CARDIAC ELECTROPHYSIOLOGIC STUDIES (EPS)

Transcription

1 CARDIAC ELECTROPHYSIOLOGIC STUDIES (EPS) INTRODUCTION Cardiac Electrophysiology is the study of normal and abnormal electrical behaviour of the heart. In order to understand the nature and risks of the procedure (an EPS) it will be helpful if we discuss the electrical workings of the heart, how that may go wrong (producing an arrhythmia) and finally how we might explore the issue and treat the problem. Throughout the discussion the medical terms will be shown in italics to draw your attention to the words that you will hear used. THE NORMAL ELECTRICAL BEHAVIOUR OF THE HEART The heart is a remarkably efficient pump its only function is just that, to pump the blood to the lungs to receive oxygen and to the rest of the body to supply nutrients to the tissues. The heart has two top chambers, the right and left atrium (which receive the blood from the body and lungs respectively, like an atrium in a hotel receives guests, albeit with more flair) and two more powerful chambers the right and left ventricles which pump the blood to the lungs and body respectively. At a microscopic level heart muscle like other muscle is made up of cells organized into fibres. When electrical current is applied to the fibres, they contract. Whilst the muscles responsible for moving arms, legs etc receive an electrical discharge from nerves, cardiac muscle has the unique property of self generation of an electrical signal. Furthermore the muscle fibres conduct the electrical signal along their length and as the signal passes, so does a wave of fibre contraction. Whilst all zones of the heart are capable of generating an electrical signal an area of specialized tissue in the top right part of the heart, called the sinus node, fires off the quickest and thus generally acts as the natural pacemaker. Hence normal rhythm is called sinus rhythm. The rate of firing at rest will generally be between times per minute

2 which will be revved up by activity/stress and lowered when asleep for instance. The electrical activity spreads out from the sinus node across the top chambers (atria) which squeeze and then relax. The signal is then transmitted to the bottom chambers by a specialized electrical junction box called the atrioventricular or AV node. The signal is then rapidly conducted around the ventricles which squeeze in a coordinated fashion. ABNORMAL HEART RHYTHM (CARDIAC ARRHYTHMIAS) Most abnormal rhythms are organized into 2 categories tachycardias (greek for fast heart) where the speed is too fast and bradycardias where it is too slow. Tachycardias may produce: a sensation of the heart pounding fast (palpitations), chest tightness, or breathlessness (dyspnoea). Occasionally patients with rapid rhythms become dizzy (presyncope) or black out / lose consciousness (syncope). Bradycardias generally produce dizziness (presyncope) or loss of consciousness (syncope) because the blood flow to the brain is inadequate. By the way syncope is pronounced singkopee. Bradycardias arise when either the sinus node or AV node misbehaves. The sinus node may fail to initiate electrical firing at an adequate speed (runs too slow) or may go to sleep for a bit, causing a pause in heart beating, these problems are termed (sinus node dysfunction or sick sinus syndrome). The other mechanism of bradycardia is the AV node failing to conduct the electrical signal to the ventricles. This is called heart block. If the bottom chambers fail to beat it doesn t matter how hard or fast the atria contract, they are unable to pump the blood out of the heart. Tachycardias are more complex. There is a little overlap in the classification but we divide them into mechanisms of origin in the bottom chambers (ventricular tachycardia or VT) and those arising in the top chambers. All tachycardias occur for one of 2 reasons 1. a short circuit develops leading to electrical activity whizzing around in a circle. These used to be termed circus rhythms but this process is called reentry. 2. a spot fires off out of control, we call this an ectopic focus and the rhythms ectopic or automatic tachycardias. The term ectopic may be familiar to you as in ectopic pregnancy meaning a pregnancy occurring in the wrong place ie in the Fallopian tube instead of the womb. Of the rhythms arising in the top chambers we have subsets based on their appearance on the electrocardiogram (ECG): Sinus tachycardia arising in the sinus node, usually not an abnormal rhythm but the sinus node just

