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ECG grammar part 2. section A ."Understanding the genesis of ECG changes in acute myocardial ischemi PowerPoint Presentation

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Description : “Understanding the genesis of ECG changes in acute myocardial ischemia ”. ECG patterns in various cardiac diseases are readily available in the text books. In our conventional teaching we have been taught and teaching others to cram these. ECG grammar is an attempt to change this trend to “understanding the concepts , what, why and how these changes are recorded in ECG.” This can bring about a paradigm shift in clinical teaching. Objective of this presentation is to explain some unrevealed concepts of ECG changes is acute myocardial ischemia , often overlooked in conventional teaching.

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Slide 1-

“ECG grammar” part 2. “understanding the genesis of abnormal ECG “. Part 2 will be uploaded in two sections . Section A. Ischemic heart disease. Section B Non Ischemic heart diseases. (This will be uploaded later ). Part 2 Section A. “Understanding the genesis of ECG changes in acute myocardial ischemia ”. By Asadullah Kundi
Slide 2-

“Understanding the genesis of ECG changes in acute myocardial ischemia ”. ECG patterns in various cardiac diseases are readily available in the text books. In our conventional teaching we have been taught and teaching others to cram these. ECG grammar is an attempt to change this trend to “understanding the concepts , what, why and how these changes are recorded in ECG.” This can bring about a paradigm shift in clinical teaching. Objective of this presentation is to explain some unrevealed concepts of ECG changes is acute myocardial ischemia , often overlooked in conventional teaching.
Slide 3-

Pre test . “Understanding the genesis of ECG changes in acute myocardial ischemia ”. Don’t guess if you don’t know , select “ I don’t know”. Don't worry about your pretest score. You are expected to achieve >80 % in post test if you read the article carefully. 1.Among the following , select the most appropriate option regarding electrophysiological & ECG changes in the ST elevation myocardial Infarction (STEMI). a. In STEMI, zero voltage line drifts above the zero voltage level at TP segment. b. In STEMI, zero voltage line drifts below the zero voltage level at TP segment. c. In STEMI, zero voltage line doesn’t drift at TP segment. d. I don’t know. 2. Among the following , select the most appropriate option regarding electrophysiological & ECG changes in non ST elevation myocardial infarction (NSTEMI ) & unstable angina (USA). a. In NSTEMI &USA, zero voltage line drifts above the zero voltage level at TP segment. b. In NSTEMI & USA, zero voltage line drifts below the zero voltage level at TP segment. c. In NSTEMI & USA, zero voltage line doesn’t drift at TP segment. d. I don’t know.
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Pre test . “Understanding the genesis of ECG changes in acute myocardial ischemia ”. Don’t guess if you don’t know , select “ I don’t know”. 3. 1.Among the following , select the most appropriate option regarding electrophysiological & ECG changes in the ST elevation myocardial Infarction (STEMI). a. In STEMI, zero voltage line drifts above the zero voltage level at ST segment. b. In STEMI, zero voltage line drifts below the zero voltage level at ST segment. c. In STEMI, zero voltage line doesn't drift at ST segment , it remains at the zero voltage level. d. I don’t know. 4. Among the following , select the most appropriate option regarding electrophysiological & ECG changes in non ST elevation myocardial infarction (NSTEMI ) unstable angina (USA). a. In NSTEMI & USA , zero voltage line drifts above the zero voltage level at ST segment. b. In NSTEMI & USA , zero voltage line drifts below the zero voltage level at ST segment. c. In NSTEMI & USA, zero voltage line doesn't drift at ST segment , it remains at the zero voltage level. d. I don’t know.
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Pre test . “Understanding the genesis of ECG changes in acute myocardial ischemia ”. Don’t guess if you don’t know , select “ I don’t know”. 5. Among the following , select the most appropriate option regarding electrophysiological & ECG changes in non ST elevation myocardial infarction (NSTEMI ) or unstable angina (USA). a. In majority of NSTEMI or USA patients , T waves are concordant with the QRS complex. b. In majority of NSTEMI or USA patients T waves are discordant with the QRS complex. c. I don’t know. 6. Among the following , select the most appropriate option regarding electrophysiological & ECG changes in non ST elevation myocardial infarction (NSTEMI ) or unstable angina (USA). a. In most patients with NSTEMI or USA, vector of repolarization is directed from epicardium towards endocardium. b. In most of patient with NSTEMI or USA, vector of repolarization is directed from endocardium towards epicardium.. C . In most of patient with NSTEMI or USA, direction of vector of repolarization remains unchanged. d. I don’t know.
