Pharmacology for Anaesthesia and Intensive Care

P1: PSB Printer: Yet To Come
9780521704632c14 CUFX213A/Peck 9780521618168 December 28, 2007 12:32

14 Anti-arrhythmics

Arrhythmias

Tachyarrhythmias


These may originate fromenhanced automaticitywhere the resting potential

of contractile tissue loses its stability and may reach its threshold for depolari-

zation before that of the SA node. This is seen during ischaemia and hypo-

kalaemia.


Ischaemic myocardium may result in oscillations of the membrane potential.

Theseafter-potentialsmay reach the threshold potential and precipitate tach-

yarrhythmias.


Re-entryorcircusmechanisms describe how an ectopic focus may originate, lead-

ing to tachyarrhythmias (Figure14.2).

Bradyarrhythmias

These are due to failure of conduction from the SA node to surrounding tissue.

Second- and third-degree block becomes clinically significant. Atropine,βstimula-

tion or pacing may be required.

Classification of anti-arrhythmics

Traditionally anti-arrhythmics have been classified according to the Vaughan–

Williams classification. However, it does not include digoxin and more recently intro-

duced drugs such as adenosine. In addition, individual agents do not fall neatly into

one category, e.g. sotalol has class I, II and III activity.

Anti-arrhythmics may also be divided on the basis of their clinical use in the

treatment of:

0

30

60

90

0 100 200 300 400 Time (msec)

4

3

2

1

0

(a) (b)

Membrane potential (mV)

Figure 14.1.Action potentials of (a) pacemaker and (b) contractile tissue.