Index
Ep Defined | Getting Started | Working in the EP Lab
Right Atrium | Right Ventricle | Left Atrium | Left Ventricule | Cardiac Conduction | Cardiac Cell Properties | Action Potential | Sympathetic or Not | Med Page
Electrograms Defined | Recording Modes | Electrode Spacing | Filters | EGM Interpretation | Arrhythmia Analysis
The Physical Lab | Tools of the Trade
Setting Up | Catheter Placement | Baseline Measurement | SNRT | Conduction Study | Arrhythmia Induction | Pacing Protocols | Ablation | Tilt Table | Secrets to Success
Bradycardia | Atrial Tach | Atrial Flutter | Atrial Fibrillation | AVNRT | AVRT | Ventricular Tachycardia
Surface ECG's | Intracardiac Questions | Med Challenge | Advanced

Cardiac Anatomy

Image Courtesy of St. Jude Medical

Know the Basics...*

   

       The human heart is truly an amazing organ. From the time we are born until the time we pass away, the heart muscle continues to pump blood through the body ensuring that every cell recieves enough oxygen and nutrients to continue functioning normally. This is quite an impressive achievment. To begin to understand how special the heart is, let us try a simple experiment. Hold one of your arms out in front of you. Extend your fingers straight out so that your palm is flat. It can be facing either up or down.

          Now begin closing your hand into a fist and then opening it again. Do this at a rate of once per second. What you are doing is mimicking the heart contracting at a rate of 60 times per second. Try to keep opening and closing your hand continuously at this rate for as long as possible. You may notice, after a short period of time, that your arm and hand begin to become sore. Keep up the exercise until you can no longer continue, or until you have realized the extent to which your heart must work to continue to pump blood uninterrupted year after year. Fortunately, the heart muscle does not fatigue in a manner similar to the other muscles of the body. Even with that in mind, that power of the heart is an impressive thing to contemplate. If you take a mean heart rate of 80 times per minute (the average range is between 60-100 beats per minute) and multiply by 60 minutes, you arrive at a total of 4,800 beats in an hour. Over the period of a full day, the heart may beat 115,200 times. Extend that time frame over one year and you arrive at a total number of contractions at around 42,076,800. If we extrapolate this rate over the average lifetime of 72 years, the heart would beat a total of 3,029,529,600 times before we passed away. During this time, we very rarely take notice of the work the heart does. Now that we have stopped to consider just how hard the heart does work, let us take a closer look at this amazing organ.

Cardiac Chambers

          The heart is a muscular structure that lies roughly in the center of the chest. The different layers of cardiac tissue comprizes four chambers, two on the top and two on the bottom. The top chambers are called atria and the bottom chambers are known as the ventricles. The atria are smaller than the ventricles and recieve blood as it returns from either the body or the lungs. The ventricles are larger and are responsible for most of the pumping action that keeps the blood circulating in your body.  The top and bottom regions of the heart are further subdivided into right and left sides.  Thus, the top of the heart contains the right and left atria and the bottom regions of the heart are made up of the right and left ventricles.

Detailed information on the specific cardiac structures listed below may be accessed by clicking on the highlighted text.

Right Atrium: The right atrium is the first chamber of the heart to recieve blood flow returning from the body as it is pumped towards the lungs to be reoxygenated. It is also home to one of the most important cardiac structures, the sinus node. The sinus node, or sino-atrial (SA) node is the primary pacemaker of the heart. It is this small region of the heart that helps to regulate how fast or slow the heart contracts. Some other structures of interest include the crista terminalus, the fossa ovalis, the eustachion ridge, Bachman's Bundle and the ostium of the coronary sinus. For the student of electrophysiology, each of these structures is important in how they affect or interact with the cardiac electrical system. It is important to learn how each of these structures is involved in both normal and abnormal cardiac rhythms. Some arrhythmias that are associated with the right atrium include atrial tachycardia, atrial flutter, AVNRT and some forms of AVRT.

