Physiology of the Heart

Anatomy
1. Illustrate and describe the gross anatomy of the heart. Include the layers of the heart wall, chambers, valves, structures and major blood vessels connected to the heart. Indicate direction of flow. Include your own diagram.
The heart is a complex biological electrical pump. It is found in mediastinum of the thorax. Surrounding the heart is the pericardium, which contains serous fluid, allowing the heart to move freely within the membrane. (1) The outside surface of the heart is known as the epicardium, the inner surface of the heart muscle; the myocardium and the innermost surface; the endocardium.

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The heart itself can be separated into 4 chambers which are filled with blood when the heart is relaxed, and pumped out of when the heart contracts. (1) They are separated into the left and right side, which are distinct, and into atria (singular atrium) and ventricles. The atria and ventricles are separated by the coronary sinus or AV groove. Between the right atrium and right ventricle is the Tricuspid Valve which is made up of 3 leaflets. Deoxygenated blood feeds into the right atrium from the Vena Cava (which is separated into the superior and inferior vena cava – superior from the head, neck and arms and the inferior from the lower limbs and the abdomen). (1) The right ventricle feeds into the pulmonary artery which carries deoxygenated blood to the lungs. Blood is then oxygenated and fed back to the heart through the pulmonary vein. This fills the left atrium and subsequently, flows through the Mitral Valve into the left ventricle.. (1) As the heart contracts, this pushes the blood into the aorta, which feeds through to the major arteries in the body. Within the base of the aorta lies some very small arteries known as the coronary arteries. These feed the heart tissue with oxygenated blood and drain into the right atrium, with the systemic deoxygenated blood. (1)

2. Briefly describe the function of the pericardial cavity
As mentioned before, the heart and roots of the great vessels (aorta, vena cava, pulmonary vein and artery) is surrounded with a very strong membrane known as the pericardium. It is a double walled structure, made up of the fibrous pericardium on the outermost surface of the heart, and an inner serous pericardium. (1)The fibrous pericardium is made of very dense connective tissue, and contains many collagen fibres. It prevents overfilling of the heart and anchors it to the surrounding walls of the thoracic cavity.(2) The serous pericardium can be differentiated into two layers, the parietal layer, which is fused and continuous with the fibrous pericardium, and the visceral pericardium which can also be known as the epicardium. Between these layers is a potential space known as the pericardial cavity, filled with about 50mls of serous fluid. (2) This potential space is extremely important as it allows the heart to move freely within the space by keeping the transmural cardiac pressures very low, as well as facilitating atrial filling during ventricular systole by maintaining a negative pericardial pressure. It also prevents hypertrophy of the heart under strenuous exercise, keeping the heart muscle a relatively constant size. (2) The membranes completely isolate the heart from the thoracic cavity which prevents spread of disease or infection. Its importance is particularly obvious when there are cases of pericardial tamponade – build-up of fluid in the pericardial cavity which causes compression of the heart. (2) Without the pericardial cavity, the heart would not be able to pump as efficiently as it would have to overcome the pressures exerted on it by the surroundings, which would just add to the work of the cardiac muscle.(2)
3. Illustrate and describe the anatomy of the electrical conduction system of the heart. Briefly describe the blood supply to the electrical conduction system.
The electrical impulse originates at the Sino Atrial Node in the right atrium. This impulse travels through the cardiac muscle – through the many gap junctions, as well as through the intermodal pathways or Bachman’s bundle. (1) At the level of the atrio-ventricular valves, there is another node called the Atrio-Ventricular Node (AV Node) that has properties that delay the stimulus. Following this, the impulse travels down the left and right bundle branch fibres in the ventricular septum, into the bundles of His which travel up the ventricular walls and branch into Purkinje fibres. The stimulus reaches the apex of the heart first, and then travels up towards the outflow tracts resulting in coordinated depolarisation and contraction. (1) This coordination is a result of both the coordination of the stimulus as well as the layout of the myocytes, as well as the ease at which the electrical signal is able to propagate through cardiac muscle.(3) Sino Atrial Node is usually supplied oxygenated blood by the Right Coronary Artery (RCA) or the Left Coronary Artery (LCA) though this is variable. In most people, the AV Node is supplied by the AV Node Artery branch of the Posterior Descending Artery which is a branch off the RCA, though in some it will be supplied by the same artery, just as a branch of the left Circumflex Artery. (3) All of the fibres downstream from this point are supplied by the Left Anterior Descending artery with exception of the His fibres, which are also supplied by the AV Node Artery. (3)

