What is the relationship between the conduction system of the heart and cardioplegia in cardiac surgery?

Relationship Between the Conduction System of the Heart and Cardioplegia in Cardiac Surgery

Cardioplegia significantly impacts the cardiac conduction system during cardiac surgery, with different methods of cardioplegia affecting conduction recovery differently. Normothermic blood cardioplegia provides better electrophysiologic recovery of the cardiac conduction system compared to cold crystalloid cardioplegia, resulting in fewer postoperative conduction disturbances and arrhythmias. 1

Cardiac Conduction System Anatomy and Vulnerability

• The cardiac conduction system (sinoatrial node, atrioventricular node, His bundle, bundle branches) is particularly vulnerable to ischemia-reperfusion injury during cardiac surgery 2
• Conduction tissue is more susceptible to cold injury than working myocardium, explaining why hypothermic cardioplegia techniques may cause more conduction disturbances 3
• The anatomical relationship between surgical procedures and the conduction system significantly impacts the risk of postoperative bradycardia and heart blocks 2

Effects of Different Cardioplegia Types on Conduction

Temperature Effects

• Cold crystalloid cardioplegia (4-16°C) causes significantly more fascicular blocks (24% vs 8%) compared to tepid blood cardioplegia 3
• Normothermic blood cardioplegia (33.5°C) allows spontaneous resumption of sinus rhythm in 95% of patients compared to only 25% with hypothermic cardioplegia 1
• Hypothermic cardioplegia is associated with higher incidence of:
  • Ventricular fibrillation during reperfusion 1
  • Complete atrioventricular block requiring temporary pacing 1
  • Ventricular premature beats in the first 16 hours post-bypass 1

Composition Effects

• Blood vs. crystalloid cardioplegia:
  • Blood-based cardioplegia provides better protection of the conduction system 3
  • No single cardioplegia solution (including del Nido, histidine-tryptophan-ketoglutarate, St. Thomas) has shown clear superiority in myocardial protection 4
  • Terminal warm blood cardioplegia before crossclamp removal may reduce arrhythmias in complex congenital heart surgery 5

Clinical Implications of Conduction Disturbances After Cardioplegia

• Conduction disturbances occur in 18-45% of patients after coronary artery bypass grafting 3

• Risk factors for conduction disturbances include:

  • Long aortic cross-clamp time 3
  • Method of cardioplegia (cold crystalloid having higher risk) 3
  • Depth of hypothermia 3
  • Patient age 3
  • Inadequate myocardial protection 6

• Clinical consequences of conduction disturbances:

  • Need for temporary pacing 1
  • Prolonged hospital stay (14.4 days with cold crystalloid vs. 8.4 days with tepid blood cardioplegia) 3
  • Hemodynamic instability requiring inotropic support 3
  • Increased mortality risk, particularly late mortality 6

Minimizing Conduction System Damage During Cardiac Surgery

• Routine placement of temporary epicardial pacing wires during coronary artery bypass surgery is reasonable (Class IIa recommendation) 2
• For patients who develop new postoperative sinus node dysfunction or atrioventricular block with persistent symptoms or hemodynamic instability after isolated coronary artery bypass, permanent pacing is recommended before discharge (Class I recommendation) 2
• Minimally invasive extracorporeal circulation techniques reduce postoperative atrial fibrillation by 34-38% compared to conventional extracorporeal circulation 2

Cardioplegia Selection and Conduction System Protection

• No specific type of cardioplegia (warm vs. cold, blood vs. crystalloid, antegrade vs. retrograde, continuous vs. intermittent) has shown clear superiority in preventing conduction disturbances 2
• However, temperature appears to be a critical factor, with normothermic or tepid techniques showing better conduction system recovery 1, 3
• The choice between beating-heart (off-pump) and on-pump techniques with cardioplegia shows conflicting results regarding postoperative atrial fibrillation 2

Monitoring and Management of Conduction Disturbances

• Most conduction disturbances with tepid blood cardioplegia resolve within the first hour 3
• Cold crystalloid cardioplegia-induced conduction disturbances may persist, with some patients discharged with left anterior hemiblock 3
• Continuous electrocardiographic monitoring is essential to detect conduction abnormalities early 1
• Temporary pacing may be required for a mean duration of 168 minutes following hypothermic cardioplegia 1

In conclusion, the cardiac conduction system is highly sensitive to the effects of cardioplegia, with temperature being a critical factor in determining electrophysiologic recovery. Normothermic or tepid blood cardioplegia appears to provide better protection of the conduction system compared to cold techniques, resulting in fewer postoperative arrhythmias and conduction disturbances.