3 doing as its told eg. under exercise, fever, excitement, contemplating a cardiac procedure Atrial fibrillation chaotic electrical activity whizzing around the top chambers usually giving rise to fast and irregular activation of the bottom chambers Atrial flutter a special type of reentry circuit looping around a large area in the top chambers at a rate between beats per minute Supraventricular tachycardia (SVT) a grouping of 3 particular rhythm types : (i) (ii) (iii) a short circuit in the AV node area AV node/junctional reentry a short circuit using an extra electrical connection between the atrium & ventricle - AV reentry. These extra connections act like a wire and are called bypass tracts. Patients with a bypass tract that conducts from top to bottom often have an abnormal ECG even when the heart is not racing. These patients are said to have the Wolff Parkinson White Syndrome. Some of the bypass tracts only conduct in reverse and don t show up on the ECG and are termed concealed. a spot or ectopic focus in the atrium fires off ectopic or automatic atrial tachycardia WHY AN ELECTROPHYSIOLOGIC STUDY? There are generally 3 reasons that an electrical study of the heart will be considered. 1. To get to the bottom of unexplained blackouts 2. To assess the mechanism and potentially cure a racing rhythm (tachycardia) 3. To assess the risk of life threatening rhythms (ventricular tachycardia / fibrillation) occurring in the future eg. in the context of prior heart muscle damage. THE PROCEDURE Before the procedure Ask any questions regarding your uncertainties Medications You may be asked to stop some medications for several days before the procedure. Please check with

4 your doctor which if any medications need to be stopped. When you are admitted to the ward you will be asked to: Have nothing to eat for 6 hours before the test. You may be required to have blood tests and an electrocardiogram (ECG). Have your right groin shaved. Have an antiseptic shower to help remove germs from your skin. Have a needle (IVC) inserted into an arm vein connected with an IV line to a bag of fluid (drip). Before the test you will receive a tablet to help you relax. WHAT IS AN ELECTROPHYSIOLOGIC STUDY? the heart. We might position them in the right atrium, near the AV node, in the ventricle and in the heart s own draining vein (coronary sinus) which runs around the back of the heart to help us identify any of the extra connections (bypass tracts). With the catheters in place we can both record electrical activity (shown on a video screen) and stimulate the heart at various locations. We can then assess the conduction properties, presence of extra connections and try to bring on the patient s tachycardia. Various drugs can be used to help make it easier to induce (bring on) the arrhythmia. In the case of curable arrhythmias (SVTs and atrial flutter) a radiofrequency ablation RFA may be performed. Let me explain that. The irritable spot in ectopic rhythms or a critical part of the circuit in reentry tachycardias (eg a bypass tract) can be cauterized (burnt) by application of radiofrequency energy (a similar concept is used in microwave ovens). In this case though the burn is very localized over a few millimetres from the tip of a specialized ablation catheter. The burns can be felt as pain in the chest or shoulder lasting less than a minute. An EP Study is an invasive electrical examination of the heart. The procedure is performed in the cardiac catheter laboratory where Xray equipment is available to guide us. We use medication to make you drowsy and comfortable, general anaesthesia is rarely required. Many patients however sleep through the whole thing. We have a CD player for background music so feel free to bring your own selection. After cleaning the skin with cold antiseptic, one or two access tubes (sheaths) are inserted through tiny nicks in the skin of the right groin into the vein (femoral vein) our road to the heart. Then 1-4 catheters (electrical cables) each about 2mm diameter are passed up and into the chambers of

5 You should be able to eat after returning to the ward post procedure. Your doctor will discuss the results of your tests and any further treatment options with you before you are discharged. Duration: Depending on whether an ablation is being performed and on its complexity, the duration of the procedure can vary from 45minutes to several hours. After the procedure After it s over the tubes are removed from the leg and a pressure bandage applied, you will have to lie flat from 2 to 4 hours (the longer time if we need to puncture the artery to access the left sided chambers). It is important for you to remain in your bed, relaxed and calm for this period, again to control any bleeding or bruising. During this time you will be transported back to your ward or the coronary care unit. You can have a drink when you get there if you wish. Frequent observations will be carried out for the first hour. You will be attached to a portable, radio-transmitted heart monitor to observe your heart rhythm overnight. ARE THERE ANY RISKS? As with any procedure, complications may unfortunately occur. For a purely diagnostic study (no ablation) the risk of major complication is incredible small. Mild groin bruising is not uncommon. If we are assessing risk of life threatening rhythm we might bring on ventricular tachycardia / fibrillation and need to shock the heart back to normal rhythm (you would be unconscious for this). For ablation procedures the risks are: injury to good conducting tissue in the AV node which may necessitate a permanent pacemaker <1/300 clot in leg vein (deep venous thrombosis or DVT) that could pass to the lung (pulmonary embolus) <1/500 heart attack, stroke, perforation and bleeding around the heart < 1/1000