Basics of electrocardiogram (ECG)

The electrocardiogram (ECG or EKG) is primarily a tool for evaluating the electrical events within the heart. The action potentials of cardiac muscle cells can be viewed as batteries that cause charge to move throughout the body fluids. These moving charges currents, in other words – are caused by all the action potentials occurring simultaneously in many individual myocardial cells and can be detected by recording electrodes at the surface of the skin.

In a typical ECG , the first deflection is called the P wave. It corresponds to current flows during atrial depolarization. It generates about 0.2 mv and lasts for 0.1s

The second deflection, the QRS complex, occurs approximately 0.15s later. It is the result of ventricular depolarization. It is a complex deflection because the paths taken by the wave of depolarization through the thick ventricular walls differ from instant to instant, and the current generated in the body fluids change direction accordingly.

The S-T interval represents the time during which the entire ventricular muscle is depolarized.

The final deflection, the T wave , is the result of ventricular repolarization. Atrial repolarization is usually not evident on the ECG because it occurs at the same time as the QRS complex.A typical ECG makes use of multiple combinations of recording locations on the limbs and the chest (called ECG leads).

It is not a direct record of the changes in membrane potential across individual cardiac muscle cells. Instead it is a measure of the currents generated in the extracellular fluid 

Cardiac Cycle

The cardiac cycle is divided into into systole (ventricular contraction) and diastole(ventricula relaxation). At an average heart rateof 72 beats /minute, each cardiac cycle lasts approximately 0.8s , with 0.3s in systole and 0.5s in diastole.

1.     At the onset of systole, ventricular pressure rapidly exceeds atrial pressure, and the atrioventricular valves close. The aortic and the pulmonary valves are not yet open, however, and so no ejection occurs during this isovolumetric ventricular contraction(constant volume of blood in ventricle).

2.     When ventricular pressures exceed aortic and pulmonary trunk pressures, the aortic and pulmonary valves open, and ventricular ejection of blood occurs. The volume of blood ejected from each ventricle during systole is termed the Stroke Volume.

3.     When the ventricles relax at the beginning of diastole, the ventricular pressures fall significantly below those in the aorta and pulmonary trunk , and the aortic and pulmonary valves close. Because the atrioventricular valves are also still closed, no change in ventricular volume occurs during this isovolumetic ventricular relaxation.

4.     When ventricular pressures fall below the pressures in the right and the left atria, the Atrioventricular valves open, and the ventricular fillinf phase of diastole begins.

5.     Filling occurs very rapidly at first so that atrial contraction , which occurs at the very end of diastole, adds only a small amount of additional blood to the ventricles.

6.     The amount of blood in the ventricles just before systole is the end-diastolic volume. The volume remaining after ejection is the end-systolic volume, and the volume ejected is the stroke volume

Heartbeat Coordination

The SA node is the normal pacemaker for the entire heart. Its depolarization normally generates the action potential that leads to depolarization of all other cardiac muscle cells, and so its discharge rate determines the heart rate, the number of times the heart contracts per minute.

The action potential initiated in the SA node spreads throughout the myocardium, passing from cell to cell by way of gap junctions. The spread throughout the right atrium and from the right atrium to the left atrium does not depend on fibers of the conducting system. The conduction through atrial muscle cells is rapid enough that the two atria are depolarized and contract at essentially the same time.

In the ventricles

The spread of the action potential to the ventricles is more complicated and involves the rest of the conducting system. The link between atrial depolarization and ventricular depolarization is a portion of the conducting system called the atrioventricular node, located at the base of right atrium. The action potential spreading through the muscle cells of the right atrium causes depolarization of the AV node.

After leaving AV node, the impulses enters the wall – the interventricular septum – between two ventricles.    This pathway has conducting-system fibers termed the bundle of His.

Within the interventricular septum the bundle of His divides into right and left bundle branches, which eventually leave the septum to enter walls of both ventricles. These fibers in turn make contact with Purkinje fibers, large conducting cells that rapidly distribute the impulse throughout much of the ventricles. 

Finally , the Purkinje fibers make contact with ventricular myocardial cells , by which the impulse spreads through the rest of the ventricles. The rapid conduction along Purkinje fibers and diffuse distribution of these fibers causes depolarization of all right and left ventricular cells more or less simultaneously and ensure a single contraction.