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Electrocardiography-ECG/EKG

Electrocardiography-ECG/EKG. Is a transthoracic interpretation of the electrical activity of the heart over time captured and externally recorded by skin electrodes. It is a noninvasive recording produced by an electrocardiographic device. Electrocardiography: Introduction

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Electrocardiography-ECG/EKG

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  1. Electrocardiography-ECG/EKG Is a transthoracic interpretation of the electrical activity of the heart over time captured and externally recorded by skin electrodes. It is a noninvasive recording produced by an electrocardiographic device

  2. Electrocardiography: Introduction • Body fluids are good conductors (the body is a volume conductor) • Fluctuations in potential (action potentials of myocardial fibers) can be recorded extracellularlywith surface electrodes • placed on the skin • The record of these potential fluctuations during the cardiac cycle is the electrocardiogram (ECG). • The ECG provides information on: - • - Heart rate and rhythm - The pattern of electrical activation of the atria and ventricles - The approximate mass of tissue being activated - Possible damage of the heart muscle - Possible changes in the body’s electrolyte composition

  3. Electrocardiography: • ECG is a complex recording representing the overall spread of activity throughout the heart during depolarization and repolarization. • The recording represents comparisons in voltage detected by electrodes at two different points on body surface, not the actual potential.

  4. ECG graph paper • Paper moves at a speed of 25mm/second • At this speed • Each horizontal small cube represents 0.04 seconds • Each vertical small- 0.1 mv • Large cube- horizontal- 0.2 seconds • Large cube- vertically- 0.5 mv

  5. Important features of the ECG are the P wave, the QRS complex and T wave. • Relevant intervals and segments are the PR interval, the RR interval, the QT segment and the ST segment.

  6. P wave: Atrial depolarization as recorded from the surface of the body P – R interval: Time taken for the wave of depolarization to move through the atria, AV node, bundle of His, Purkinje fibres to the ventricular myocardium. QRS complex: Depolarization of the ventricles. ST segment: Marks the end of the QRS complex and the beginning of the T wave. It occurs when the ventricular cells are in the plateau phase of the action potential (i.e. there is no change in potential occurring and so the ECG baseline is at zero potential) T wave: Repolarization of the ventricles (due to potential changes occurring during phase 3 of the cardiac action potential) Q – T interval: Period during which ventricular systole occurs R – R interval: This time is usually used to calculate the heart rate.

  7. Waves and normal values • P wave- Atrial depolarization 0.1 seconds 0.25 milli volts • PR interval- AV nodal delay 0.12 seconds- 0.2 seconds • QRS complex- ventricular depolarization 0.1-0.11 seconds

  8. ECG intervals 1Measured from the beginning of the P wave to the beginning of the QRS complex 2Shortens as heart rate increases from average of 0.18 at a rate of 70 beats/min to 0.14 at a rate of 130 beats/min

  9. Fig. 2. Principles of the bipolar recording of an action potential

  10. Recording the Electrocardiogram. Basic concept When the wave of depolarization moves toward the positive electrode, an upward deflection is recorded, whereas depolarization moving in the opposite direction produces a negative deflection

  11. EKG RULES: A wave of depolarization traveling toward a positive electrode results in a positive deflection in the ECG trace. 2) A wave of depolarization traveling away from a positive electrode results in a negative deflection. 3) A wave of repolarization traveling toward a positive electrode results in a negative deflection. 4) A wave of repolarization traveling away from a positive electrode results in a positive deflection.

  12. EKG RULES: continued 5)A wave of depolarization or repolarization traveling perpendicular to an electrode axis results in a biphasic deflection of equal positive and negative voltages (i.e., no net deflection). 6) The instantaneous amplitude of the measured potentials depends upon the orientation of the positive electrode relative to the Mean QRS vector. 7)The voltage amplitude is directly related to the mass of tissue undergoing depolarization or repolarization.

  13. Electrical Vectors Fig. 11. The basic direction of electrical conduction through the heart

  14. How the polarity of the waveform depends on the position of the recording electrodes relative to the heart. The effect of changing electrode position on the wave form recorded.

  15. EKG Leads Leads are electrodes which measure the difference in electrical potential between either: 1. Two different points on the body (bipolar leads) 2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads)

  16. EKG Leads The standard EKG has 12 leads: 3 Standard Bipolar Limb Leads 3 AugmentedUnipolarLimb Leads 6 Precordial Leads The axis of a particular lead represents the viewpoint from which it looks at the heart.

  17. ECG recordings from Bipolar Limb Leads

  18. Einthoven’s Triangle • Einthoven’s triangle • hypothetical triangle created around the heart when electrodes are placed on both arms and the left leg .The sides of the triangle are numbered to correspond with the three leads ("leeds"), or pairs of electrodes. Lead I at the top of the triangle, is orientated horizontally across the chest. This angle is taken as zero. Lead II is angled at 60 degrees to Lead I, and Lead III at roughly 120 degrees to Lead I.

