Choice Between Drugs And Electroconversion

Direct current (DC) electric shock applied externally is often the best way to convert cardiac arrhythmias to sinus rhythm. Many atrial or ventricular arrhythmias start as a result of transiently operating factors but, once they have begun, the abnormal mechanisms are self-sustaining. When a successful electric shock is given, the heart is depolarised, the ectopic focus is extinguished and the SA node, the part of the heart with the highest automaticity, resumes as the dominant pacemaker.

Electrical conversion has the advantage that it is immediate, unlike drugs, which may take days or longer to act; also, the effective doses and adverse effects of drugs are largely unpredictable, and can be serious.6

Uses of electrical conversion: in supraventricular and ventricular tachycardia, ventricular fibrillation and atrial fibrillation and flutter. Drugs can be useful to prevent a relapse, e.g. sotalol, amiodarone.

SPECIFICTREATMENTS7

Sinus bradycardia

Acute sinus bradycardia requires treatment if it is symptomatic e.g. where there is hypotension or escape rhythms; extreme bradycardia may allow a ventricular focus to take over and lead to ventricular tachycardia. The foot of the bed should be raised to assist venous return and atropine should be given i.v. Chronic symptomatic bradycardia is an indication for the insertion of a permanent pacemaker.

Atrial ectopic beats

Reduction in the use of tea, coffee and other methylxanthine-containing drinks, and of tobacco, may suffice for ectopic beats not due to organic

6 To the layman, 'shock' treatment could be interpreted as frights (which stimulate the vagus, as described above), or as the electrical sort. Dr James Le Fanu describes a Belfast doctor who reported a farmer with a solution that covered both possibilities. He had suffered from episodes of palpitations and dizziness for 30 years. When he first got them, he would jump from a barrel and thump his feet hard on the ground at landing. This became less effective with time. His next 'cure' was to remove his clothes, climb a ladder and jump from a considerable height into a cold water tank on the farm. Later, he discovered the best and simplest treatment was to grab hold of his 6-volt electrified cattle fence — although if he was wearing Wellington (rubber) boots he found he had to earth the shock, so besides grabbing the fence with one hand he simultaneously shoved a finger of the other hand into the ground.

7 See also UK Resuscitation Council guidelines (Fig. 24.2).

heart disease. When action is needed, a small dose of a p-adrenoceptor blocker may be effective.

Paroxysmal supraventricular (AV reentrant or atrial) tachycardia

For acute attacks, if vagal stimulation (by carotid massage, or swallowing ice-cream) is unsuccessful, adenosine has the dual advantage of being effective in most such tachycardias, while having no effect on a ventricular tachycardia. The response to adenosine is therefore of diagnostic value. Intravenous verapamil is an alternative for the acute management of a narrow complex tachycardia. If, however, the patient is in circulatory shock as a result of the tachycardia, or drug treatment fails, a DC shock should be delivered, for immediate effect. Flecainide or sotalol are the drugs of choice for preventing attacks (prophylaxis).

Atrial fibrillation (AF)

The therapeutic options are:

• Treatment vs no treatment

• Conversion vs rate control

• Immediate vs delayed conversion

• Drugs or DC conversion.

The information required is:

• Haemodynamic state ('normal' or compromised)

In many patients, AF is an incidental finding on the background of some existing cardiovascular disease, and with a large atrium. With a long history of symptoms, rate-controlling medication such as a P-blocker, digoxin or calcium antagonist is indicated. If there appears to be a short history (weeks), and the atrium is not enlarged, or there has been recent onset of heart failure or shock, cardioversion should be attempted. Electrical (DC) conversion is favoured where treatment is either urgent or likely to be successful in holding the patient in sinus rhythm. Pharmacological conversion can often be achieved over hours to days by amiodarone, and this drug is also useful in patients who revert rapidly to AF after DC conversion.

When conversion is not urgent, it should be delayed for a month to permit institution of anti coagulation by warfarin, and this should be continued for 4 weeks thereafter. In patients who have reverted to AF after previous conversions, amiodarone is the drug of choice prior to further attempts at cardioversion. Amiodarone is also used to suppress episodes of paroxysmal supraventricular tachycardia and atrial fibrillation.

Additional treatments in chronic atrial fibrillation.

Long-term treatment with warfarin is almost mandatory to reduce embolic complications. The efficacy of aspirin as an antiembolic agent is probably less in this group, but has been shown to be of value in patients where warfarin is considered inappropriate.

Atrial flutter

It is doubtful whether this differs in its origins or sequelae from atrial fibrillation. The ventricular rate is usually faster (typically, half an atrial rate of 300, where 2:1 block is present), which is too fast to leave without treatment. Since, similarly, the patient is unlikely to have been in this rhythm for a prolonged period, there is less likelihood that atrial thrombus has accumulated. Conversion without prior anticoagulation may occasionally be considered safe but anticoagulation is usually also needed. Patients should not be left in chronic atrial flutter, and DC conversion will usually restore either sinus rhythm or result in atrial fibrillation. The latter is treated as above. Patients who fail to convert, or who revert to atrial flutter should be referred for consideration of radiofrequency ablation that is highly effective and may remove the cause of the atrial flutter > 80% of cases.

