THE CARDIAC ACTION POTENTIAL

The ECG is best understood by first considering some electrical and chemical features of the myocardial cell. Resting cells have a potassium concentration (about 140mmol/litre) which is high in comparison with that in the extracellular tissues (about 4mmol/litre), whereas extracellular sodium concentration is much greater than intracellular. This ionic disequilibrium between the cell and its environment is maintained by a sodium-potassium ions into the cell and sodium ions out of the cell.

During diastole there is a relative negative potential within the cell of the order of 90mV the Trans membrane resting potential. This is primarily due to two factors, the high concentration of potassium ions intracellularly and the high permeability of the cell membrane to potassium ions. As a result, potassium ions tend to diffuse out of the cell down their concentration gradient, creating a negative charge in the interior of the cell which offsets and almost balances the concentration gradient for potassium.

The cardiac action potential arises due to a sequence of changes in permeability to sodium,calcium and potassium ions. At rest,cell membrane is relatively impermeable to sodium ions. The rapid upstroke of AP is due to a sudden increase in sodium permeability, causing a rapid influx of sodium ions. This sodium current is short-lived,because the ion channels which open to cause the increase in sodium permeability rapidly close once again. The first inward sodium current is succeeded by a slower inward current,comprised predominantly of calcium ions and to a lesser extent sodium ions. This slow inward current is responsible for the plateau phase, preventing the cell from re-polarizing rapidly like a nerve. During this phase potassium permeability is reduced. Re polarization is achieved by a gradual increase in potassium permeability once again, accompanied by a gradual decrease in the slow inward current.

CURRENT                                 ION                                    FUNCTION

1) Fast inward current                 Na+                                    Rapid depolarization

2) Slow inward current              Ca2+                                  Plateau maintenance
                                                 (mainly)                           Excitation-contraction coupling

3) Outward curves                      K+                                      Re-polarization
                                                                                         Resting membrane potential

STARLING'S LAW

The law of the heart, as enunciated by Starling, states that the more myocardial fibres are stretched (or greater the diastolic volume of the heart) with in the physiological limits, the greater the energy of the ensuing contraction. Beyond these limits, the energy of contraction falls off. Two aspects of sarcomere function contribute to Starling's law:

1) As the sarcomere is progressively stretched to its optimal length, and the diastolic fibre lengthens proportionately the force of contraction progressively increases. This is partly due to the fact that over the normal operating range the opposing actin filament over lap, interfering with actin-myosin cross bridge formation. With stretch the degree of overlap decreases and hence force generation increases.

2) Recent evidence suggests that calcium sensitivity of the contractile proteins increases with increase in length, providing a greater degree of activation for a calcium concentartion.

EXCITATION-CONTRACTION COUPLING: During systole there is a 50-fold increase in intracellular calcium concentration. The cardiac action potential is responsible for the increase in intracellular calcium in two ways:

1) Calcium ions enter the cell from the extracellular space during the plateau phase of action potential

2) The spike of the action potential triggers the release of calcium from the intracellular stores within the sarcoplasmic reticulum.

THE ENERGY SUPPLY: Energy is produced in the heart by the process of Oxidative phosphorylation. This results from the conversion of the energy produced by the oxidation of substrates such as glucose, lactate and fatty acid into the energy of Adenosine tri-phosphate (ATP) and creatine phosphate(CP). These substances provide the energy source for muscular contraction. Normally, free fatty acid are the main substrates, but in ischemia the glycolytic pathway is stimulated. This however cannot substitute for oxidative phosphorylation and inevitably ischemia leads to a fall in ATP levels and a consequent impairment of myocardial function.

CARDIAC CYCLE

The cardiac cycle 3 basic events are
1.LV contraction
2.LV relaxation
3.LV filling

The function of the heart is to maintain a constant circulation of blood throughout the body. The heart acts as a pump and its action consists of a series of events known as the cardiac cycle. During each heart beat, the heart contracts(SYSTOLE) and relaxes (DIASTOLE). Each cycle lasts about 0.8 second

LV CONTRACTION: LV pressure in the early contraction phase builds up and exceed,that in the left atrium (normally 10-15mmHg), followed about 20sec later by M1, the Mitral component of the first heart sound. Shortly there after pressure changes in the RV, cause the tricuspid valve to close there by creating T1,which is the second component of the first heart sound.
During this phase of contraction, between the MV closure and Aortic valve opening the LV volume is fixed (Isovolumetric contraction) because both the AV and MV are shut. When the pressure in the LV exceeds that in the aorta, the AV opens. Opening of the AV is followed by the phase of 'Rapid Ejection.' The LV Pressure rises to a peak and then starts to fall.