  19. Augmented Unipolar Limb Leads (aVR, aVL and aVF) Three unipolar limb leads are also used for recording ECGs. Each lead measures the potential difference between an exploring electrode and an “indifferent” electrode (V) assumed to be at zero potential. This indifferent electrode is constructed by connecting the electrodes on the right arm (R), left arm (L) and left leg or foot (F) together. This indifferent electrode is called V and is assumed to be at zero potential (since the sum of the potentials in all the leads cancel out).

  20. Augmented limb leads • Represented by aVR, aVF, aVR. a- augmented V-unipolar Last letter represents the part of body • aVR- between right arm and left arm+ left leg • aVL- between left arm and rt arm+ left leg • aVF- between left foot and rt arm+ lt arm

  21. Precordial Leads These are unipolar leads measuring the potential difference between an electrode placed on the chest and an indifferent electrode, again made up by connecting the RA, LA and LL electrodes (i.e. the V electrode). There are 6 locations to place the chest electrode and so there are 6 chest electrodes (V1 – V6). With the chest leads, if the chest electrode is in an area of positivity, which occurs if the wave of depolarization is approaching this electrode, then an upward deflection is recorded. Adapted from: www.numed.co.uk/electrodepl.html

  22. Precordial Leads

  23. Pre cordial leads • V1- 4th intercoastal space, rt side sternal boarder • V2- 4th intercoastal space lt side of sternal boarder • V3- between V2 and V4 • V4- 5th intercoastal space in the mid clavicular space • V5- 5th intercoastal space in the anterior axillary line • V6- 5th intercoastal space in the mid axillary line.

  24. Summary of Leads

  25. SitusInversus with dextrocardia

  26. Arrangement of Leads on the EKG

  27. Anatomic Groups(Septum)

  28. Anatomic Groups(Anterior Wall)

  29. Anatomic Groups(Lateral Wall)

  30. Anatomic Groups(Inferior Wall)

  31. Anatomic Groups(Summary)

  32. INTERPRETATION OF THE ELECTROCARDIOGRAM What to inspect in an ECG 1. Heart Rate 2. Rhythm 3. Duration, segments and intervals.(P wave duration, PR interval, QRS duration, QT interval) 4. Mean QRS Axis (mean electrical axis, mean QRS vector) 5. P wave abnormalities Inspect the P waves in leads II and V1 for left atrial or right atrial enlargement. Left atrial hypertrophy would result in a taller P wave in Lead II RA hypertrophy – taller P wave in V1. 6. QRS wave abnormalities 7. ST segment / T wave abnormalities

  33. Determining the Heart Rate Rule of 300 Take the number of “big boxes” between neighboring QRS complexes( R – R interval), and divide this by 300. The result will be approximately equal to the rate Although fast, this method only works for regular rhythms. (300 / 6) = 50 bpm (1500/30) = 50 bpm

  34. What is the heart rate? www.uptodate.com (300 / ~ 4) = ~ 75 bpm (1500/20 ) = 75 bpm

  35. What is the heart rate? (300 / 1.5) = 200 bpm Heart Rate < 60 beats / min  Bradycardia Heart Rate > 100 beats / min Tachycardia

  36. The Rule of 300 It may be easiest to memorize the following table:

  37. 2. Rhythm Is the rhythm determined by the SA node pacemaker? i.e. is it a “sinus rhythm”? If normal, the following should be present: · The P wave should be upright in leads I, II and III. · Each QRS complex should follow a P wave

  38. Einthoven’s Triangle • Einthoven’s triangle • hypothetical triangle created around the heart when electrodes are placed on both arms and the left leg .The sides of the triangle are numbered to correspond with the three leads ("leeds"), or pairs of electrodes. Lead I at the top of the triangle, is orientated horizontally across the chest. This angle is taken as zero. Lead II is angled at 60 degrees to Lead I, and Lead III at roughly 120 degrees to Lead I.

  39. All Limb Leads

  40. The QRS Axis The QRS axis represents the net overall direction of the heart’s electrical activity. Abnormalities of axis can hint at: Ventricular enlargement Conduction blocks (i.e. hemiblocks)

  41. The QRS Axis By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°. -30° to -90° is referred to as a left axis deviation (LAD) +90° to +180° is referred to as a right axis deviation (RAD)

  42. Determining the Axis • The Quadrant Approach • The Geometric method.

  43. Determining the Axis Predominantly Positive Predominantly Negative Equiphasic

  44. The Quadrant Approach 1. Examine the QRS complex in leads I and aVF to determine if they are predominantly positive or predominantly negative. The combination should place the axis into one of the 4 quadrants below.

  45. The Quadrant Approach 2. In the event that LAD is present, examine lead II to determine if this deviation is pathologic. If the QRS in II is predominantly positive, the LAD is non-pathologic (in other words, the axis is normal). If it is predominantly negative, it is pathologic.

  46. Quadrant Approach: Example 1 The Alan E. Lindsay ECG Learning Center http://medstat.med.utah.edu/kw/ecg/ Negative in I, positive in aVF  RAD

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