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We will discuss “understanding the genesis of ECG changes in acute myocardial ischemia ” under the following headings. Classification & pathological zones of myocardial transformation in acute myocardial ischemia. Different phases of action potential & their correlation with ECG. Why normally perfused myocardium has uniform electrical charges during ST & TP segments . What is zero voltage line & Correlation of ST and TP segments with cardiac cycle. Sequence of electrophysiological changes in resting versus ischemic myocardial cell. Why is Ischemic myocardium in partially depolarized state during diastole ? What is current of injury and what are the electrophysiological alteration due to current of injury ? What are the ECG manifestation of current of injury ? To highlight their importance & facilitate understanding , some of the statements are repeated . Commonest cause of myocardial ischemia /infarction is partial or total occlusion of coronary artery. Therefore we will briefly review pathological and clinical classification of acute myocardial ischemia.
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1. Clinical classification of acute coronary syndrome (ACS). Only ECG criteria is mentioned. Acute coronary syndrome (ACS ) includes ST elevation myocardial infarction (STEMI ), non-ST elevation myocardial infarction (NSTEMI ) and unstable angina (USA). 1.ST elevation myocardial infarction (STEMI ). Manifested in ECG as ST elevation with T wave changes. 2.Non ST elevation myocardial infarction(NSTEMI ).It is characterized by ST depression & T wave changes. 3.USA.It is commonly manifested as T wave changes with or without ST segment changes .In some cases ECG may be unremarkable. 2.Pathological classification of acute coronary syndrome (ACS). 1. Transmural infarction. It involves full thickness of myocardium ,from endocardium to epicardium. 2. Nontransmural myocardial infarction refers to a myocardial infarction that does not involve the full thickness of the myocardium. Also called subendocardial infarction. Because the Transmural depth of necrosis cannot be precisely determined clinically, Transmural & nontransmural myocardial infarction (MI )are no longer recommended for use in clinical cardiology. However we will briefly discuss correlation ECG findings with pathological classification of acute MI.
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Pathological transformation of myocardium in acute myocardial infarction . Total Coronary artery occlusion results in acute myocardial infarction. Figure below is showing diagrammatic representation of three zones of pathological transformation of myocardial tissue in acute myocardial ischemia. Copied & modified from https://za.pinterest.com/nishen72/pins/ Zone of ischemia. Zone of necrosis. Zone of injury. Diagrammatic representation of Transmural myocardial infarction.
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Total Coronary artery occlusion. It commonly but not always, results in Transmural MI. (Fig A). It has a central area of necrosis with adjacent areas of myocardial injury & Ischemia. Common but not invariably ECG changes are Q waves ,T wave changes with ST elevation & ST depression in adjacent areas of necrosis. Partial coronary artery occlusion(Fig B). It is said to be associated with nontransmural infarction with T wave changes & with or without ST depression. Copied & modified from https://za.pinterest.com/nishen72/pins/ Zone of ischemia. Zone of necrosis. Zone of injury. Diagrammatic representation of Transmural myocardial infarction. Copied & modified from Acute Coronary Syndrome (ACS) by Ali Al-Hadithi | 20 May, 2020 https://www.clinicianrevision.com/courses/cardiology/lessons/ischaemic-heart-disease/topic/acute-coronary-syndrome/ A B Diagrammatic representation nontransmural MI (Blue color ). Can ECG changes differentiate Transmural from nontransmural myocardial infarction (MI ) ?. *Above description is to facilitate the understanding of electrophysiological changes in acute myocardial ischemia. Studies have shown that ECG changes are not reliable to differentiate between Transmural & nontransmural infarction. . *Z.Antalóczy et al in their study concluded that “ To distinguish with certitude between subendocardial infarct and Transmural myocardial infarct on the basis of the ECG does not seem possible. Q wave infarct as “Transmural” and non-Q wave infarct as “subendocardial” does not correspond to the pathologic evidence.” *Z.Antalóczy et al, Correlation of electrocardiologic and pathologic findings in 100 cases of Q wave and non-Q wave myocardial infarction, Journal of Electrocardiology Volume 21, Issue 4, 1988, Pages 331-33
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Pathophysiology of three zones of acute myocardial infarction. Figure is showing various zones of myocardial transformation in acute myocardial infarction. Zone of necrosis. (Black colored ) zone with dead myocardial cells and it is usually not reversible. Most commonly involves the whole thickness of the myocardium. Zone of Injury. Peripheral zone in which myocardial cells are injured but viable with severe Ischemia. Zones of ischemia. Cells are viable with lesser Ischemia. Zones of ischemia & injury have viable tissue & is usually reversible particularly with early intervention. However if left untreated can result in cell death , necrosis. Next few slides we will discuss the genesis of ECG changes in each of the above conditions. Zone of ischemia. Zone of necrosis. Zone of injury. Copied & modified from https://za.pinterest.com/nishen72/pins/ Diagrammatic representation of Transmural myocardial infarction.
