- why the T wave is deflected upwards although it's a repolarization wave?

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Simon Chapman
5 years ago
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1 Cardiac Electrograph: - why the T wave is deflected upwards although it's a repolarization wave? After depolarization the ventricle contracts but since the heart is a volume conductor (3D not 2D), when it contracts the contraction is towards the centre hence too much pressure develops in the endocardium that it drainages the electrolytes to the extent it delays repolarization, that it starts first in the pericardium. The conclusion is : its positivity is due to reversed direction from pericardium to endocardium. Note that this can be proven by atrial repolarization, as it's negative deflection because they lack significant pressure. Or it can be explained intrinsically as endocardial muscles have longer action potential compared to pericardium, therefore repolarization wave starts in the pericardium. - For calculations sake it's worth knowing that the machine moves 25mm/s on the X axis and 10mm/s on the Y axis. Time wise : Each small square 1mm 25 squares 1 second 1 square 1/25=0.04 s Or we can count the large squares ( 5 large 1 s)

2 Voltage wise: 10 small squares 1s Or 2 large square 1s -To calculate the rate ( how many cardiac cycles per minute ) : Take the time for one cardiac cycle, to determine it, take R-R interval as it's the easiest, you can take other intervals, it's supposed to give the same end result but this one is easiest to obtain. Count the small squares and multiply by 0.04 Rate of beats per minute =60/time for one cycle. that if the heart rate is regular, If it's irregular we obtain rhythm. to decide whether the heart rate is regular or irregular we match the cycle, we cut off the ECG into small pieces one for each cycle and match them ( time wise ) if they do match, then it's a regular rate if they don't as cycles maybe portionsof a second less or more, then it's an irregular rate. -how to calculate the rhythm of irregular heart: we do an ECG for 20s which equals 100 large square as each second equals 5, then we count how many cycles there are and multiply by 3 to get the number of cycles per minute. we can do the ECG for 15 s and multiply by 4. Irregularity itself can be regular ( repeated ) as 5s.6s,7s and so on.

3 Or irregularity can be irregular ( no distinct pattern ). Intervals : P-R interval : the beginning of P till the beginning of Q if clear or the beginning of R if Q isn't that clear. Usually we use R as again it's easier to obtain. Count small squares, multiply by 0.04, it should be less than 0.2s. otherwise, it indicates something wrong in the AV node. QRS interval : from the beginning of Q to the end of S.it represent ventricular depolarization. It should be less than 0.12s. if prolonged it means slow depolarization, which could happen for 2 reasons : Ventricular hypertrophy - Left bundle branch block due to ischemia - Note that the conduction is slower in the ventricular (0.5m/s )muscles than the purkinje (4-5m/s) which results in prolonged QRS interval. Q-T interval : beginning of Q till the end of T, this is variable according to the heart rate.

4 Segments : concerned with iso-electric lines, could be elevated or depressed and denote Ischemia. As Ischemic myocardium can be either depolarized which shows on ECG as depressed segment or hyperpolarized as an elevated segment. Note there might be a T wave followed by a U wave, this U denotes K+ abnormally. K+ is critical because it's equivalence is usually 4-5 mill moral, that's a 20% change, comparing it to Na mmolar. In K+ 1mmolar makes a difference, therefore any change in K+ affects ECG. Remember : for teaching purposes we take 0.8s as the time for each cycle and the rate as 75 beats /minute. Remember : ECG reflects electrical changes but we correlate it with contraction and relaxation. Flow of electrical current : AV node AV bundle interventricular septum and spreads from there. At any point there is a vector of current on the depolarized area and still polarized area, we can calculate a resultant of the two (value, direction ) The last part to depolarize is the posterior aspect of the left ventricle, at this point there's a vector between the

5 posterior ( still polarized )and the heart (depolarized part ). We calculate a mean resultant to all those momental resultants as there is a different victor with each moment but the final vector is obtained when the depolarization is complete. Then he repeated we calculate the mean vector through the partial depolarized heart! Note : atria and ventricles diastole might happen at the same time but never systole at the same time, systole should start in the atria and continue to ventricles. Please go through the slides, as the dr just read them. Lead I : - on right arm, + on left arm Lead II : - on right arm, + on left foot Lead III: - on left arm, +on left foot Einthoven's law states : the algebraic summation of QRS ( signs considered ) of lead II= Lead I + lead III. or the sum of any two leads equals the third ( signs considered ) ex. Lead I = lead II lead III remember : Einthoven connected the right arm to the negative electrode and the left arm to the negative electrode in lead I and so on, only to obtain positive quantities in all leads, as positive is always preferable.

6 note : the values can be negative in parts of the QRS wave, which is abnormal within this arrangement of electrodes.by algebraic summation of the small squares on the voltage (Y) axis ( positivity being above 0 point and negativity below ) we obtain the final value of the lead. note : we take the values of QRS as it's easier but we can use both T and P waves to get values of the leads. note :if the current is going towards the positive electrode it's a positive value. if the current is going away from the positive electrode it's a negative value. remember electrical circuits and kirchoff's law, if the current is moving in a close circuit the resultant is zero. therefore Einthoven applied kirchoff's 2 nd law, he only reversed the direction of lead II. according to kirchoff's lead I + Lead III lead II = zero according to Einthoven's lead I + Lead III = Lead II Remember : the mean vector QRS flows to the left and anterior unless there's dextrocardia, the current flows in the opposite direction. Left was analyzed by the frontal plane. Anterior on the other hand was analyzed by the transverse plane ( chest leads )

7 To obtain Lead I, II, III values that contributed to the mean value, we draw perpendicular columns from the borders of the mean on the the line that represents the lead itself. We can obtain the mean vector from the values of the leads as well. Einthoven's triangle : By drawing lines parallel ( to preserve the same values and direction ) to the 3 leads that cross at the centre of the triangle with an angle of 60 degrees between them we obtain what so called trigonal axis. Augmented leads : They are obtained by applying very high resistance on the limbs. By putting the + electrode on the right arm after eliminating the high resistance on the right arm and putting the electode on the other two, we obtain avr Same concept for avl and avf. Chest leads : In surface anatomy we have to use 2 different lines, vertical and transverse. Transverse sternal angle, 2 nd intercostals space. Perpendicular parasternal.

8 V1 4 th intercostals space, parastenral on the right side V2 4 th intercostals space, parasternal on the left side. V3 midway between V2 and V 4. V 4 5 th intercostals space, midclavicular line, left side. V 5 5 th intercostals space, anterior axillary line, left side. V6 5 th intercostal space, mid axillary line, left side. Please just go through the slides, he finished slide 30 Done By : Marina Zawaideh

Electrocardiography negative zero LA/VL RA/VR LL/VF recording electrode exploring electrode Wilson right arm right arm, left arm left arm

Electrocardiography negative zero LA/VL RA/VR LL/VF recording electrode exploring electrode Wilson right arm right arm, left arm left arm Electrocardiography In the previous lecture, we were talking about the unipolar limb leads. We said that to make the unipolar lead, you have to make the negative electrode as zero electrode, this is done