Right Ventricle: Blood leaves the right atrium and enters the right ventricle through the tricuspid valve (see below). The right ventricle is the final chamber the blood passess through before it enters the pulmonary arteries and then the lung field. As the blood exits the main protion of the right ventricle, it passess through the right ventricular outflow track, referred to as the RVOT. Both the RV and the RVOT have specific types of arrhythmias that are associated with these structures. For sepcific information on these rhythms, see the section on ARVD and RVOT PVC's.

Left Atrium: As oxygenated blood returns from the lungs, it enters the left atrium through the pulmonary veins (see below). Over the last several years, the left atrium has become a primary target of interest to many EP labs. This is due to improved techniques in dealing with atrial fibrillation, which is primarily associated with the left atrium. It is also possible to have atrial tachycardias and some types of atypical flutter associated with the left atrium.

Left Ventricle:The left ventricle (LV) recieves blood flow from the left atrium through the mitral valve (see below). As the primary pump for the circulatory system, the left ventricle plays a dominant role in cardiac function. When arrhythmias originate within the LV it rarely goes unnoticed. Ventricular tachycardia is one of the most significant and dangerous types of arrhythmias. Only ventricular fibrillation is more life threatening.

Base vs Apex: When referring to the ventricles, the top of the ventricles where the valves are located are known as the base of the ventricles. The lower ends are called the apex of the ventricles. While this terminology may seem a bit confusing at first, it does make sense to use this terminology. The base refers to both the seat of the valves and the location where the left and right bundles enter the ventricles.

Cardiac Layers

          There are four layers of tissue that comprise the heart. Each of these layers is made up of different tissue and serves a specific purpose. Understanding the construct of the heart is an important step in understanding many of the different arrhythmia processes.

Pericardium: The outer most layer is the pericardium. The pericardial sac helps to anchor the heart in place inside the chest cavity. Inside the pericardial space is a fluid that is referred to as (you guessed it!!!) pericardial fluid. This fluid helps to support the heart as well as cushion it. The importance of the pericardium and the pericardial fluid from an EP perspective has to do primarily with two aspects; epicardial mapping and cardiac perforation.

Cardiac Perforation:  One of the major risks during any procedure where catheters are placed inside the heart is the risk of perforation. A perforation occurs when a catheter, or similar object, penetrates the cardiac tissue creating a hole between the blood pool and the pericardial fluid. When this occurs, there is the potential for blood to fill the pericardial space and cause a condition known as cardiac tamponade. This is a serious, even life threatening complication and thus represents one of the most serious complications of an EP procedure. As blood fills the pericardium, the amount of fluid begins to restrict the motion of the heart preventing it from fully dilating in the diastolic phase. Patient restlessness is usually the first indication, followed by decrease O2 sats and blood pressure. A pericardial tap must be performed immediately to relieve fluid pressure from around the heart. If the patient is on heparin, it must be immediately discontinued and reversed. In cases where bleeding can not be controlled, cardiac surgery may be required.

Epicardial Mapping: One of the more difficult arrhythmia types to deal with are those with epicardial origins, or those where a critical portion of the reentry circuit is located in the outer myocardial and epicardial layers. In some cases, endocardial ablation will not be sufficient to terminate these rhythms. It is possible to deal with these rare cases by introducing a catheter into the pericardial space, generally using a sub-zyphoid approach. Once a catheter has been introduced into the pericardium, the mapping and ablation process proceeds as any conventional mapping procedure would.

Epicardium:  The outermost layer that resides within the pericardial space is the epicardium. This outer layer is rarely accessed during EP procedures except during cases where endocaridal mapping is unsuccessful.

Myocardium: The myocardium is the primary muscle of the heart. This is the layer that provides the majority of the contractile strength during depolarization. Some of the general aspects of the myocardium that are intrinsically involved with cardiac electrophysiology are tissue thickness and myocardial fiber alignment.