LV RELAXATION: The rate of ejection of blood from the LV into the aorta falls (phase of reduced ejection). The Pressure in the Aorta exceeds the falling pressure in the LV. The AV closes, creating the first component of the second heart sound A2.

LV FILLING: As the LV pressure drops below that in the LA, the MV opens, the phase of rapid filling or early filling occurs to account for most of the ventricular filling. Active diastolic relaxation of the ventricle also contributes to early filling such rapid filling may cause the physiological Third heart sound(S3) particularly when there is hyper kinetic circulation. As pressure in the atria and ventricles equalize,LV filling partially stop (Diastasis).

EVENTS:
Atrial systole-0.1s
Atrial diastole-0.7s
Ventricular systole-0.3s
ICT-0.05s(closure of AV valve to opening of SL valve)
Max.ejection-0.11s
Reduced Ejection-0.14s
Ventricular Diastole-0.5s
Protodiastole-0.04s(combined atrial and ventricular diastole)
IVRT-0.08s(closure of SL valve to opening of AV valve)
First rapid filling-0.113s
Diastasis(slow filling)-0.167s
Last rapid filling-0.1s

CONDUCTION SYSTEM OF THE HEART

The rhythmic contractile process originates spontaneously in the conducting system and travels to different regions of the heart.The conduction system consists of specialised cardiac muscle present in the sino atrial node., the Atrio ventricular node, the Atrio ventricular bundle and its right and left terminal branches and the subendocardial plexus of Purkinjee fibres

SINOATRIAL NODE: The SA node is located in the wall of the RA in the upper part of the Sulcus terminalis just to the right of opening of the SVC. The node spontaneously gives origin to rhythmical electrical impulse that spread in all directions through the cardiac muscle of the atria and cause the muscle to contract.

ATRIO VENTRICULAR NODE: The AV node is placed on the lower art of the atrial septum just above the attachment of the septal cusp of tricuspid valve. From it, the cardiac impulse is conducted to the ventricles by the AV bundle. The speed of conduction of impulse through the AV node is 0.11s

ATRIO VENTRICULAR BUNDLE: The AV bundle or bundle of HIS is the only pathway of cardiac muscle that connects the myocardium of atria and ventricles. The bundle descends through the fibrous skeleton of the heart. The AV bundle descends behind the septal cusp of the tricuspid valve to reach the inferior border of the membranous part of the ventricular septum. At the upper border of the muscular part of the septum it divides into two branches, one for each ventricle. The Right bundle branch (RBB) passes down the right side of ventricular septum to reach the moderator band, where it crosses to the anterior wall of RV. Here it becomes continuous with the fibres of purkinjee plexus. The Left bundle branch (LBB) pierces the septum into two branches (anterior and posterior), which eventually become continuous with the fibres of the purkinjee plexus.

INTERNODAL CONDUCTION PATHS: Impulse from the SA node have been shown to travel to the AV node more rapidly than they can travel by passing along the ordinary myocardium. This phenomenon has been explained by the description of special pathways in the atrial wall, having a structure consisting of mixture of purkinjee fibres and ordinary cardiac muscle cells. The pathways are
>Anterior internodal pathways
Middle internodal pathways
Posterior internodal pathways

ANATOMY OF THE HEART

The heart is a hollow muscular organ that lies within the pericardium in the mediastinum. The pericardium is a fibroserous sac that encloses the heart and the roots of the great vessels. Its function is to restrict excessive movements of the heart as a whole and to serve as a lubricated container in which the different parts of the heart can contract.

SURFACE OF THE HEART:
The heart has 3 surfaces:
1.Sternocostal surface (anterior): Formed mainly by the right atrium (RA) and right ventricle(RV), which are separated from each other by the atrioventricular (AV)groove.
2.Diaphragmatic surface (inferior): Formed by the right ventricle and the left ventricle(LV), which are separated by the posterior inter ventricular groove. The inferior part of the RA, into which the IVC opens, also forms part of this surface.
3.Base (posterior): Formed by the left atrium (LA), in to which the pulmonary veins open.
Apex of the heart is formed by the left ventricle and is directed downward, forward and to the left. It lies at the level of the fifth intercostal space along the mid clavicular line.

