2.01 - Cellular Reactions To Myocardial Ischaemia

Question 1

What electrochemical event occurs during early ischaemia?

Answer 1

Disordered calcium handling. Failure to remove calcium from the cell Calcium is important as it plays a role in conduction and action potential, in excitation-contraction coupling and contraction and contractility

Question 2

What are the consequences of increased intracellular calcium?

Answer 2

Electrophysiological afterdepolarisations (early and late afterdpolarisations). Both caused by calcium overload Functional: relaxation failure, contraction failure, cardiac contracutre. “If you fail to remove calcium, relaxation fails, this then leads to a failure of contraction”

Question 3

What are afterdepolarisations?

Answer 3

Afterdepolarizations are abnormal depolarizations of cardiac myocytes that interrupt phase 2, phase 3, or phase 4 of the cardiac action potential in the electrical conduction system of the heart. Afterdepolarizations may lead to cardiac arrhythmias.

Question 4

What are early afterdepolarisations?

Answer 4

Early afterdepolarizations (EADs) occur with abnormal depolarization during phase 2 or phase 3, and are caused by an increase in the frequency of abortive action potentials before normal repolarization is completed.

Phase 2 may be
interrupted due to
augmented opening of calcium channels, while phase 3 interruptions are due to the opening of sodium channels. Early afterdepolarizations can result in torsades de pointes, tachycardia, and other arrhythmias.

Question 5

What are delayed afterdepolarisations?

Answer 5

Delayed afterdepolarizations (DADs), on the other hand, begin during phase 4 - after repolarization is completed, but before another action potential would normally occur. They are due to elevated cytosolic calcium concentrations

Question 6

Describe the nature of contracile failure in ischaemia

Answer 6

Systolic failure declines relatively rapidly after ischaemia

Diastolic failure occurs much quicker but is less obvious to see

There is a threshold for reversibility

However if ischaemia is continued for too long, the myocardium becomes a contractile and no amount of pressure can cause it to contract.

Question 7

Describe Myocardial Stunning

Answer 7

Usually short duration, mild, sublethal ischaemia. Good examples of this is in coronary artery bypass grafting

Persistent contraction depression despite. Restoration of blood flow. No irreversible damage

Short duration: Transient – 2 hours to several days

Two most likely mechanisms: Calcium overload, Oxygen free radicals

Question 8

Describe Myocardial Hibernation

Answer 8

Prlonged contractile depression. Occurs in people with prolonged ischaemia. Cellular integrity maintained in spite of damage. May be protective.