ca2+ channel block

8/3/2019 Ca2+ Channel Block

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CaCa2+2+ Channel BlockChannel Block

Donnah Laizabeth A. Dones


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The major electrophysiological effects

resulting from block of cardiac Ca2+

channels are in slow-response tissues:

a. sinus nodes

b. AV nodes.


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nifedipine

Dihydropyridines

used commonly in angina and hypertension

preferentially block Ca

2+

channels in vascularsmooth muscle;

their cardiac electrophysiological effects,

such as heart rate acceleration, result

principally from reflex sympathetic activationsecondary to peripheral vasodilation.


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verapamil, diltiazem, and

bepridil block Ca2+ channels in cardiac cells at clinically useddoses.

These drugs generally slow heart rate although

hypotension, if marked, can cause reflex sympatheticactivation and tachycardia.

The velocity of AV nodal conduction decreases,sothe PR interval increases.

AV nodal block -occurs as a result of decrementalconduction, as well as increased AV nodalrefractoriness.


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Another important indication forantiarrhythmic therapy is to reduceventricular rate in atrial flutter or

fibr

illat

ion

.

Unlike adrenergic receptor antagonists,Ca2+

channel blockers have not been shown toreduce mortality after myocardialinfarction (Singh, 1990).


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Bepridil

increases action potential duration in

many tissues

can exert an antiarrhythmic effect in atriaand ventricles.

However, it can cause torsades de pointes

it is not prescribed widely and has beendiscontinued in the United States.


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Verapamil and Diltiazem

The major adverse effect ofintravenous verapamilor diltiazem is hypotension,particularly with bolusadministration.

Hypotension - is a particular problem if the drugs are used mistakenly in

patients with ventricular tachycardia (in which Ca2+

channel blockers usually are not effective) misdiagnosedas AV nodal re-entrant tachycardia (Stewart et al., 1986).

-also is frequent in patients receiving other vasodilators,including quinidine, and in patients with underlying leftventricular dysfunction, which the drugs can exacerbate.


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Parenteral verapamil and diltiazem-

approved for rapid conversion of PSVTs tosinus rhythm and for temporary control of

rapid ventricular rate in atrial flutter or

fibrillation.


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Severe sinus bradycardia or AV block

also occurs, especially in susceptible

patients, such as those also receiving

blockers.

With oral therapy, these adverse effects

tend to be less severe.

Constipation can occur with oral

verapamil


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Verapamil

(CALAN, ISOPTIN, VERELAN, COVERA-HS) isprescribed as a racemate.

L

-Verapamil is a more potent calcium channelblocker than is D-verapamil.

However, with oral therapy, the L-enantiomerundergoes more extensive first-pass hepatic

metabolism. --For this reason, a givenconcentration of verapamil prolongs the PRinterval to a greater extent when administeredintravenously than when administered orally


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Oral verapamil

- may be used in conjunction with digoxin to

control ventricular rate in chronic atrial flutter

or fibrillation

for prophylaxis of repetitive PSVT.


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Diltiazem

(CARDIZEM, TIAZAC, DILACORXR, and

others)

also undergoes extensive first-pass

hepatic metabolism

have metabolites that exert Ca2+ channel

blocking actions.


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In clinical practice, adverse effects during therapy withverapamil or diltiazem are determined largely by:

a. underlying heart disease

b. concomitant therapy

plasma concentrations of these agents are notmeasured routinely

Both drugs can increase serum digoxin concentration,although the magnitude of this effect is variable

excess slowing of ventricular response may occur inpatients with atrial fibrillation.

ca2+ channel block

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