Biology CARDIAC CYCLE AND ECG

### KEY TOPICS

star Cardiac Cycle
star Electrocardiogram

### Cardiac Cycle

● To begin with, all the color{violet}("four chambers") of color{violet}("heart") are in a relaxed state, i.e., they are in color{brown}("joint diastole.")

● As the color{brown}("tricuspid and bicuspid valves") are open, color{violet}("blood") from the color{violet}("pulmonary veins") and color{violet}("vena cava flows") into the left and the right ventricle respectively through the left and right color{violet}("atria.")

● The color{violet}("semilunar valves") are closed at this stage.

● The color{brown}("SAN") now generates an color{violet}("action potential") which stimulates both the atria to undergo a simultaneous contraction – the color{brown}("atrial systole.")

● This increases the color{violet}("flow of blood") into the ventricles by about 30%.

● The color{violet}("action potential") is conducted to the ventricular side by the color{violet}("AVN") and color{violet}("AV bundle") from where the color{violet}("bundle of HIS") transmits it through the color{brown}("entire ventricular musculature.")

● This causes the color{violet}("ventricular muscles") to contract, (color{brown}("ventricular systole")), the atria undergoes relaxation (color{brown}("diastole")), coinciding with the color{violet}("ventricular systole.")

● color{violet}("Ventricular systole") increases the color{violet}("ventricular pressure") causing the color{brown}("closure of tricuspid and bicuspid valves") due to attempted backflow of blood into the color{violet}("atria.")

● As the color{violet}("ventricular pressure") increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing the blood in the ventricles to flow through these vessels into the color{violet}("circulatory pathways.")

● The ventricles now relax (color{brown}("ventricular diastole")) and thecolor{violet}(" ventricular pressure") falls causing the closure of semilunar valves which prevents the color{violet}("backflow of blood") into the color{violet}("ventricles.")

● As the color{violet}("ventricular pressure") declines further, the color{violet}("tricuspid and bicuspid") valves are pushed open by the pressure in the color{violet}("atria exerted") by the blood which was being emptied into them by the veins.

● The color{violet}("blood") now once again moves freely to the color{violet}("ventricles").

● The color{violet}("ventricles and atria") are now again in a color{brown}("relaxed (joint diastole)") state, as earlier.

● Soon the color{violet}("SAN") generates a new color{violet}("action potential") and the events described above are repeated in that sequence and the process continues.

● This sequential event in the color{violet}("heart") which is color{violet}("cyclically repeated") is called the color{violet}("cardiac cycle") and it consists of systole and color{violet}("diastole") of both the color{violet}("atria and ventricles").

● As mentioned earlier, the color{violet}("heart beats 72 times per minute,") i.e., that many color{violet}("cardiac cycles") are performed per minute.

● From this it could be deduced that the duration of a color{violet}("cardiac cycle") is color{brown}("0.8 seconds.")

● During a color{violet}("cardiac cycle,") each color{violet}("ventricle pumps") out approximately color{brown}("70 mL of blood") which is called the color{brown}("stroke volume.")

● The color{violet}("stroke volume") multiplied by the heart rate (no. of beats per min.) gives the color{brown}("cardiac output.")

● Therefore, the color{violet}("cardiac output") can be defined as the volume of color{violet}("blood pumped") out by each ventricle per minute and averages color{violet}("5000 mL or 5 litres") in a healthy individual.

● The body has the ability to alter the color{violet}("stroke volume") as well as the color{violet}("heart rate") and thereby the cardiac output.

● For example, the color{violet}("cardiac output") of an athlete will be much higher than that of an ordinary man.

● During each color{violet}("cardiac cycle") two prominent sounds are produced which can be easily heard through a stethoscope.

● The first heart sound (color{violet}("lub")) is associated with the closure of the color{violet}("tricuspid and bicuspid") valves whereas the second color{violet}("heart sound (dub)") is associated with the closure of the semilunar valves.

● These sounds are of color{violet}("clinical diagnostic significance.")

### Electrocardiograph (ECG)

● In a hospital, a patient can be seen hooked up to a color{violet}("monitoring machine") that shows voltage traces on a screen and makes the color{violet}("sound “... pip... pip... pip..... peeeeeeeeeeeeeeeeeeeeee”") as the patient goes into color{brown}("cardiac arrest.")

● This type of color{violet}("machine (electro-cardiograph)") is used to obtain an color{brown}("electrocardiogram (ECG).")

● color{violet}("ECG") is a graphical representation of the electrical activity of the heart during a color{violet}("cardiac cycle.")

● To obtain a standard ECG, a patient is connected to the machine with color{brown}("three electrical leads") (one to each
wrist and to the left ankle) that continuously monitor the color{violet}("heart activity.")

● For a detailed evaluation of the heart’s function, multiple leads are attached to the color{violet}("chest region.")

● Each peak in the standard color{violet}("ECG") is identified with a letter from color{brown}("P") to color{brown}("T") that corresponds to a color{violet}("specific electrical activity of the heart.")

● The color{brown}("P-wave") represents the electrical color{brown}("excitation (or depolarisation) of the atria,") which leads to the contraction of both the color{violet}("atria.")

● The color{brown}("QRS complex") represents the color{brown}("depolarisation of the ventricles,") which initiates the color{violet}("ventricular contraction.")

● The contraction starts shortly after color{violet}(Q) and marks the beginning of the systole.

● The color{brown}("T-wave") represents the return of the ventricles from excited to normal state (color{brown}("repolarisation")).

● The end of the color{brown}("T-wave") marks the end of systole.

● Obviously, by counting the number of color{violet}("QRS complexes") that occur in a given time period, one can determine the color{violet}("heart beat") rate of an individual.

● Since the color{violet}("ECGs") obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible color{brown}("abnormality or disease.")

● Hence, it is of a color{violet}("great clinical significance.")