Conduction
1. Illustrate and describe the propagation of a single beat through the electrical conduction system and the relationship to the surface ECG. Include in your answer a discussion on conduction velocity through the various components and list the normal ECG intervals.
An ECG works by detecting the electrical change in the heart through sensors that are put on the surface of the skin. Direction is determined through the use of electrical vectors generated by many hundreds of individual cells. (4)
The P wave is the first small wave in the ECG. It reflects the spread of depolarisation through the atria from the SA node. The normal range is ¬0.08-0.1seconds. After the P wave there is a brief isoelectric stage when the current is flowing through the AV node, and the conduction is slowed. This is known as the PR interval and it is usually 0.12-0.2seconds. (4) The QRS complex shows the very strong electrical signal and resulting contraction that forces blood into the aorta and pulmonary artery. It is about 0.06-1seconds, which shows just how fast depolarisation spreads through the ventricles (its shape has been idealised on the schematic below). (1) After the QRS complex there is another isoelectric period which indicates plateau phase of depolarisation. The T wave is the repolarisation of the ventricles – in preparation for the next beat the duration between the P and T waves usually approximately 0.2-0.4seconds, though this is dependent on heart rate.(4) The U wave is a very rarely seen artefact and is thought to reflect the repolarisation of the papillary muscles that control the valves. (1)
Figure 3: Electrocardiogram schematic. Based on the diagram from Bruce Shade: Fast and Easy ECGs (4)

 
2. Briefly describe the ionic movement that occurs during each phase of the myocardial and SA node action potential. Include a labelled illustration of both action potentials in your answer.
The myocardial action potential is quite complex with influxes and effluxes of 3 different ions, changing the membrane potential as contraction occurs. There are 5 distinct phases of the myocardial action potential. (1) These are shown in the figure 4 below. Between phase 0 and 2 there is an absolute refractory period where it is impossible to invoke another action potential. This allows even more coordination of the spread of a stimulus.(1) The ECG trace shown below the action potential shows where the stages of contraction occur that can be extrapolated out into the ventricular depolarisation (QRS complex) and the ventricular repolarisation (T wave)