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Copied & modified from https://za.pinterest.com/nishen72/pins/ Zone of ischemia. Zone of necrosis. Zone of injury. Understanding the genesis of ECG changes in the necrotic zone of myocardial infarction. Fig. is showing diagrammatic representation of various pathological zones of myocardial transformation in acute myocardial infraction. Zone of necrosis (black) involves the whole thickness of the myocardium .It consists of necrotic (Dead ) cells that cannot be depolarized. Thus, positive electrode overlying necrotic tissue will record the electrical activity on the opposite side of the myocardium (Black arrow). As the negative (Tail end) vector of depolarization is facing the positive electrode ECG will record Q wave. ECG (ST –T )changes in zones of injury & ischemia will be discussed in detail in later slides. + - Diagrammatic representation of Transmural myocardial infarction.
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Copeid & modified from https://ecgwaves.com/ecg-topic/ecg-normal-p-wave-qrs-complex-st-segment-t-wave-j-point/ What are physiological & pathological Q waves in ECG ? Some ECG leads may have small Q waves which are considered as normal so called physiological Q waves. What is criteria for pathological Q wave ? (Q wave depth.) In this complex Q wave is deep and wide Next few slides we will discuss the genesis of ECG changes is myocardial injury & ischemia.
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To understand the ECG changes in myocardial injury & ischemia we first need to review action potential. Action potential. We have already discussed this in ECG grammar part 1. However will review it as it will highly facilitate understanding the genesis of ECG changes in acute ischemia . What is action potential ? An action potential is a rapid rise and subsequent fall in voltage (membrane potential ) across myocardial cell membrane. It consists of two stages , depolarization and repolarization. Action potential has been classified into five phases 0 to 4. Depolarization has one phase . This by convention is designated as phase “0”. Repolarization has three phases. Designated as phases 1,2 and 3. Polarized phase (Resting state) is an additional phase , designated as phase 4.
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Copied From https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwiA9uX6pc3lAhVCzoUKHSbnDI8QjRx6BAgBEAQ&url=https%3A%2F%2Fwww.researchgate.net%2Ffigure%2FMyocardial-action-potential-Shown-is-the-action-potential-of-a-ventricular-musclecell_fig3_40022512& K+ 3 Figure showing five phases of cardiac muscle action potential (o to 4). Phase 4 –(From left side of screen) Resting phase cell is in polarized state. (With -90 mV inside the cell). Phase 0 –Depolarization. Na+ ions rapidly diffuse into the interior of the muscle cell neutralizing (Depolarizing) the polarized resting state.(Minimal or almost no difference inside &outside cell charges). Phase 1. Repolarization. It is due to transient outward K+ channel.(Reducing the positive charge inside). Phase 2. Plateau phase is due to influx of positively charged Ca ions. It interrupts repolarization which had started in phase 1. Phase 3. During this phase there is large efflux of K ions (Out of cell) resulting in increased negative charge inside the cell and restoration of resting , polarized state.(Complete repolarization phase ). 1 2 0
Slide 15-

Figures showing temporal relationships of ECG (Fig A) & ventricular action potential below (Fig B). QRS corresponds to the upstrokes (phase 0), depolarization phase ,ventricular systole(Dual headed arrow). ST segment corresponds to the plateau phase (2). During this phase influx of positively charge Ca ++ ions arrests the process of repolarization & keep it in state of depolarization. T wave is generated during phase 3 (Repolarization phase ) . TP segment is inscribed during ventricular diastole of complete repolarization (phase 4). Illustrative summary of ECG correlation with different phases of normal action potential. Figure copied from https://thoracickey.com theelectrocardiogram-2/ plateau
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Copied & modified From https://image.slidesharecdn.com/chapter6introductionto12leadinterpretation-140910083335-phpapp01/95/chapter-6-introduction-to-12-lead-interpretation-25-638.jpg?cb=1410358109 ST TP Iso means same. It refers to state of cell membrane having uniform (same ) electric charge. When the myocardial cell has same (negative or positive ) charge all along the cell membrane , cell surface has zero voltage. In ECG this generate a straight horizontal line called isoelectric line. Isoelectric line (horizontal line ) between the end of QRS & onset of T wave of same beat is ST segment. Isoelectric lines between the end of T wave of previous cycle & the onset of P wave of the subsequent cardiac cycle is called the TP segment. Both TP & ST segments are isoelectric lines .However by convention TP segment is considered as an Isoelectric line. What is an isoelectric line.?
