Cardiac Arrhythmias

Your heart rate is controlled by a complex electrical system within the heart muscle which drives it to go faster when you exert yourself and slower when you rest. A number of conditions can affect the heart rate or rhythm. Heart rate simply refers to how fast your heart is beating. Heart rhythm refers to the electrical source that is driving the heart rate and whether or not it is regular or irregular.

As some types of arrhythmias can cause you to faint without warning, your doctor may restrict your driving until the condition is controlled.

Some common terms:

• Sinus rhythm is the normal rhythm
• Arrhythmia means abnormal rhythm
• Fibrillation means irregular rhythm or quivering of one part of the heart
• Bradycardia means slow heart rate
• Tachycardia means fast heart rate
• Paroxysmal means the arrhythmia comes and goes

Tachycardia

The most common of these is atrial fibrillation. This is where your heart rhythm is irregular and often too fast. Symptoms include fatigue, palpitations (where you are aware of your heart racing or pounding), dizziness and breathlessness.

Other tachycardias include supraventricular tachycardia (SVT) or ventricular tachycardia (VT). These have similar symptoms to atrial fibrillation but can also cause you to lose consciousness (faint).

Bradycardia

The most common form of this is called heart block. This is because messages from the electrical generator of the heart don’t get through efficiently to the rest of the heart and hence it goes very slowly or can pause. Symptoms of the heart going too slowly include feeling tired, breathless or fainting.

Tests

As well as having the following tests to diagnose what sort of arrhythmia you have, you might be investigated for evidence of heart diseases that cause arrhythmias with echocardiography, blood tests, or other tests looking for evidence of cardiovascular disease.

The first test you will have will be a resting electrocardiogram (ECG). The trace of the heart’s electrical activity gives the diagnosis of the source of the arrhythmia. The resting ECG is often normal at rest and more extensive prolonged testing is needed to try and catch the arrhythmia especially if it is intermittent.

Ambulatory ECG (Holter monitor)
This test is used to monitor your heart for rhythm abnormalities during normal activity for an uninterrupted 24-hour period. During the test, electrodes attached to your chest are connected to a portable recorder – about the size of a matchbox – that is suspended in a pouch around your neck.

Event recorder
This is a test covering 1-2 weeks. You wear a small monitor and if you have any symptoms, such as dizziness, you press a button on a recording device which saves the recording of your heart rhythm made in the minutes leading up to and during your symptoms. Because you can wear this for a longer period of time it has a higher chance of catching your abnormal rhythm.

Treatment

Tachycardia
Most treatments for these conditions consist of medication to stop the abnormal rhythm or make it slower if and when it occurs.

Bradycardia
Depending on how slow your heart goes and what symptoms you have you may be referred for a pacemaker. This is a small operation where a battery powered device is placed under the skin with wires that lead to your heart and provide it with electrical stimulation to prevent the heart from going too slow. You can’t feel anything when the pacemaker is working but will be aware of a small flat lump under your skin just below your collar bone where the pacemaker is implanted.

Atrial Fibrillation/Flutter

Atrial fibrillation (AF) is the most common arrhythmia. The prevalence of permanent atrial fibrillation/flutter is 1% and reaches > 5% in octogenarians. Brief episodes of atrial fibrillation/flutter (“paroxysmal”) are much more common. A detailed evaluation of the management and investigation of atrial fibrillation has been released by the New Zealand Guidelines Group and is available on the website

There should be a high index of suspicion in patients with an irregular heart beat and the diagnosis should be confirmed with an ECG. Initial evaluation includes history, exam, laboratory investigation – thyroid function tests, renal function, INR, electrolytes, liver function, full blood count.

Echocardiography is a central investigation and should be undertaken in nearly all patients to look for underlying unrecognised structural heart disease, assess left ventricular (LV) size, function, hypertrophy and left atrial (LA) size. This information can be important in assessing thromboembolic risk and is necessary prior to initiating antiarrhythmic therapy. Ideally the ventricular rate should be < 100 bpm at the time of the scan to get accurate measurements.

Every patient with atrial fibrillation should have a thromboembolic risk assessment. The overall risk of stroke is 5%/year (3-5 fold higher than those without AF) but is different in various populations (note: there is no difference between chronic and paroxysmal AF). Very high risk populations include rheumatic heart disease (up to 20%/year), prosthetic heart valves, and previous/recent stroke/TIA (12%/year). Traditional intermediate risk factors for stoke include age > 65 , hypertension, heart failure, diabetes, coronary artery disease, and LV impairment on echocardiography. Younger patients with no structural heart disease (lone AF) are at very low risk. Warfarin reduces the risk by 2/3 and aspirin by 1/5. A risk table based on the Framingham database is available in the NZGG Practice Guideline. Warfarin use should be considered prior to a complete risk evaluation, as it can be stopped at a later stage if the risk is low.

The initial assessment of a patient with atrial fibrillation must include considering the probable underlying cause. There are many known causes; these may require urgent treatment in addition to treatment of the rhythm disturbance which has been caused by the problem.

The primary approach to control symptoms in atrial fibrillation is ventricular rate control. Beta-blockers are first line therapy, they are most effective and have proven prognostic benefit in hypertension, IHD and heart failure (which are frequently associated with AF). Verapamil (avoid concomitant use with beta-blockers) or Diltiazem should be used if beta blockers are contraindicated. Digoxin is very useful as a synergistic therapy and can be used in inactive elderly patients as first line therapy, but not used alone for most patients. Rate control is essential even in the absence of symptoms to prevent rate related cardiomyopathy. This should be evaluated at rest and with exercise. The target on clinical evaluation is an apical rate <= 80/min at rest, and <= 110/min with 5-6 minutes gentle exercise. Holter monitoring can be very helpful to assess rate control and should have a target average rate <100.

Attempted maintenance of sinus rhythm (cardioversion-electrical or pharmacologic, anti-arrhythmic therapy) is considered with highly symptomatic patients despite attempts at rate control and often in younger patients. This should be under the supervision of a Cardiologist.

Other Tachyarrhythmias

As many as one person in a hundred may experience an episode of sudden racing of the heart. Typically the onset is abrupt and the person may notice the clothing over the chest moving rapidly as well as feeling the fast heart beat. Typical heart rates are 160 to 220 beats per minute. Sometimes there is associated tightness in the chest, lightheadedness or breathlessness. The episode may stop spontaneously, or the patient may have learnt some “tricks” to stop it, such as breath holding or massaging their neck.

Reassuringly, although a first occurrence may be alarming, such episodes are rarely due to structural heart disease, but rather are an electrical “short-circuit” somewhere in the heart. If episodes last more than 15 minutes it is prudent to seek medical attention and very helpful for subsequent management to have the rhythm documented on a full electrocardiogram. No special treatment may be necessary for infrequent episodes. When episodes are recurrent and intrusive, the cause may usually be discovered with a procedure called an electrophysiology study. This involves recording electrical signals from inside the heart with electrode catheters using local anaesthesia.

The two most common causes found are a short-circuit in the filter in the middle of the heart (called the AV node and the problem AV nodal re-entrant tachycardia) or an extra electrical cable called an accessory pathway. When evidence of an accessory pathway is visible on the ECG, the problem is called Wolf-Parkinson-White syndrome. The problem is often fixed at the same time as diagnosis by “knocking out “ a small part of the faulty electrical circuit with a treatment called radiofrequency ablation. Patients can often go home the same day after such a procedure or may stay overnight.

Uncommonly the fast heart beat may arise from the lower pumping chambers (the ventricles) and additional investigations may then be warranted.