Depolarisation Repolarisation

ECG
Cells in the SA node are pacemaker cells and have a property which is known as automaticity. They do not need activation to fire an action potential.(1) They are very similar to myocytes but have several key differences in their action potentials (see figure 5). Phase 0 is significantly slower in the pacemaker cells of the SA node as it is dependent on the activation of L-type calcium channels instead of Sodium channels, which makes the depolarisation significantly slower at this phase.(1) During Phase 1, repolarisation of the membrane occurs leading to a period of pacemaker potential, where the membrane potential gradually depolarises through constant Na2+ leakage into the cell. When the action potential is triggered automatically, Phase 0 commences. Pacemaker cells do not have phase 1 and 2. (1)
Figure 5: SA Node Action Potential
3. Describe the role of escape pacemakers in the conduction system.
The SA node is entirely autonomous which means that it does not need external innervation or activation to fire. Other areas of the heart are heteronomous which means they need an external source of action potential to stimulate them to produce one. (1) Some specific cells along the conduction fibres possess both of these properties. This is so that if the SA Node fails for some reason, they can activate themselves and this allows the heart to beat, even without a functional SA Node. (1) Each area will have a slower rate of autonomy as it gets further downstream from the SA Node to prevent competition between the different areas. (5) This is very useful when the SA Node fails, and the AV Node takes over as the AV Node can maintain a BPM of about 40-60 BPM which is still slow but can maintain life for a reasonable amount of time. Further downstream the Bundles of His can maintain about 25-40 BPM and the Purkinje fibers about 15-30BPM which cannot maintain life for any reasonable period of time, though it can help during some forms of arrhythmias which prevent the signal reaching the Purkinje fibers/Bundles of His. (6) This is also where Escape beats originate, and this is seen on the ECG as a widened QRS Complex.
4. Discuss the role of decremental conduction in the AV node.
Decremental conduction means the more the AV node is stimulated, the slower it conducts the stimulus. This allows a control over how fast the blood is pumped out. The faster the contractions, the less time between them for filling of the heart and therefore less blood is pumped out. (1) The AV Node slows down the stimulus so that there is an element of control of how fast the signal reaches the apex of the heart and prevents the ventricles from contracting so fast that the cardiac output drops too low.(7) It is extremely important that the AV node is able to slow down the conduction velocity, even as it gets activated more and more frequently. It is even able to block out some signals. This is clearly seen in patients with atrial fibrillation. (7) The only way for the signal to travel to the ventricles is through the AV node (due to the insulating fibrous skeleton that prevents atrial cell – ventricular cell depolarisation spread). (7) If the AV node allowed conduction of every single depolarisation, an atrial fibrillation would be fatal as the cardiac output would become too low, and the ventricle would go into ventricular fibrillation, which is fatal without intervention. It is the decremental properties of the AV node that prevent this from occurring. (7)
5. Explain the term ‘functional syncytium’ and its significance in the cardiac muscle contraction.
A functional syncytium by definition is a group of cells that are both mechanically and electrically bound to one another, so they are able to function as one. This is extremely important in cardiac muscle contraction.(1) One of the main reasons that the heart is able to work so effectively is that the contractions and depolarisations are always coordinated. (8) There is no coordination without communication, which are the electrical signals that are passed between the myocytes. (1) This extremely effective communication is completely useless unless the cardiac myocytes are mechanically bound, so that when they do contract, it is as a whole. The specialization, which is unique to cardiac muscle, that allows it to be a functional syncytium is a structure called intercalated discs. (8) They contain three types of intercellular junctions; many fascia adherens and desmosomes, for mechanical connection, and many gap junctions allowing for direct communication between neighbouring cells. (1) Another interesting property that contributes to the functional syncytium is the fact that cardiac muscle has an innate rhythmicity. This means that at the level of the muscle, the myocytes will exhibit the rhythm of the cell with the fastest rhythm. This makes regulating and coordinating the speed of the heart beat very easy and effective. (8)
References
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3. Futami C, Tanuma K, Tanuma Y, Saito T. The arterial blood supply of the conducting system in normal human hearts. Surg Radiol Anat [Internet]. 2003 Apr [cited 2014 Apr 9];25(1):42–9. Available from: http://www.ncbi.nlm.nih.gov/pubmed/12819949
4. Shade BR, Wesley K. Fast and Easy ECGs [Internet]. McGraw-Hill Higher Education; 2007 [cited 2014 Apr 9]. Available from: http://books.google.com/books?id=hibqIAAACAAJ&pgis=1
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6. Vassalle M. On the mechanisms underlying cardiac standstill: Factors determining success or failure of escape pacemakers in the heart. J Am Coll Cardiol [Internet]. Journal of the American College of Cardiology; 1985 Jun 1 [cited 2014 Apr 9];5(6):35B–42B. Available from: http://content.onlinejacc.org/article.aspx?articleid=1111307
7. Cardiac Electrophysiology: From Cell to Bedside 5th edition – ISBN: 9781416059738| US Elsevier Health Bookshop [Internet]. [cited 2014 Apr 9]. Available from: http://www.us.elsevierhealth.com/cardiology/cardiac-electrophysiology-from-cell-to-bedside-expert-consult/9781416059738/
8. Cardiac Muscle | histologyolm.stevegallik.org [Internet]. [cited 2014 Apr 9]. Available from: http://histologyolm.stevegallik.org/node/146