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What is definition of segments and intervals in ECG ?. Copied from https://litfl.com/wp-content/uploads/2018/10/ECG-waves-segments-and-intervals-LITFL-ECG-library-3.jpg By convention segments are defined as staright horizontal lines in the ECG without waves & Intervals are horizontal lines with one or more ECG waves. As seen in the figure PR and QT are intervals as they are intercepted by P wave and T waves respectively. ST & TP are segments as they are not interrupted by any ECG waves. However sometimes U wave may be recorded in in TP segment & therefore in such ECG , TP may be labeled as interval as shown in figure.
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Electrophysiological changes during two phases of action potential have vital role in the ECG manifestation of acute myocardial ischemia/injury. 1. Phase 2 ( Plateau phase ), corresponding to ST segment on surface ECG. 2, Phase 4, corresponding to the TP segment on surface ECG. In next few slides we will discuss these in detail.
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Copied From https://www.google.com/url?sa=i&source=images&cd=&ved=2ahUKEwiA9uX6pc3lAhVCzoUKHSbnDI8QjRx6BAgBEAQ&url=https%3A%2F%2Fwww.researchgate.net%2Ffigure%2FMyocardial-action-potential-Shown-is-the-action-potential-of-a-ventricular-musclecell_fig3_40022512& K+ Plateau phase ( 2 ) delays repolarization and maintains the cell in depolarization phase. Repolarization is initiated in phase 1 caused by transient outward K+ channel. (Reducing positive charge inside cell). However it is important to note that during phase 2 , repolarization ( Plateau ) phase is interrupted due to influx of positively charge Ca ++ ions. In other words plateau phase delays repolarization , thus during plateau phase cell is fully depolarized. 3 2 0 1
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Depolarizing cell. + - B Depolarized cell. Action potential phase 1 and phase 2. Diagram A above is showing myocardial cell in depolarizing state (Phase 1). Diagram B above is showing myocardial cell which is in fully depolarized state (Phase 2). Note that repolarization starts in phase 1 but it is interrupted during phase 2 & therefore the myocardial cell is kept in fully depolarized state.(Fig. B). Through out the membrane surface (Cell exterior) has negative charge with zero voltage (Fig B). This is why phase 2 is recorded in ECG as horizontal line , ST segment . A Electrophysiological correlation of phase 1 & 2 with surface ECG in normal myocardial cell. Figure copied from https://chemistryoftaste.weebly.com/uploads/2/4/5/7/24576351/9821524_orig.jpg Figure copied from https://thoracickey.com theelectrocardiogram-2/
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Figure copied from https://chemistryoftaste.weebly.com/uploads/2/4/5/7/24576351/9821524_orig.jpg Repolarized cell (Resting cell) Action potential phase 4 . Above cell (Fig .A) is in resting state , during diastolic 4th phase of action potential. Through out the length exterior of cell membrane is positively charged . Thus myocardial surface has zero voltage. Therefore phase 4 is recorded in ECG as horizontal line , TP segment. Electrophysiological correlation of phase 4 action potential with surface ECG ,in normal myocardial cell. Figure copied & modified from https://thoracickey.com theelectrocardiogram-2/ A
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Totally depolarized Total repolarized A During ST segment (phase 2),all myocardial surface has same (negative ) charges.(B1 ). Similarly during T-P segment (phase 4 )all myocardial surface has same(positive) charges.(B2 ). Thus during ST & TP segments therefore no current is generated ( zero voltage )on the myocardial surface of normal myocardium. Therefore during both these segments are recorded as straight horizontal lines. ( Fig. A) - - - + - - - - Zero voltage during ST & T-P segments in normally perfused myocardial surface. Figure modified from https://www.cvphysiology.com/ Cardiovascular Physiology Concepts. Prof. Richard E. Klabunde, Revised 12/6/16 Figure copied from https://thoracickey.com theelectrocardiogram-2/ Figure copied from https://thoracickey.com theelectrocardiogram-2/ + + + + + + B1 B2
