FRCA Notes

Cardioplegia

The 2019 SAQ on cardioplegia (71% pass rate) was deemed 'straightforward to candidates who had rotated through a cardiac module'.

Resources

  • Cardioplegic solutions are administered to cause myocardial arrest in diastole, leading to:
    1. Better operative conditions with a still, relaxed heart
    2. Myocardial protection, preserving myocardial function and preventing cell death

Main features

  • The primary component is ~20mmol/L potassium
    • The potassium shifts the cardiac myocyte membrane potential to -50mV
    • This causes inactivation of If Na+ channels
    • It thus prevents upstroke of the myocyte action potential
    • It renders the myocardium unexcitable and therefore in an arrested state

  • Hyper-osmolar to reduce myocardial oedema
  • Alkalotic to offset metabolic acidosis

Crystalloid vs. blood

  • Cardioplegia solution is either formed in crystalloid or blood/crystalloid mix (typically in a 4:1 ratio)

    • Advantages of blood-based cardioplegia solution
    Oxygen carrying capacity; delivers O2 to myocytes (Although [Hb] 50g/L and Oxy-Hb curve shifted left)
    Delivers glucose & other nutrients
    Scavenges free radicals
    Buffers H+ ions
    Improves microvascular flow
    Reduces myocardial oedema
    Preserves myocyte & endothelial cell function

  • Blood-based cardioplegia solutions are therefore associated with a reduced incidence of:
    • Myocardial infarction
    • LV failure
    • Low cardiac output syndrome
    • Mortality

Other contents

Component Rationale
Calcium Maintains cell membrane integrity
Lower-than-plasma concentration avoids myocardial activity
Magnesium Prevents magnesium loss from the cells
Maintains its role as an enzymatic cofactor intracellularly
Competes with calcium, reducing calcium-induced contraction
Sodium bicarbonate Offsets metabolic acidosis associated with ischaemia
Mannitol Raises solution osmolarity, reducing tissue oedema
Procaine (sometimes other LA) Reduces arrhythmia risk at reperfusion by decreasing excitability
Na+ and Cl- At near plasma concentrations
Others Aspartate | Glutamate | L-arginine
Adenosine | NAC | Nicorandil

Temperature

  • Cold cardiplegia (4°C - 10°C)
    • Promotes electromechanical inactivity thus impeding apoptosis
    • Reduces oxygen consumption (from 80ml/100g/min to 0.3ml/100g/min at 22°C)
    • May accentuate reperfusion injury

  • Tepid cardioplegia (27-30°C)
    • May provide the best overall protection and recovery

  • Warmed cardioplegia (37-38°C)
    • Associated with lesser increases in post-operative CK-MB
    • Associated with reduced length of stay
    • May render myocardium susceptible to warm ischaemic injury if continuously infused

  • In the CPB circuit, a separate mixing system mixes cardioplegia solution with blood taken from the arterial line
  • It is either administered:
    • As an initial bolus dose is followed by subsequent doses every 20mins or so
    • As a continuous infusion

Route of administration

  • Anterograde administration into the coronary artery ostia (i.e. into the aortic root proximal to the cross-clamp)
    • Benefits from quick arrest and good LV protection
    • Requires a competent aortic valve and relatively patent coronary arteries

  • Retrograde administration into the coronary venous sinus
    • E.g. if concerns about coronary artery patency or aortic valve competency

  • Direct damage associated with the cardioplegia infusion cannulae
  • Failure to attain total cardiac perfusion with the cardioplegia solution; may leave areas of myocardium active but ischaemic
  • Fluid overload
  • Myocardial injury/haemorrhage from high infusion pressures
  • Myocardial oedema
  • Air bubble embolisation into the coronary arteries (reduced using a bubble trap)