Drugs for PSVT

DRUGS FOR PSVT

An attack of PSVT can be terminated by i.v. injection of verapamil, diltiazem, esmolol or digoxin; but most cardiologists now prefer adenosine. Maintenance therapy with oral digoxin/ verapamil/β blockers can prevent recurrences.

Adenosine

Administered by rapid i.v. injection (over 1–3 sec) either as the free base (6–12 mg) or as ATP (10–20 mg), adenosine terminates within 30 sec. more than 90% episodes of PSVT involving the AV node. It activates ACh sensitive K+ channels and causes membrane hyperpolarization through interaction with A1 type of G protein coupled adenosine receptors on SA node (pacemaker depression → bradycardia), AV node (prolongation of ERP → slowing of conduction) and atrium (shortening of AP, reduced excitability). It indirectly reduces Ca2+ current in AV node; depression of the reentrant circuit through AV node is responsible for termination of PSVT. Adrenergically induced DADs in ventricle are also suppressed. Coronary dilatation occurs transiently.

ADENOJECT, ADENOCOR 3 mg adenosine (base) per ml in 2 ml and 10 ml amp.

Adenosine has a very short t½ in blood (~10 sec) due to uptake into RBCs and endothelial cells where it is converted to 5AMP and inosine. Almost complete elimination occurs in a single passage through coronary circulation. Injected ATP is rapidly converted to adenosine.

Dipyridamole potentiates its action by inhibiting uptake, while theophylline/ caffeine antagonize its action by blocking adenosine receptors. Higher doses may be required in heavy tea/coffee drinkers. Patients on carbamazepine are at greater risk of developing heart block. Advantages of adenosine for termination of PSVT are:

·      Efficacy equivalent to or better than verapamil.

·      Action lasts < 1 min; adverse effects (even cardiac arrest, if it occurs) are transient.

·   No haemodynamic deterioration; can be given to patients with hypotension, CHF or those receiving β blockers. Verapamil is contraindicated in these situations.

·      Safe in wide QRS tachycardia (verapamil is unsafe).

·      Effective in patients not responding to verapamil.

However,  adenosine  produces  transient dyspnoea, chest pain, fall in BP and flushing in 30–60% patients; ventricular standstill for few sec or VF occurs in some patients. Bronchospasm may be precipitated in asthmatics. Adenosine has to be rapidly injected in a large vein and has brief action, not suitable for recurrent cases. It is expensive and cannot be used to prevent recurrences.

Other Uses of Adenosine

ü Diagnosis of tachycardias dependent on AV node.

ü To induce brief coronary vasodilatation during certain diagnostic/interventional procedures.

ü To produce controlled hypotension during surgery.