Pharmacology for Anaesthesia and Intensive Care

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9780521704632c14 CUFX213A/Peck 9780521618168 December 28, 2007 12:32

Section IIICardiovascular drugs

AVN

(a) (b) (c) (d) (e)

Figure 14.2.Atrioventricular nodal re-entrant tachycardia ( action potential followed by

refractory period). In this situation, there are two anatomically and physiologically distinct

conduction pathways within the atrioventricular node (AVN). The fast pathway has a long

refractory period, whereas the slow pathway has a short refractory period. (a) A normal atrial

action potential (AP) travels at different velocities through the two pathways. The AP in the

slow pathway arrives at the refractory final common pathway and is therefore terminated (b).

(b) also demonstrates how the slow pathway recovers from its refractory state more quickly

than the fast pathway. If a premature atrial impulse arrives at the origins of the two pathways

and finds the fast pathway still refractory it will only travel down the slow pathway (c).

Because it travels slowly down the slow pathway it is not terminated by refractory tissue as in

(b) and may travel on into the ventricles but also retrogradely up the fast pathway (d).

Because of the short refractory period of the slow pathway the impulse may travel down the

slow pathway (e) to continue the circus movement thereby generating the self-perpetuating

tachycardia. The atrioventricular re-enterant tachcardia seen in WPW syndrome is generated in

asimilar manner except that the accessory pathway (bundle of Kent) is distinct from the AVN.

Supraventricular tachyarrhythmias(SVT)(digoxin, adenosine, verapamil,β-

blockers, quinidine)


Ventricular tachyarrhythmias(VT)(lidocaine, mexiletine)


Both SVT and VT (amiodarone, flecainide, procainamide, disopyramide,

propafenone, sotalol)


Digoxin toxicity(phenytoin)

Supraventricular tachyarrhythmias

Digoxin

Presentation

Digoxin is a glycoside that is extracted from the leaves of the foxglove (Digitalis lanata)

and is available as oral (tablets of 62.5–250μg, elixir 50μg.ml−^1 )and intravenous

(100–250μg.ml−^1 )preparations. The intramuscular route is associated with variable

absorption, pain and tissue necrosis.