2.6 Cardioplegia

Question 1

a) What are the purposes (3 marks) of cardioplegia solutions?

Answer 1

Purposes:
» Myocardial protection:

• Cardiac arrest in diastole and manipulation of the extracellular environment to minimise ongoing metabolic activity and its deleterious consequences during a period of suboptimal perfusion.
• Cooling of the heart.

> > Facilitation of surgery:

• Still, relaxed heart.
• Bloodless field.

Question 2

typical composition (4 marks) of cardioplegia solutions?

Physiological actions (5 marks) of cardioplegia solutions?

Answer 2

1. High potassium concentration,
   approximately 20 mmol/l.

Arrest of heart in diastole –
high extracellular potassium levels
prevent repolarisation of myocytes,
causing inactivation of the fast inward voltage
sensitive sodium channels that are important in
phase 0 of the action potential.

2. Calcium at a lower concentration
   than plasma.

Calcium is required to maintain
cell membrane integrity,
but keeping the concentration low reduces
the amount of calcium available for contraction,
thus avoiding myocardial activity.

3. Magnesium concentration exceeding
   normal plasma level.

Prevents magnesium loss from the cells,
thus maintaining its role as enzymatic cofactor,
and competes with calcium,
thus reducing calcium induced contraction.

4. Sodium and chloride usually at levels
   near those found in plasma.

(Alternatively, low sodium concentration can be used
as the mechanism to induce cardiac arrest.)

5. Bicarbonate, histidine or other buffer.
   To offset tendency to metabolic acidosis associated
   with ischaemia.
6. Mannitol.
   To raise the osmolarity of the solution,
   thus reducing tissue oedema.

Other additives:
7. Procaine.
8. Blood.
Reduction of arrhythmia at reperfusion.
Oxygen carrying capacity.

Question 3

b) By which routes can solutions of cardioplegia be administered? (2 marks)

Answer 3

1. >> Anterograde: 
cannula into ascending aorta 
or coronary ostia 
(dependent on adequate root pressure, 
good coronary perfusion and competent
aortic valve to reach all of the myocardium).

2. > > Retrograde: cannula into the coronary sinus

Question 4

c) What are the possible complications of cardioplegia solution administration? (6 marks)

Answer 4

Remember to include the issues that
may arise from the method of
administration as well as from the cardioplegia itself.

> > Direct damage associated with the cannulae.

> > Failure to attain widespread cardiac perfusion with the cardioplegia, leaving areas of myocardium warm and active whilst ischaemic.

> > Fluid overload.

> > Myocardial oedema,
haemorrhage and injury resulting
from high infusing pressures.

> > Postoperative electrolyte derangement
with consequent risk of arrhythmia.

> > Air bubbles in the cardioplegia solution can cause air emboli in the coronary arteries – bubble trap used.