Myocardial tissue thickness: The thickness of the myocardial tissue is import to the practice of cardiac electrophysiology for two reasons. The first deals with the potential risk of perfortation. In regions of the heart where the myocardium is thinner, the possability of cardiac perforation is increased. This is why many physicians will exercise caution when placing a catheter into either the right or left atrial appendage. (Note that in the auricles (another term for appendage), flow is reduced and there is an increased risk of clotting. This is another reason why the atrial appendages are often avoided.) Note that tissue thickness is not the only factor that has a significant impact on cardiac perforation. The rate of blood flow is also a major factor that determines how severely a perforation will affect the patient. This is why the RVOT is approached with great caution. A perforation here will result in rapid onset of cardiac tamponade.
          The second aspect of myocardial thickness and its affect on EP procedures has to do with the ablation process. A standard RF lesion will normally be able to produce a lesion that is around 5mm in size. If the tissue thickness is greater than 5mm and the origin or critical path of the arrhythmia targeted lies beyond that range, it may not be possible to provide a successful ablation solution without using an epicardial approach.

Myocardial fiber alignment:  One of the concepts that must be grapsed in order to understand some of the mechanisms of arrhythmias is that of fiber alignment and its affects on cardiac conduction.  Fiber alignment can, in a simplistic way, be compared to our roads and highways. If the fiber alignment is strong and the "road" is in good condition, electrical conduction proceeds on it at an unimpeded rate. This would be compareable to an multilane highway in good condition. Traffic will normally traverse the terrain quickly and without interuption. Think of the right atrium and the internodal conduction pathways that govern conduction through the atrial tissue. In specific, let us turn our attention to Bachman's Bundle which is the primary conduction pathway between the right and left atria. Under normal circumstances, conduction passes from the right atrium to the left with little or no noticeable delay. If however, Bachman's Bundle were damaged or destroyed, the electrical wavefront would have to traverse the septum on a cellular level dramatically increasing intraatrial conduction time. This would be similar to what happened if a major freeway were damaged or closed. Traffic would have to detour on alternate routes and could only procede as fast as conditions would allow.
          When this happens in the heart, it sets up the perfect conditions for macro reentry arrhythmias to occur. Ask yourself what causes typical right atrial isthmus dependant flutter. Many people would suggest that slow conduction through the isthmus is responsible for this rhythm. If this were true, we should all be in atrial flutter. Under normal circumstances, the tissue in the isthmus conducts slower than most other tissue in the atrium. For the reentry circuit that would be necessary for atrial flutter to occur using the isthmus, conduction must be changed somewhere else in the atrium. If this were due to damage of the intranodal tracks, then atrial flutter become easy to explain. It also becomes easier to understand why more of us are not walking around in flutter.

Endocardium (the surface you map):  The innermost layer of cardiac tissue is the endocardium. This is the surface where most EP procedures collect information. It is the endocardial surface that the catheters contact when intracardiac electrograms are recorded.  When radiofrequency energy is applied, the endocardial and nearby myocardial tissues are those that are eradicated by the resistive energy that is delivered.

Cardiac Valves

       There are four valves in the heart, two that lie between the atria and the ventricles and two that lie between the heart and the blood vessels that contain flow that is moving away from the heart. In terms of cardiac electrophysiology, these valves are important in terms of the landmarks they provide. This section will discuss the functional aspects and physical characteristics of the four valves. 1

Atrioventricular Valves: The two valves that separate the atria and the ventricles are referred to as the atrioventricular valves. These valves have a smooth atrial side that is similar in appearance to the tissue surrounding the valve. The ventricular side is marked by the insertion of the chordae tendinae which attach to the ventricular papillary muscles. The leaflets of the valves are separated by spaces referred to as commisures. The commisures do not fully separate the cusps giving the valve a complete basal ring at the annulus.

The Tricuspid Valve - Atrioventricular valve between the Right Atrium and Right Ventricle. This valve, in accordance with its name, has three cusps. There is the anterior cusp, the medial or septal cusp and the posterior cusp.

