High P BaroR detects ↓ stretch
    • Remove inhibition of symp. outflow from vasomotor centre (RVLM)
  1. ↑symp & ↓ parasymp. outflow
    • H → ↑HR, ↑FoC
    • Resistance Vessels → VC → ↑SVR
    • Capacitance Vessels → VC → ↑VR

→ Unable to compensate for large blood vol. pooling in capacitance system

2. β1 activation of RAAS

    • ↑renin → ↑AII → vaso & veno constriction

3. ↑ADH from PPG → ↑H2O absorption & direct CV

Low Pressure Afferents

      • BaroR in atria, Great Veins → ↓stretch → ↓ANP → ↓inhibition of ADH → ↓natriuresis & diuresis

Juxtaglomerular Reflex

    • ↓GFR → ↓flow through DCT → activation JG cells → ↑renin → ↑AII → vaso & veno constriction