What is Electrocardiograph? Md. Saifur Rahman 26 Nov, 2020 The human heart beats about 70 times per minute. Each beat beginswith a contraction of the atria which is followed a moment later by thecontraction of the muscle. The contractions of the heart muscle, likethe contractions of other muscles, are triggered by electric signals. Butin contrast to other muscles, where the electric signals travel alongnerve fibers, the electric signals in heart muscle travel along the musclefibers. These electric signals involve changes of the electric potentialin and around the muscle fibers. The changes of the electric potentialassociated with the heartbeats are strongest in the immediate vicinity ofthe muscle fibers; but, with a sensitive detector, the changes in thepotential can be measured at some distance from the heart, on thesurface of the skin. The detector used to measure such changes ofpotential is called electrocardiograph; it is widely used by physiciansto monitor the operation of the heart and to discover defects in theheart muscle. The mechanism for the propagation of electric signals along musclefibers is almost exactly the same as for the propagation along nervefibers. Each muscle fiber is a single, long cell, sheathed in amembrane. When the muscle fiber is at rest (inactive), the membranecarries a layer of positive charge on its outside surface and a layer ofnegative charge on its inside surface. A membrane with such layers ofopposite charges is said to be polarized. The positive charges aremainly K+ ions, and the negative charges are mainly Cl- ions. Betweenthese layers of charge there is a uniform electric field betweenoppositely charged parallel plates consequently there is a potentialdifference between the outside and the inside of the cell. The potentialinduced of the cell is negative. This resting potential is about 90 millivolts. When a muscle fiber is stimulated by an electric signal from anadjacent muscle fiber, positive charges (mostly Na+ ions) from theinterstitial fluid surrounding the fiber flows through the membrane intothe fiber. The accumulation of these positive charges reverse thepotential difference, from - 90 millivolts to about + 30 millivolts. Themembrane is then said to be depolarized (actually, polarized in thereverse direction). The depolarization propagates along the length ofthe muscle fiber and constitutes the electric signal. This depolarizationtriggers the contraction process of the fiber. Within a short time,chemical processes within the fiber pump the positive charges out ofthe fiber and restore the charge distribution to the initial resting state.The depolarized fiber is then ready for the next depolarization and thenext contraction. The depolarization of muscle fibers can be detected at some distancefrom the fibers by the changes in the electric potential. Consider amuscle fiber that is initially polarized, and progressively becomesdepolarized from left to right. Initially the positive charge distributionis centered on the negative charge distribution. The potential at a pointM beyond one end of the fiber is nearly zero, since the averagepositions of the positive and the negative charge distributions areslightly different - the average position of the positive chargedistribution is slightly to the right of the average position of thenegative charge distribution. The separation between the averagepositions of the charge distributions implies that the muscle fiber has adipole moment. The electric fields then do not cancel, and thepotential at the exterior point is not zero. In the heart, the depolarization and contraction of the muscle fibers ofthe atria and the ventricles are initiated respectively by the sinus nodeand the atrioventricular node (A-V node). These nodes are smallclumps of muscle fibers. The sinus node is the primary pacemaker ofthe heart. Its muscle cells depolarize and contract spontaneously atregular intervals. The electric signals produced during this activity aretransmitted to the muscles of the atria and initiate their contraction.When the electric signals reach the A-V node, it relays these signals tothe ventricles and thereby initiates their contraction. To observe the potential changes generated during the depolarizationof the muscle fibers in the heart, electrodes (contacts) are placed on theskin and connected by wire leads to a sensitive voltmeter. The potentialchanges on the skin are of the order of a few millivolts, and to detectsuch small potentials, the voltmeters used in the electrocardiograph areequipped with amplifier circuits. The standard procedure forelectrocardiography is to attach electrodes to the left wrist, the rightwrist and the left ankle (other electrodes are attached). The sinus node is the primary pacemaker of the heart to the chest wall,but we will ignore these. The electrodes are covered with a salt paste,for good electric contact. The choice of location of the electrodes onthe arms and legs is a matter of convenience. The arms and the legs actas conducting segments, and attaching electrodes to the ends of thearms or legs. Measurements with the wrist and ankle electrodes areroughly equivalent to measurements at the vertices of a trianglecentered on the heart called the Einthoven triangle. With these three electrodes at the vertices of the triangle, sixmeasurements are routinely made and plotted by theelectrocardiograph: three “unipolar” measurements and three “bipolar”measurements. The unipolar measurements simply give the values ofthe potential at the three vertices of the triangle. These potentials aredesignated VR (for right wrist), VL (for left wrist), and VF (for leftankle or foot). The plus signs included next to these symbols indicatethat the +e terminal of the voltmeter is connected to this electrodeduring the measurement. The -e terminal of the voltmeter is connectedto a reference terminal, or “indifferent terminal,” whose potential iszero. The bipolar measurements give the potential differences betweenadjacent vertices. These potential differences are designated I, II, andIII. The plus and minus signs indicate where the + and - terminals ofthe voltmeter are connected during the measurement. Note that I, II,and III can be expressed as differences between VR, VL, and VF.I = VL - VII = VF - VRIII = VF - VL.Figure 2 : is an example of an electrocardiogram (ECG) measures on ahealthy individual. This ECG displays the potentials I, II, III, VR, VL,and VF as a function of time. Figure 2 is an enlarged sketch of thepotential II. The main features of this plot are the maxim andminimum, called the P, Q, R, S, and T “waves”. These waves indicatethe potentials generated by depolarizations and repolarizations ofmuscle fibers in different parts of the heart. The P wave indicates thedepolarization of the atria; the Q wave that of the septum, the R wavethat of the main mass of the ventricles, and the S wave that of asmaller, lateral portion of the right ventricle. The T wave indicates thedepolarization of the ventricles. These waves are of different magnitudes because the muscle mass isdifferent (largest for ventricle, smallest for septum), and because thedirections of the progressive depolarization of the muscle fibers aredifferent. The direction of depolarization in the ventricle is on theaverage, aligned with the direction to which the potential II belongsand therefore this depolarization gives a large contribution to thispotential. You May Like👉 What is green chemistry?👉 What is meant by C4 plants?👉 What is salt bridge?👉 What is glass fire?👉 Why chemistry is called the key science?👉 What is diaphragm?👉 Since when chemistry is being practiced?👉 What is interphase?👉 What is Al-Chemi?