Clinical sequelae of infarction

4 major causes of death post acute MI:

1) Decreased cardiac output — cardiogenic shock

• Myocardial infarct —> reduced number of contracting myocardial fibres + systolic stretch [dyskenesis: bulging of a portion of myocardial wall during systole]

—> reduced cardiac output

—> cardiogenic shock [insufficient blood flow to tissues]

2) Damming of blood in venous system

• Myocardial infarct

—> reduced cardiac output

—> i) damming of blood in venous system of lungs and systemic circulation

—> pulmonary congestion

ii) retention of fluid by kidneys.—> pulmonary congestion —> pulmonary oedema —> respiratory failure

3) Rupture of the infarcted area [wall rupture, VSD, papillary muscle rupture]

• During first day of myocardial infarct

—> little danger of rupture of ischaemic area of heart

• A few days post large MI (within 2 weeks)

—> dead muscle fibres begin to degenerate

—> thinning of wall in region of MI

—> progressive worsening of systolic bulging

—> rupture of heart wall

—> loss of blood into pericardial space

—> cardiac tamponade

—> blood is unable to enter compressed heart

—> cardiogenic shock

4) Ventricular dysrrhythmias post MI

• Danger periods for VF:

i) within first 10 minutes post MI

ii) 3 - 5 hours post MI lasting for hours to days

• Factors leading to tendency to fibrillate:

i) ischaemic myocardium —> release of intracellular K+ —> local increase in extracellular K+ —> increased myocardial irritability

ii) ischaemic myocardium —> unable to repolarise—> injury currents

iii) MI —> reduced CO —> intense sympathetic stimulation —> increased myocardial irritability

iv) MI —> reduced CO —> dilatation of heart —> increased pathway length for conduction —> allow impulse to re-enter muscle that is recovering from refractoriness —> increased potential for circus movements —> VT

Complications of MI

1. Early Arrhythmias

· 95% of patients of some disturbance of rate, rhythm or conduction

· 80% are VEB’s; 15% AF

2. Cardiac Failure

· 50% develop some degree of LV failure

3. Cardiogenic Shock

· 5% develop shock

4. Rupture

· 1.5% rupture LV wall —> tamponade & death

· VSD & papillary muscle rupture —> sudden worsening of cardiac failure

5. Thromboembolism

· Intraventricular thrombosis —> systemic embolisation

6. Myocardial Reinfarction

· 10% of cases within first few weeks

· Due to reocclusion of a reperfused coronary artery (usually)

7. LV Aneurysm & Cardiac Dilatation

· Especially extensive anterior infarct —> thinning of injured myocardium —> aneurysm formation ± general dilatation of ventricle

8. Persistent Angina

· Angina may arise from margins of infarction or an area distant to the infarction

9. Mortality

· 20-25% of patients with MI die before reaching hospital.

· After hospitalisation, the risk of death in the first month is 10-15%, with a further risk in the subsequent year of 10%.

· In subsequent years, the risk drops to ?% per annum.