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Cardiac action potential is a brief change in voltage across the cell membrane of myocardial cells. ECG is a graphic record of changes in the voltage on the myocardial surface & displayed on horizontal line (black dotted line ) also called Isoelectric line (Figure ). It is recorded in two forms 1If there is difference in the electric charge along the myocardial surface it is recorded as Wave, deflection. 2. If the myocardial surface has uniform (same )electric charge all over , having zero voltage , it is recorded as short horizontal lines (without any deflection), called segments. The isoelectric line ( black dotted line) is called zero voltage line “o mv” because in normal myocardium it represents zero voltage activity level. What is zero voltage line ? ……………….. …………………………………………………………………..…………………………………………………. 0 voltage baseline (0 mv) Figure copied modified from https://thoracickey.com theelectrocardiogram-2
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Figure showing the ST and TP segment in normal ECG. ST segment is a depolarized state with negative charge all over myocardial surface ( Having zero voltage) . TP segment, is totally repolarized state with positive charge all over myocardial surface (Having zero voltage) . In normally perfused myocardium , as both have zero voltage therefore they are recorded as straight horizontal lines (red color ) superimposed over the zero voltage baseline. Thus they are considered as potential baseline (or potential zero line ) . Temporal relationships of TP and ST segments with true zero voltage base line in normal myocardial cell. ………………..…………………………….………………………………………………………………………..……………………… 0 voltage baseline (0 mv) Figure copied modified from https://thoracickey.com theelectrocardiogram-2
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ST segment. During ventricular systole, ECG records horizontal straight line between end of QRS & onset of T wave, called ST segment. TP segment . During ventricular diastole , ECG records horizontal straight line between end of T wave of preceding beat & onset of P wave of next beat called TP segment. ST TP Correlation of ST and TP segments with cardiac cycle. Copied and modified from https://static.cambridge.org/binary/version/id/urn:cambridge.org:id:binary:20170118084331100-0062:9781139626798:61238fig14_14.png?pub-status=live
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Diastole (Completely repolarized cell ). Resting cell A 1 B3 Diastole partially depolarized state. Figure B1 to B3 shows sequence of electrophysiological changes in ischemic myocardium during diastole (resting cell ) & systole. Normal myocardium is in depolarized state only during systole (Fig A2) . Where as the ischemic myocardium is in depolarized state during systole as well as partially depolarized during diastole. (Fig B1 to B3). Why is the Ischemic myocardium in depolarized state during diastole (Fig B1 & B3)?. Sequence of electrophysiological changes in resting versus ischemic myocardial cell. Figures copied & modified from https://chemistryoftaste.weebly.com/uploads/2/4/5/7/24576351/9821524_orig.jpg Electrical changes in ischemic myocardium . Systole (Depolarized state) A2 A3 Electrical changes in normally perfused myocardium. Resting (partially depolarized ) B2 B1 Systole (Depolarized state). Figure A1 to A3 shows sequence of electrophysiological changes in normally perfused myocardial cell during diastole (resting cell ) and systole.
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Why is Ischemic myocardium in partially depolarized state during diastole ? Ischemic cells has reduced blood supply and get hypoxic which results in reduction in ATP production. Reduction of ATP impairs the activity of ATP-dependent transport systems, including the Na+/K+-ATPase pump that normally transports K+ into the cell and Na+ out of the cell. In simple words due to Ischemia ,myocardium cell are energy deficient & cannot maintain polarity of cell & therefore remain persistently partially depolarized state in diastole. What is current of injury ? In ischemic myocardium , left ventricle has two electrical zones. Normally perfused region and localized area of ischemic myocardium. As discussed in previous slide the ischemic myocardium remain depolarized in diastole . Thus wave of depolarization travels from the ischemic zone towards normally perfused myocardium during diastole. This flow of current between injured & normal myocardium during diastole is known as current of injury. What are electrophysiological consequences of current of injury ?