BORDERS OF THE HEART:
Right border formed by RA
Left border formed by LA and LV
Lower border mainly formed by RV, also by RA
Apex formed by LV

CHAMBERS OF THE HEART: the heart is divided by vertical septa into 4 chambers
• Right atrium
• Left atrium
• Right ventricle
• Left ventricle
The walls of the heart composed of cardiac muscle, the myocardium, covered externally with serous pericardium called the epicardium and lined internally with a layer of endothelium, the endocardium.

RIGHT ATRIUM: It consists of a main cavity and a small out pouching, the auricle. On the outside of the heart at the junction between the right atrium and the right auricle is a vertical groove, the sulcus terminalis, which on the inside forms a ridge, the crista terminalis. This part of the RA anterior to this ridge is trabeculated by bundle of muscle fibres, the musculi pectinati, which run from the crista terminalis to the auricle>.

ANTERIOR SURFACE OF THE HEART

POSTERIOR SURFACE OF THE HEART

OPENING INTO RA:

SUPERIOR VENA CAVA (SVC): Opens in to the upper part of the right atrium, drains the upper part of the body and has no valve.


INFERIOR VENA CAVA(IVC): Opens in to the lower part of the RA, drains the lower part of the body. A rudimentary non-functioning valve called the Eustachian valve guards its opening in to the RA.

CORONARY SINUS: Drains blood from the heart walls opens in to RA between the IVC and AV orifice. It is guarded by a rudimentary non-functioning valve.

ATRIO-VENTRICULAR ORIFICE (AV ORIFICE): Lies anterior to the IVC opening and is guarded by the tricuspid valve.

FETAL REMNANTS IN THE RA:

FOSSA OVALIS: It is a shallow depression which is the site of the foramen ovale in the fetus.

ANNULUS OVALIS: It lies on the atrial septum that separates the RA and LA. It forms the upper margin of the fossa ovalis.
The floor of the fossa ovalis represents the persistent primum of the embryo.

INTERIOR SURFACE OF THE HEART

RIGHT VENTRICLE: The RV communicates with the RA through the AV orifice and with the pulmonary trunk through the pulmonary orifice. As the cavity approaches the pulmonary orifice it becomes funnel shapped, at which point it is referred to as the infundibulum. The walls of the RV are much thicker with projecting ridges known as trabeculae carneae. The moderator band is one such trabeculation, which crosses the ventricular cavity from the septal to the anterior wall. It conveys the right branch of the AV bundle, which is the part of the conducting system of the heart. The tricuspid valve guards the atrioventricular orifice and consists of 3 cusps formed by a fold of endocardium. Anterior, septal, inferior (posterior). The bases of these cusps are attached to the fibrous ring of the skeleton of the heart, while the free edges and ventricular surface are attached to the chordae tendinae. The chordae tendinae connect the cusps to the papillary muscles. The pulmonary valve guards the pulmonary orifice and consists of three semilunar cusps. At the root of the pulmonary trunk are 3 dilatations called the sinuses, the 3 cusps are arranged with one posterior and 2 anterior.

LEFT ATRIUM: It consists of a main cavity and a left auricle. The interior of the LA is smooth, but the left auricle possesses muscular ridges as in the right auricle.

OPENINGS INTO THE LA: The four pulmonary veins, two from each lung, open through the posterior wall and have no valves. The left AV orifice is guarded by the mitral valve.

LEFT VENTRICLE: The LV communicates with the LA through the AV orifice and with the aorta through the aortic orifice. The walls of the LV are three times thicker than those of the RV. There are well developed trabeculae carneae and 2 large papillary muscles, but no moderator band. The part of the LV below the aortic orifice is called aortic vestibulae. The Mitral valve (MV) guards the AV orifice. It consists of 2 cusps-anterior mitral leaflet (AML) and posterior mitral leaflet (PML). The attachment of the chordate tendinae to the cusps and the papillary muscles is similar to that of tricuspid valve. The aortic valve guards the aortic orifice and it has 3 cusps.
Right coronary cusps-situated on anterior wall
Left coronary cusps and non-coronary cusps – situated on posterior wall
Behind aortic sinus gives origin to the right coronary artery and the left posterior sinus gives origin to the left main coronary artery.

AORTA: The aorta is the main arterial trunk that delivers oxygenated blood from the left ventricle to the tissues of the body. It is divided in to ascending aorta, arch of aorta, descending thoracic aorta and abdominal aorta.

ARCH OF AORTA: It is the continuation of the ascending aorta. It gives rise to the following branches.
Right brachio cephalic artery: It is the first branch of the arch of the aorta and it divides in to the right subclavian and right common carotid arteries.
Left common carotid
Left subclavian artery