The Mitral Valve - Atrioventricular valve that separates the Left Atrium and Left Ventricle. The Mitral Valve is often referred to as the Bicuspid Valve as it appears to have two primary leaflets. It actually has two large leaflets, the anterior or aortic cusp and the posteior or mural cusp. There are also two smaller commisural cusps.

Arterial or Semilunar Valves: The two valves that separate the ventricular outflow tracks from the pulmonary artery and aorta are most commonly referred to as semilunar valves. Each of these valves are anchored on a ridge of tissue that encircle the base of the arterial connection to the outflow tracks. This ridge forms three pocket like cusps that are known as the sinus of valsalva from which each of the leaflets extends.

The Pulmonic Valve - The pulmonic valve lies between the right ventricular outflow track and the pulmonary artery.

The Aortic Valve - This valve separates the left ventricular outflow track from the Aortic Root where the coronary arteries are located.

Cardiac Circulation

          As blood circulates through your body, it travels through a network of arteries and veins. The term arteries refers to vessels that transport blood away from the heart. As the circulating blood travels out into the body, it passes into smaller and smaller arteries until it reaches the capillary beds where most of the oxygen and nutrients carried in the blood are delivered to the different cells of the body. After the blood deposites the nourishment and oxygen, it picks up waste products and carbon dioxide (CO2). The waste products will eventually be filtered out by the liver and the CO2 will be taken back to the right side of the heart and delivered to the lungs where it will be exchanged for oxygen.

          Blood flow returns to the heart after passing through the body by way of two large veins. The superior vena cava (SVC) provides blood return from the upper portion of the body while blood flowing from the lower portion of the body enters the heart through the inferior vena cava (IVC). Both of the vena cava connect to the right atrium.

SVC/IVC – Deoxygenated blood returns to the heart from the body through the Superior and Inferior Vena Cava.

RA The Right Atrium recieves the blood from the IVC and SVC. This blood passes through the Tricuspid Valve.

RV – The Right Ventricle is the smaller of the two pumping chambers of the heart. Blood leaves the Right Ventricle passing through the Right Ventricular Outflow Track.

RVOT – Blood exits the rights side of the heart through the Pulmonic Valve, one of the two semilunar valves.

PA – The Pulmonary Artery recieves blood from the RVOT and has the distinction of being the only artery in the body that carries venous or deoxygenated blood.

The Lungs - Deoxygenated blood is delivered to the lungs where exhange carbon dioxide is exchanged for oxygen.

PV’s – The Pulmonary Veins carry the oxygenated blood back to the heart.

LA – From the four (usually) pulmonary veins, blood rich in oxygen is delivered to the Left Atrium.

LV – Blood from the Left Atrium crosses the Mitral Valve into the main pumping chamber of the heart, the Left Ventricle.

LVOT – Oxygenated blood exits the Left Ventricle through the Left Ventricular Outflow Track.

AO – Blood from the LVOT cross the Aortic Valve into the Arotic Root.

The Coronary Arteries, located in the Arotic Root are the first vessels that deliver oxygen rich blood to tissue of the body. These arteries feed the tissue of the heart including the main muscle layer of the heart, the myocaridum. (See image below)

Coronary Sinus – Blood returning from the heart muscle dumps back into the Right Atrium by way of the Coronary Sinus.

          

          This image shows a schematic of the general locations of the coronary arteries. It should be noted that like much of the cardiac anatomy, variations from person to person will cover a wide range of possibilities.

          It is important to have a good understanding of the location of the coronary arteries during LV ablations. This is especially true when using an epicardial approach. Applying RF energy too close to a coronary artery may have disasterous results.


         *  The image at the top of the page was segmented using the Verismo segmentation software and provided courtesy of St. Jude Medical. The images provide by segmenting a CT or MRI scan provide a great tool to help understand the shape of cardiac structures by allowing us to see how the endocardial surfaces of the cardiac chambers appear inside the body. For information regarding the Verismo product, please click on www.SJM.com.

1: Information found in The Netter Collection of Medical Illustrations, Volume 5; The Heart

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