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+ - + + + + - - - - - - Electrophysiological alteration in ischemic myocardium during ventricular systole ( ST segment). All the myocardial cells are totally depolarized during systole , ST segment.(Fig A2). As whole myocardial surface has negative charge therefore current of injury ceases. Electrophysiological alternation in ischemic myocardium during ventricular diastole ( TP segment). Fig A1 is showing Diagrammatic representation of Transmural myocardial infarction during diastole. As shown in fig .ischemic myocardium is in state of partial depolarization during diastole ( T-P segment). Thus Current of injury generated during diastole propagates from the ischemic zone towards normal myocardium.(Fig. A1 green arrow heads ). ST T P A1 A 2 Figures copied & modified from Cardiovascular Physiology Concepts. Prof. Richard E. Klabunde, In conclusion electrophysiological alteration in ischemic myocardium generate current of injury during diastole that is TP segment , which cease during systole that is ST segment. What is Impact of current of injury on ECG recoding will be discussed in next slide. + Electrophysiological changes due to impact of current of injury on TP & ST segments, (in ischemia ). Figures copied & modified from Cardiovascular Physiology Concepts. Prof. Richard E. Klabunde,
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A 1 - - - Figure modified from https://www.cvphysiology.com/ Cardiovascular Physiology Concepts. Prof. Richard E. Klabunde, Revised 12/6/16 TP TP 0 mV zero voltage line - - - - A 2 A 3 Figures showing left ventricle electrophysiological & ECG changes in patient with ST elevation MI (STEMI) . Upper row , A1 to A3 of the figures are showing ECG changes (ST elevation ) . Lower row, B1 to B3 is showing electrophysiological changes during diastole & systole in above three beats. In STEMI , current of injury keeps the ischemic myocardium depolarized during diastole(Figures B1–B4 diastole). Current of injury initiated in the depolarized area of MI (red color) is propagating toward normal myocardium white color. It is directed away from overlying positive electrode (Figures B1-B4 diastole , green arrow heads). Therefore zero voltage (dotted ) line drifts below the zero voltage level at TP segment (red line Figs A1- A3). During systole current of injury ceases at the J point (Figures B1 –B3 systole). Therefore zero voltage line returns to zero voltage level (drifts up ) at ST segment (Curved arrow Figures. A1-A3). Systole Diastole Diastole + + + + - - - - - - - - - - - - - - TP TP Diastole + + + + + - - - - - - - - - - - - - - + + + + + - - Systole Diastole Systole + + + + + + B 1 B 2 B 3 ST ST ST Correlation of electrophysiological changes with ECG manifestation in ST elevation MI (STEMI ) . B 4
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A 1 - - - Figure modified from https://www.cvphysiology.com/ Cardiovascular Physiology Concepts. Prof. Richard E. Klabunde, Revised 12/6/16 TP TP 0 mV zero voltage line - - - - A 2 A 3 Systole Diastole Diastole + + + + - - - - - - - - - - - - - - TP TP Diastole + + + + + - - - - - - - - - - - - - - + + + + + - - Systole Diastole Systole + + + + + + B 1 B 2 B 3 ST ST ST Figure showing left ventricle electrophysiological & ECG changes in patient with ST elevation MI (STEMI). Upper row , A1 to A3 of the figures are showing ECG changes (ST elevation ) . Lower row, B1 to B3 is showing electrophysiological changes during diastole & systole in above three beats. Take away message . In Fig A1 to A3 we can see that at ST segment, zero voltage line remains at zero voltage level . However at TP segment zero voltage (dotted ) line drifts below the zero voltage level. (Red line & downward directed arrow heads). Thus in STEMI, current of injury during diastole results in depression of zero voltage line at TP segment appearing in ECG as if ST segment is elevated. Correlation of electrophysiological changes with ECG manifestation in ST elevation MI (STEMI ) .
