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Vectors and EKG’s Chapters 11, 12, and 13 1 Electrocardiogram (ECG) • Depolarization wave passes through the heart and the electrical currents pass into surrounding tissues. • Small part of the extracellular current reaches the surface of the body. • The electric potential generated can be recorded from electrodes placed on the skin • An EKG is a comparison of two vectors • It compares the “heart vector” showing current flow on the heart with the reference, “recording lead vector” on the body. •(Non-invasive) •Heart Rate •Signal conduction •Heart tissue (enlarged) •Conditions (MI) •electrolyte and hormone imbalances 2 Vector diagrams • Vectors are used to describe depolarization and repolarization events • Vectors are arrows which show two things: – Direction or pathway (of charge spread) – Magnitude or size (amt of charge) • Vector analysis explains the waves on an EKG Q S 3 EKG is Extracellular Recording • Only looks at the charge on the outside of fibers! – Resting cell: outside positive – Depolarizing cell: outside negative – Repolarizing cell: outside positive +++++++++++ -----------------+++++++++++ -----------------+++++++++++ ------------------ +++++++++++ ------------------ • Depolarization: spread of surface neg charge • Repolarization: spread of surface positive charge • Vectors will always be positioned so that head of vector is in area of positive charge; tail is in area of negative charge. 4 Rest No current flow, no vector. The following vectors represent the spread of negative charge during depolarization; Then the spread of positive charge during repolarization 5 = depol SA nodal fibers, spread of neg charge over atria 6 - + 7 + 8 + 9 The atria would start to repolarize down and to the left, as the current continues downward to the ventricles We don’t detect this on the EKG, but what would the repolarizing vector look like? (review your specialized cells/contractile cells lecture!) + 10 + 11 Atria now have repolarized and now have positive surface charge again. 12 Meanwhile, as the atria are repolarizing...... We turn to the Depolarizing AV node These are small diameter fibers with few gap junctions; little or no detectable current flow 13 IV Septal Depolarization Moving down bundle of His; Current moves down R and L bundle branches from L toward R…why? 14 15 16 Apex then Lateral walls 17 18 Through the thickness of the heart, from endo- , to myo-, to epicardium 19 20 Ventricles completely depolarized, negative surface charge No current No vector 21 Begin Ventricular Repolarization Spread of positive charge + 22 23 24 25 26 27 28 29 Rest End of cycle; No current flow, no vector. 30 Recording from Lead II Standard limb lead II 31 The Rules of Vectors Analysis • • • • • • • • An EKG is a comparison of two vectors It compares the “heart vector” with the reference “recording lead vector” on the body. If the vectors run parallel (same direction) the pen moves upward from baseline If the vectors run antiparallel (opposite direction) then the pen moves downward from baseline. If the vectors are perpendicular, the pen remains on baseline. If there is no current flow, the pen remains on baseline. Each lead consists of two electrodes placed on the skin, with a voltmeter between them. The voltmeter is attached to a pen, which travels over paper running at 25 mm/sec. This produces waves called an electrocardiogram. 32 - I + - - III II Einthoven’s Triangle + + Bipolar Limb Leads 33 Atrial depolarization Pen here II V T The heart vector is parallel to the lead, but how can you confirm?34 Atrial depolarization II 1. 2. + Draw a perpendicular line to the lead vector Draw a line toward from the perpendicular vector toward your cardiac vector 35 Atrial depolarization II 36 AV nodal depolarization II 37 IV septal depol, from L to R II Anti-parallel! Pen deflects down Draw it! 38 IV septal depol, from base to apex II 39 Lateral walls depol II Draw it! 40 Depolarization complete; no current flow; pen returns to baseline II 41 Waiting to begin repolarization; no current flow II 42 Ventricular Repolarization begins II 43 Ventricular Repolarization II 44 Ventricular Repolarization complete; no current flow; pen on baseline II 45 Ventricular Repolarization complete; waiting to start all over again II End of one cardiac cycle 46 What does that tell you about the recording you obtain from each lead? • Each lead describes the events on the heart from “it’s own point of view” • Reading from several leads gives you different points of view about the same set of repeating events (depol, repol) • What if the recording lead was oriented this way? Use the words “down” or “up” to note the deflection compared to the five cardiac vectors above 47 Body Cross-section at Heart Level Heart V6 12 Lead EKG’s • Read from each lead independently; one at a time over several heartbeats. • See what each lead shows. • 12 leads – 3 bipolar limb leads (I, II, III) – 3 augmented unipolar limb leads • (aVR, aVL, aVF) – 6 precordial leads (chest leads, V1V6) V5 V1 V2 V3 V4 48 6 Leads- bipolar and augmented; all of these are in flat plane Augmented- Obtained by using the average voltage of any two points on skin as ground (neg pole) and reading from the third electrode (pos pole.) 49 Bipolar Leads and Einthoven’s Law • Lead I - The negative terminal of the electrocardiograph is connected to the right arm, and the positive terminal is connected to the left arm. • Lead II - The negative terminal of the electrocardiograph is connected to the right arm, and the positive terminal is connected to the left leg. • Lead III - The negative terminal of the electrocardiograph is connected to the left arm, and the positive terminal is connected to the left leg. • Einthoven’s Law states that the electrical potential of any limb equals the sum of the other two (+ and - signs of leads must be observed). Lead I Lead III Lead II LA – RA LL- LA LL- RA 50 Summary of Events • • • P wave – atrial depolarization- SA node to the AV node – (mechanical event that will result: atrial systole) QRS complex- depolarization of ventricles – Q wave- due to left to right depolarization at bundle branch (right has “detour”) – atrial repolarization and diastole (signal obscured) – AV node fires, ventricular depolarization – (mechanical event that will result: ventricular systole) T wave – ventricular repolarization – (mechanical event that will result: ventricular diastole. ventricles remain somewhat contracted until a few milliseconds after the end of the T repolarization wave.) 51 52 Intervals & Segments • Segments are flat lines, do not include waves: PR segment, ST segment. • Intervals include at least one wave • P-R interval- from beginning of P to the Q wave. Is time for atrial depolarization plus delay from AV node. Also, time of atrial contraction (more than .2 sec could be 1st degree block) • P-R segment- delay in impulse through AV node. 53 Phases of EKG • Q-T interval- includes Q and T waves, total time for ventricular depolarization and repolarization; this approximates the time of total ventricular contraction. • T-P segment - end of one cycle to beginning of next • P-P interval - time for one complete cycle (could also use R-R or T-T, etc.) • S-T segment: time between ventricular depolarization and repolarization; time of peak ventricular contraction (maximum 54 tension) Cardiac Arrhythmias • Tachycardia: abnormally fast heart rate • Bradycardia: Abnormally slow heart rate • Incomplete Atrioventricular Block: Prolonged P-R interval (1st degree) • Complete Atrioventricular Block: P waves and QRS complexes become dissociated (3rd degree) • Fibrillation: Complete lack of coordination Arrhythmia: conduction failure at AV node No pumping action occurs No P waves 55 Electrolyte imbalance • Hypernatremia: – Inhibits calcium entry into the cell – Depresses overall heart activity and becomes flaccid; (negative inotropy) • Hypercalcemia: – – – – (-, +) Increased heart irritability More calcium into cytoplasm What reflex could augment the decreased chronotropy? • Hyperkalemia: – Peaked T waves. 56 Electrolyte imbalance • Hyponatremia: – Depolarization delay – Decreased heart rate • Hypocalcemia: – (+,-) – Less heart contractility – What reflex could augment the increased chronotropy? • Hypokalemia: – Lowers RMP (makes it more negative) – Decreases heart rate – U waves 57 Determining the MEA Vector This presentation aims to teach you the trick to visually determine the position of the MEA from the EKG 58 Know the Orientation of All Leads • Use any two leads on an EKG • Given: limb leads I 210o and avF 180o • Goal: to find the MEA vector + 120 o - 90 o aVR aVL - 30 o I III 0o II aVF +60 o -90 o 59 Step 1: Visually, Lead I examine the profiles of leads I and aVF aVF 60 Lead I: Make note of the SIGN of the net deflection + 10 mm Lead I - 2mm Lead I Net deflection (+ 8) is POSITIVE. Where would this fall in the graph? 61 - + -90o 180o 0 +90o Lead I 62 Lead aVF: Make note of the SIGN of the net deflection + 1mm - 8 mm aVF Lead aVF Net deflection (- 7) is NEGATIVE. Where would this fall in the graph? 63 -90o 180o 0 - +90o + Lead aVF 64 Step 2: • Superimpose the two diagrams of the heart, and see where the hatched areas overlap. • This will be the area which must contain the MEA vector 65 - + -90o 180o 0 - +90o + MEA vector must lie in the zone of overlap 66 - + -90o 180o 0 - +90o + MEA vector must lie in the zone of overlap 67 - + -90o 180o 0 - +90o + Conclusion: LAD 68 Here’s Step 1: Visually, Lead I examine the profiles of leads I and aVF another example: aVF 69 Lead I: Make note of the SIGN of the net deflection Lead I Lead I Net deflection is VERY NEGATIVE. Where would this fall in the graph? 70 - + -90o 180o 0 +90o Lead I 71 Lead aVF: Make note of the SIGN of the net deflection aVF Lead aVF Net deflection is a SOMEWHAT POSITIVE. Where would this fall in the graph? 72 -90o 180o 0 - +90o + Lead aVF 73 Step 2: • Superimpose the two diagrams of the heart, and see where the hatched areas overlap. • This will be the area which must contain the MEA vector 74 - + -90o 180o 0 - +90o + 75 - + -90o 180o 0 - +90o + MEA vector must lie in the zone of overlap 76 - + -90o 180o 0 - +90o + Conclusion: RAD 77 Mean Axial Shift • Left axis deviations – – – – endomorph- short stature Pregnancy Left ventricle hypertrophy LBBB • Right axis deviations – Ectomorph- tall /thin – Hypertrophy of right ventricle – RBBB How does the current normally flow down the IV septum? Left to right? OR Right to Left? How would this change if there was a LBBB? RBBB? Why does a LBBB cause a LAD? (think about the vector!) 78 76