A-Textbook-of-Clinical-Pharmacology-and-Therapeutics-5th-edition
A-Textbook-of-Clinical-Pharmacology-and-Therapeutics-5th-edition
A-Textbook-of-Clinical-Pharmacology-and-Therapeutics-5th-edition
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226 CARDIAC DYSRHYTHMIAS<br />
(as is not infrequently the case) <strong>and</strong> the patient actually has<br />
VT, little or no harm results, in contrast to the use <strong>of</strong> verapamil<br />
in VT.<br />
Mechanism <strong>of</strong> action<br />
Adenosine acts on specific adenosine receptors. A 1 -receptors<br />
block AV nodal conduction. Adenosine also constricts bronchial<br />
smooth muscle by an A 1 effect, especially in asthmatics. It<br />
relaxes vascular smooth muscle, stimulates nociceptive afferent<br />
neurones in the heart <strong>and</strong> inhibits platelet aggregation via<br />
A 2 -receptors.<br />
Adverse effects <strong>and</strong> contraindications<br />
Chest pain, flushing, shortness <strong>of</strong> breath, dizziness <strong>and</strong> nausea<br />
are common but short-lived. Chest pain can be alarming if<br />
the patient is not warned <strong>of</strong> its benign nature before the drug<br />
is administered. Adenosine is contraindicated in patients<br />
with asthma or heart block (unless already paced) <strong>and</strong> should<br />
be used with care in patients with WPW syndrome in whom<br />
the ventricular rate during atrial fibrillation may be accelerated<br />
as a result <strong>of</strong> blocking the normal AV nodal pathway <strong>and</strong><br />
hence favouring conduction through the abnormal pathway.<br />
This theoretically increases the risk <strong>of</strong> ventricular fibrillation;<br />
however, this risk is probably small <strong>and</strong> should not discourage<br />
the use <strong>of</strong> adenosine in patients with broad complex<br />
tachycardias <strong>of</strong> uncertain origin.<br />
Pharmacokinetics<br />
Adenosine is rapidly cleared from the circulation by uptake<br />
into red blood cells <strong>and</strong> by enzymes on the luminal surface <strong>of</strong><br />
endothelial cells. It is deaminated to inosine. The circulatory<br />
effects <strong>of</strong> a bolus therapeutic dose <strong>of</strong> adenosine last for 20–30<br />
seconds, although effects on the airways in asthmatics persist<br />
for longer.<br />
Drug interactions<br />
Dipyridamole blocks cellular adenosine uptake <strong>and</strong> potentiates<br />
its action. Theophylline blocks adenosine receptors <strong>and</strong><br />
inhibits its action.<br />
DIGOXIN<br />
For more information on digoxin, see also Chapter 31.<br />
Use<br />
The main use <strong>of</strong> digoxin is to control the ventricular rate (<strong>and</strong><br />
hence improve cardiac output) in patients with atrial fibrillation.<br />
Digoxin is usually given orally, but if this is impossible, or<br />
if a rapid effect is needed, it can be given intravenously. Since<br />
the t 1/2 is approximately one to two days in patients with normal<br />
renal function, repeated administration <strong>of</strong> a maintenance<br />
dose results in a plateau concentration within about three to six<br />
days. This is acceptable in many settings, but if clinical circumstances<br />
are more urgent, a therapeutic plasma concentration<br />
can be achieved more rapidly by administering a loading dose.<br />
The dose is adjusted according to the response, sometimes supplemented<br />
by plasma concentration measurement.<br />
Mechanism <strong>of</strong> action<br />
1. Digoxin inhibits membrane Na /K adenosine<br />
triphosphatase (Na K ATPase), which is responsible for<br />
the active extrusion <strong>of</strong> Na from myocardial, as well as<br />
other cells. This results in accumulation <strong>of</strong> intracellular<br />
Na , which indirectly increases the intracellular Ca 2<br />
content via Na /Ca 2 exchange <strong>and</strong> intracellular<br />
Ca 2 storage. The rise in availability <strong>of</strong> intracellular Ca 2<br />
accounts for the positive inotropic effect <strong>of</strong> digoxin.<br />
2. Slowing <strong>of</strong> the ventricular rate results from several<br />
mechanisms, particularly increased vagal activity:<br />
• delayed conduction through the atrioventricular node<br />
<strong>and</strong> bundle <strong>of</strong> His;<br />
• increased cardiac output due to the positive inotropic<br />
effect <strong>of</strong> digoxin reduces reflex sympathetic tone;<br />
• small doses <strong>of</strong> digitalis sensitize the sinoatrial node to<br />
vagal impulses. The cellular mechanism <strong>of</strong> this effect is<br />
not known.<br />
ATROPINE<br />
Use<br />
Atropine is administered intravenously to patients with<br />
haemodynamic compromise due to inappropriate sinus<br />
bradycardia. (It is also used for several other non-cardiological<br />
indications, including anaesthetic premedication, topical<br />
application to the eye to produce mydriasis <strong>and</strong> for patients<br />
who have been poisoned with organophosphorous anticholinesterase<br />
drugs; see Chapter 54).<br />
Mechanism <strong>of</strong> action<br />
Acetylcholine released by the vagus nerve acts on muscarinic<br />
receptors in atrial <strong>and</strong> cardiac conducting tissues. This<br />
increases K permeability, thereby shortening the cardiac<br />
action potential <strong>and</strong> slowing the rate <strong>of</strong> increase <strong>of</strong> pacemaker<br />
potentials <strong>and</strong> cardiac rate. Atropine is a selective antagonist<br />
<strong>of</strong> acetylcholine at muscarinic receptors, <strong>and</strong> it thereby counters<br />
these actions <strong>of</strong> acetylcholine, accelerating the heart rate<br />
in patients with sinus bradycardia by inhibiting excessive<br />
vagal tone.<br />
Adverse effects <strong>and</strong> contraindications<br />
Parasympathetic blockade by atropine produces widespread<br />
effects, including reduced salivation, lachrymation <strong>and</strong> sweating,<br />
decreased secretions in the gut <strong>and</strong> respiratory tract,<br />
tachycardia, urinary retention in men, constipation, pupillary<br />
dilatation <strong>and</strong> ciliary paralysis. It is contraindicated in<br />
patients with narrow-angle glaucoma. Atropine can cause<br />
central nervous system effects, including hallucinations.<br />
Pharmacokinetics<br />
Although atropine is completely absorbed after oral administration,<br />
it is administered intravenously to obtain a rapid