Atrial tachycardia with variable AV block

The atrial rate is 120-250/min, and commonly there is AV block. If the patient is taking digoxin, it should be suspected as the possible cause of the arrhythmia, and stopped. If the patient is not taking digoxin, it may be used to control the ventricular rate. These patients should be considered for referral for radiofrequency ablation.

Heart block

The use of permanent pacemakers is beyond the scope of this book. In an emergency, AV conduction may be improved by atropine (antimuscarinic vagal block) (0.6 mg i.v.) or by isoprenaline (p-adrenoceptor agonist) (0.5-10 micrograms/min, i.v.). Temporary pacing wires may be needed prior to referral for pacemaker implantation.

Pre-excitation (Wolff-Parkinson-White) syndrome

This occurs in otherwise healthy individuals, who possess an anomalous (accessory) atrioventricular pathway; they often experience attacks of paroxysmal AV re-entrant tachycardia or atrial fibrillation. Drugs that both suppress the initiating ectopic beats and delay conduction through the accessory pathway are used to prevent attacks e.g. flecainide, sotalol or amiodarone. Verapamil and digoxin may increase conduction through the anomalous pathway and should not be used. Electrical conversion may be needed to restore sinus rhythm when the ventricular rate is very rapid. Radiofrequency ablation of aberrant pathways will almost certainly provide a cure.

Ventricular premature beats

These are common after myocardial infarction. Their particular significance is that the R-wave (ECG) of an ectopic beat, developing during the early or peak phases of the T-wave of a normal beat, may precipitate ventricular tachycardia or fibrillation (the R-on-T phenomenon). About 80% of patients with myocardial infarction who proceed to ventricular fibrillation have preceding ventricular premature beats. Lignocaine (lidocaine) is effective in suppression of ectopic ventricular beats but is not often used as its addition increases overall risk.

Ventricular tachycardia

Ventricular tachycardia demands urgent treatment since it frequently leads to ventricular fibrillation and circulatory arrest. A powerful thump of the fist on the mid-sternum or precordium may very occasionally stop a tachycardia. If there is rapid haemodynamic deterioration, electrical conversion is the treatment of choice. If the patient is in good cardiovascular condition, treatment may begin with lignocaine (lidocaine) i.v. or, should that fail, amiodarone i.v. For recurrent ventricular tachycardia, amiodarone or sotalol are preferred. Mexiletine, disopyramide, procainamide, quinidine and propafenone are not usually indicated. These patients should be referred for consideration of the implantation of an implantable cardioverter defibrillator (ICD).

Ventricular fibrillation and cardiac arrest

Ventricular fibrillation is usually caused by myocardial infarction or ischaemia, or serious organic heart disease and is the main cause of cardiac arrest. Guidelines for the management of peri-arrest arrythmias and cardiac arrest are issued by the UK Resuscitation Council and appear in Fig. 24.2 and 24.3. Patients suffering failed sudden cardiac death (SCD) should be referred for consideration of the implantation of an ICD.

Long QT syndromes

These are caused by malfunction of ion channels, leading to impaired ventricular repolarisation (expressed as prolongation of the QT interval) and a characteristic ventricular tachycardia, torsade de pointes.8 The symptoms range from episodes of syncope to cardiac arrest. An enlarging number and variety of drugs are responsible for the acquired form of the condition (including antiarrhythmic drugs, antibimicrobials, histamine Hj-receptor antagonists, serotonin receptor antagonists), and predisposing factors are female sex, recent heart-rate slowing, and hypokalemia.9 Congenital forms of the long QT syndrome are due to mutations in the genes encoding for ion channels, some of which may be revealed by exposure to drugs.

Summary

• The treatment of arrhythmias can be directly physical, electrical, pharmacological or

8 Fr. torsade, twist + pointe, point. 'Twisting of the points', referring to the characteristic sequence of 'up', followed by 'down' QRS complexes. The appearance has been referred to as 'cardiac ballet'.

9 Viskin S 1999 Lancet 354: 1625-1633.

surgical. Radiofrequency ablation and the use of devices such as permanent pacemakers and ICDs is increasing massively and the use of drugs by themselves declining in relative terms. Drugs are often now used as adjunctive treatments.

The choice among drugs is influenced partly by theoretical predictions from their action on the cardiac cycle but largely by short and long-term observations of their efficacy and safety.

All antiarrhythmics can be dangerous, and should not be used unless patients are symptomatic or haemodynamically compromised.

Adenosine is the treatment of choice for diagnosis and reversal of supraventricular arrhythmias. Verapamil is an alternative for the management of narrow complex tachycardias. Amiodarone is the most effective drug at reversing atrial fibrillation, and in prevention of ventricular arrhythmias, but has several adverse effects.

Digoxin retains a unique role as a positively inotropic antiarrhythmic, being most useful in slowing atrioventricular conduction in atrial fibrillation.

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