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Diastole Systole Diastole ST ST ST TP TP TP Diastole Systole Figures showing electrophysiological & ECG changes in patient with USA & NSTEMI (ST depression ). Upper row, A1 to A3 of the figure is showing ECG changes (ST depression ). Lower row,B1 to B3 is showing electrophysiological changes during diastole & systole in above three beats. In such patients coronary blood flow is insufficient , particularly subendocardium is vulnerable to ischemia . Current of injury keeps the ischemic myocardium (subendocardial area)depolarized during diastole Fig B1-B3. During diastole Current of injury initiated in subendocardial area is propagating toward normal myocardium with positive end of the vector towards positive electrode ( Figure B1 to B3 diastole , green arrow heads). Therefore zero voltage (dotted)line drifts above zero voltage level at TP segment (Red line & arrows Fig A1- A3). However during systole (B1 –B3,)current of injury ceases at the J point . Therefore zero voltage lines returns to zero voltage level (Drift down )at ST segment (Curved arrow Fig. A1-A3 ). A 1 A 2 A 3 B 1 B 2 B 3 Systole Copied & modified From https://www.cvphysiology.com/ Cardiovascular Physiology Concepts. Prof. Richard E. Klabunde, Revised 12/6/16 Correlation of Electrophysiological changes & ECG manifestation of *USA & NSTEMI (ST depression ). 0 mV zero voltage line *USA= Unstable angina , NSTEMI = Non ST elevation myocardial infarction.
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Diastole Systole Diastole ST ST ST TP TP TP Diastole Systole Figure showing electrophysiological & ECG changes in patient with USA & NSTEMI (ST depression ).. Upper row, A1 to A3 of the figure is showing ECG changes (ST depression ) in patient with USA/*NSTEMI. Lower row,B1 to B3 is showing electrophysiological changes during diastole & systole in above three beats. Take away message . In Fig A1 to A3 we can see that at ST segment, zero voltage (dotted ) line remains at zero voltage level . However at TP segment , zero voltage(dotted ) line drifts above the zero voltage level. (Red line & upward directed arrow heads). Thus in USA /NSTEMI current of injury during diastole results in elevation of zero voltage line during TP segment, appearing in ECG as if ST segment is depressed. A 1 A 2 A 3 B 1 B 2 B 3 Systole Copied & modified From https://www.cvphysiology.com/ Cardiovascular Physiology Concepts. Prof. Richard E. Klabunde, Revised 12/6/16 0 mV zero voltage line Correlation of Electrophysiological changes & ECG manifestation of *USA & NSTEMI (ST depression ). ------------------------------------------------------------------------------------------------------------- *USA= Unstable angina , NSTEMI = Non ST elevation myocardial infarction.
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Figure A is showing depolarization vector with its positive end directed towards the positive electrode . This will generate upright QRS complex. (Fig. D ). Sub epicardial cells have a shorter duration of action potential than subendocardial cells. This causes sub epicardial cells to repolarize earlier than subendocardial cells ( Fig B ). Thus repolarization vector is directed from epicardium towards endocardium .Note that tail end of repolarization vector has positive charge. Therefore positive end of the repolarization vector is directed towards the positive electrode(Fig C ). That’s why in normal myocardium, T wave is upright in leads with upright QRS i.e. it is concordant with QRS. + - + - Ventricular repolarization. Genesis of T wave in normal and Ischemic myocardium. Electrophysiogical explanation of T wave morphology in normal myocardium. Copied & modified from. https://i0.wp.com/thoracickey.com/wp-content/uploads/2018/11/C7-FF6-2.gif?w=960 C B D Copied & modified From https://www.cvphysiology.com/ CVS Physiology Concepts. Action potential
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T wave morphology in subject with normal myocardium.(Already discussed ). In healthy myocardium repolarization is initiated from epicardium & is directed towards sub endocardium.. This results in upright T wave in leads with upright QRS. T wave morphology in subject with ischemic myocardium. Subendocardial ischemia results in shorter action potential & early repolarization of subendocardial cells. Thus repolarization is initiated from endocardium & is directed towards epicardium.( Fig B2). Note that head end of repolarization vector has negative charge which is directed towards the positive electrode(Fig B 2). This results in T wave inversion in leads with upright QRS. T wave is discordant to the QRS. + - + - + - Copied & modified from. https://i0.wp.com/thoracickey.com/wp-content/uploads/2018/11/C7-FF6-2.gif?w=960 Copied & modified from. https://i0.wp.com/thoracickey.com/wp-content/uploads/2018/11/C7-FF6-2.gif?w=960 - + B2 B1
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Take away message ST segment depression and elevation recorded in ECG in acute myocardial ischemia is apparent not real. Zero voltage line does not drift at ST segment. However at T-P segment , zero voltage line drifts below or above the zero voltage level in patient with STEMI or NSTEMI injury respectively. Appearing in ECG as if ST segment is elevated or depressed.