Clinical Pharmacology and Therapeutics
A Textbook of Clinical Pharmacology and ... - clinicalevidence
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202 ISCHAEMIC HEART DISEASE<br />
maximizing its cardiovascular benefits. Aspirin acetylates<br />
platelet COX as platelets circulate through portal venous<br />
blood (where the acetylsalicylic acid concentration is high<br />
during absorption of aspirin from the gastro-intestinal tract),<br />
whereas systemic endothelial cells are exposed to much lower<br />
concentrations because at low doses hepatic esterases result in<br />
little or no aspirin entering systemic blood. This has been<br />
demonstrated experimentally, but the strategy has yet to be<br />
shown to result in increased antithrombotic efficacy of very<br />
low doses. In practice, even much higher doses given once<br />
daily or every other day achieve considerable selectivity for<br />
platelet vs. endothelial COX, because platelets (being anucleate)<br />
do not synthesize new COX after their existing supply has<br />
been irreversibly inhibited by covalent acetylation by aspirin,<br />
whereas endothelial cells regenerate new enzyme rapidly<br />
(within six hours in healthy human subjects). Consequently,<br />
there is selective inhibition of platelet COX for most of the<br />
dose interval if a regular dose of aspirin is administered every<br />
24 or 48 hours.<br />
FIBRINOLYTIC DRUGS<br />
Several fibrinolytic drugs are used in acute myocardial<br />
infarction, including streptokinase, alteplase, reteplase <strong>and</strong><br />
tenecteplase. Streptokinase works indirectly, combining with<br />
plasminogen to form an activator complex that converts the<br />
remaining free plasminogen to plasmin which dissolves fibrin<br />
clots. Alteplase, reteplase <strong>and</strong> tenecteplase are direct-acting<br />
plasminogen activators. Fibrinolytic therapy is indicated,<br />
when angioplasty is not available, for STEMI patients with STsegment<br />
elevation or bundle-branch block on the ECG. The<br />
maximum benefit is obtained if treatment is given within 90<br />
minutes of the onset of pain. Treatment using streptokinase<br />
with aspirin is effective, safe <strong>and</strong> relatively inexpensive.<br />
Alteplase, reteplase <strong>and</strong> tenecteplase, which do not produce<br />
a generalized fibrinolytic state, but selectively dissolve<br />
recently formed clot, are also safe <strong>and</strong> effective; reteplase <strong>and</strong><br />
tenecteplase can be given by bolus injection (two injections<br />
intravenously separated by 30 minutes for reteplase, one single<br />
intravenous injection for tenecteplase), whereas alteplase<br />
has to be given by intravenous infusion. Despite their higher<br />
cost than streptokinase, such drugs have been used increasingly<br />
over streptokinase in recent years, because of the<br />
occurrence of immune reactions <strong>and</strong> of hypotension with<br />
streptokinase. Being a streptococcal protein, individuals who<br />
have been exposed to it synthesize antibodies that can cause<br />
allergic reactions or (much more commonly) loss of efficacy<br />
due to binding to <strong>and</strong> neutralization of the drug. Individuals<br />
who have previously received streptokinase (more than a few<br />
days ago) should not be retreated with this drug if they<br />
reinfarct. The situation regarding previous streptococcal infection<br />
is less certain. Such infections (usually in the form of sore<br />
throats) are quite common <strong>and</strong> often go undiagnosed; the<br />
impact that such infections (along with more severe streptococcal<br />
infections, such as cellullitis or septicaemia) have on the<br />
efficacy of streptokinase treatment is uncertain, but likely to<br />
be significant. Hypotension may occur during infusion of<br />
streptokinase, partly as a result of activation of kinins <strong>and</strong><br />
other vasodilator peptides. The important thing is tissue perfusion<br />
rather than the blood pressure per se, <strong>and</strong> as long as the<br />
patient is warm <strong>and</strong> well perfused, the occurrence of hypotension<br />
is not an absolute contraindication to the use of fibrinolytic<br />
therapy, although it does indicate the need for particularly<br />
careful monitoring <strong>and</strong> perhaps for changing to an alternative<br />
(non-streptokinase) fibrinolytic agent.<br />
Key points<br />
Ischaemic heart disease: pathophysiology <strong>and</strong><br />
management<br />
• Ischaemic heart disease is caused by atheroma in<br />
coronary arteries. Primary <strong>and</strong> secondary prevention<br />
involves strict attention to dyslipidaemia, hypertension<br />
<strong>and</strong> other modifiable risk factors (smoking, obesity,<br />
diabetes).<br />
• Stable angina is caused by narrowing of a coronary<br />
artery leading to inadequate myocardial perfusion<br />
during exercise. Symptoms may be relieved or<br />
prevented (prophylaxis) by drugs that alter the balance<br />
between myocardial oxygen supply <strong>and</strong> dem<strong>and</strong> by<br />
influencing haemodynamics. Organic nitrates,<br />
nicor<strong>and</strong>il <strong>and</strong> Ca 2 -antagonists do this by relaxing<br />
vascular smooth muscle, whereas β-adrenoceptor<br />
antagonists slow the heart.<br />
• In most cases, the part played by coronary spasm is<br />
uncertain. Organic nitrates <strong>and</strong> Ca 2 -antagonists<br />
oppose such spasm.<br />
• Unstable angina <strong>and</strong> NSTEMI are caused by fissuring of<br />
an atheromatous plaque leading to thrombosis, in the<br />
latter case causing some degree of myocardial necrosis.<br />
They are treated with aspirin, clopidogrel <strong>and</strong> heparin<br />
(usually low-molecular-weight heparin nowadays),<br />
which improve outcome, <strong>and</strong> with intravenous glyceryl<br />
trinitrate if necessary for relief of anginal pain; most<br />
cases should undergo coronary angiography at some<br />
stage to delineate the extent/degree of disease <strong>and</strong><br />
suitability for PCI or CABG, <strong>and</strong> this should be done<br />
early in patients who fail to settle on medical therapy.<br />
• STEMI is caused by complete occlusion of a coronary<br />
artery by thrombus arising from an atheromatous<br />
plaque, <strong>and</strong> is more extensive <strong>and</strong>/or involves a greater<br />
thickness of the myocardium than NSTEMI. It is treated<br />
by early (primary) angioplasty where this is available;<br />
where not available, fibrinolytic drugs (with or without<br />
heparin/low-molecular-weight heparin) should be<br />
given. Important adjunctive therapy includes aspirin<br />
<strong>and</strong> clopidogrel, inhaled oxygen <strong>and</strong> opoids.<br />
Angiotensin-converting enzyme inhibition, angiotensin<br />
receptor blockade <strong>and</strong> aldosterone antagonism (with<br />
eplerenone) each improve outcome in patients with<br />
ventricular dysfunction; whether the use of all three of<br />
these treatment modalities in combination confers<br />
additional benefit over maximal dosage with one of<br />
these agents remains a matter of debate.<br />
• After recovery from myocardial infarction, secondary<br />
prophylaxis is directed against atheroma, thrombosis<br />
(aspirin) <strong>and</strong> dysrhythmia (β-adrenoceptor antagonists,<br />
which also prevent re-infarction) <strong>and</strong> in some patients<br />
is used to improve haemodynamics (angiotensinconverting<br />
enzyme inhibitors, angiotensin receptor<br />
blockers <strong>and</strong>/or eplerenone).