Cardiac Cycle and phases of heartbeat
One contraction and one relaxation is called a cardiac cycle. Contraction of heart is termed as systole and relaxation as diastole. One complete cycle takes 0.8 Lub-dub Sound of Heart seconds. The contraction and relaxation are cyclic and rhythmic. In first phase called diastole blood flows in all the four chambers passively. In second phase.
In second phase i.e. systole both the auricles contract together for about 0.1 second filling the ventricles completely with blood.Then in third phase the ventricles contract together for about 0.3 second pouring blood in to aorta and pulmonary artery.Normal rate of heart beat in a healthy human being is 72 beats per minute. This rate decreases when a person is resting or sleeping and increases to 120 beats per minutes during strong muscular exercise like running swimming etc. and with some medicines like caffeine. Every time heart pours about 85 milliliters of blood in to aorta with a great pressure. Blood travels, in all different types of vessels, at different speeds. Speed is fastest in arteries, slower in arterioles and slowest in capillaries. From here it starts getting collected in venules. Its speed starts increasing in venules and faster again in veins. The speed of flow of blood fluctuates from 425 cm/sec in the aorta to about less than 1 mm/min in capillaries.The continuous working of heart is due to certain specialized structures like SA node (also called pacemaker), AV node and some specific type of fibers called purkinji fibers.
The pace maker or SA node ( node (sino-atrial node) is the impulse-generating
(pacemaker) tissue located in the upper dorsal wall of the right atrium of the heart, near the entrance of the superior vena cava. SA node initiates the electrical impulses for
heart beat and keeps the heart in motion. The cells of the SA node generate electrical
impulses faster than other cells and are responsible for the rest of the heart's
Dysfunction of SA node causes disturbance in heart beat. Either it becomes very fast
electrical activity. SA node is thus sometimes called the primary pacemaker.
Another very important structure is AV node (atrio-ventricular node) which is the part
of electrical control system of the heart. Its main function is to co-ordinates heart rate.
It electrically connects atrial and ventricular chambers. According to encyclopedia
Britannica it is an electrical relay station between the atria and the ventricles.
or very slow or some times a combination of both.Electrical signals from the atria must pass through the AV node to reach the
ventricles. This node slows down the
speed of the electrical signals to delay the
contraction of ventricles until the atria are
not fully contracted.Purkinji fibers were discovered in 1839 by Jan Evangelista Purkyně. These fibers extend in the form of a branching tree in the heart and play an important role in its continuous working. Purkinjil fibers are the extension of the autonomic nervous system. They are found in the inner wall of the ventricle just beneath the endocardium. These are specialized myocardial fibers which conduct clectrical signals (nerve impulses) to different areas of heart to enable it to work in a coordinated way.
Electrocardiogram(ECG)
The Electrocardiogram (ECG) is a medical device used for recording the
electrical activity of the heart. For this purpose its electrodes are placed on the chest skin at specific locations. Doctors use it to monitor the electrical workings of the heart. The information is used to discover heart rate, arrhythmias, myocardial infarctions, atrial enlargements, ventricular hypertrophies, and bundle branch blocks etc.The principle implied for recording the electrical impulse ECG is quite simple. When
the overall electrical current of the heart goes towards a particular lead, it registers a
positive deflection. Those that go away from the lead register a negative deflection.
Those which are at 90 degrees or perpendicular to the vector of the lead registers 0,i.eis seen as an isoelectric line.
Reading of ECG requires following conventions to be kept in mind.
The rate of paper (i.e. of recording of the ECG) is 25 mm/s which results in:
Reading ECG
The phases of a heart beat can also be divided into sections relating to the shape of
the electrical signals produced when viewing the heart beat via an ECG
(Electrocardiogram). This traces the electrical activity of the heart. The wave shape produced is called the QRS wave, with each part of the wave being labelled to help
describe what is happening at each stage.
**TP Interval (Ventricular Diastole)
Atria and ventricles are relaxed; blood is flowing into the atria from the veins. As
the atrial pressure increases above that of the ventricle, the AV valves open, allowing
blood to flow into the ventricle
**P Wave (Atrial Systole)The SA node fires and the atria contract causing atrial systole which forces all blood into the ventricles, empyting the atria.
**QR Interval (End of Ventricular Diastole)
The AV valves remain open as all remaining blood is squeezed into the ventricles
The electrical impulse from the SA node reaches the AV node which spreads the signa
throughout the walls of the ventricles via specialised cells called bundles of His and
Purkinje fibres. The R peak is the end of ventricular diastole and the start of systole.
**RS Interval (Ventricular Systole)
As the blood is now all within the ventricles and so pressure is higher here than ir
the atria, the AV valves close. The ventricles start to contract although pressure is noyet high enough to open the SL valves
**ST Segment (Ventricular Systole)Pressure increases until it equals Aortic pressure, when the SL valves Open.The blood is ejected into the Aorta (and pulmonary artery) as the ventricles contract. Atthis time the atria are in diastole and filling with blood returning from the veins
**T Wave (Ventricular Diastole)Ventricles relax, the ventricular pressure is once again less than the aortic pressure and the SL valves close. The cycle continues. satima
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