12 Lead Basics

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Understanding the waveform essentials that make up the cardiac cycle

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12 Lead Basics

  1. 1. Understanding the 12-lead ECG
  2. 2. The heart’s electrical conduction <ul><li>Electrical activity </li></ul><ul><ul><li>precedes mechanical activity </li></ul></ul><ul><ul><li>coordinates all parts of the heart to contract at the same time </li></ul></ul><ul><ul><li>initiated in the sinoatrial (SA) node </li></ul></ul><ul><ul><ul><li>specialized group of cells in the right atrium </li></ul></ul></ul><ul><ul><ul><li>depolarizes at a rate of 60 to 100 times/minute </li></ul></ul></ul><ul><ul><ul><li>makes the atria contract and propel blood into the ventricles </li></ul></ul></ul>
  3. 3. Normal ECG waveform
  4. 4. Atrial depolarization <ul><li>SA Node produces the P wave </li></ul><ul><ul><li>first element on the ECG </li></ul></ul><ul><ul><li>first part of the cardiac cycle </li></ul></ul><ul><ul><li>appears as a small semicircular bump </li></ul></ul><ul><ul><li>1 st half represents right atrial depolarization </li></ul></ul><ul><ul><li>2 nd half represents left atrial depolarization </li></ul></ul><ul><ul><li>Normal P wave size: </li></ul></ul><ul><li>2.5-3 mm in height </li></ul><ul><li>< 0.10 seconds in width </li></ul>
  5. 5. PR interval <ul><li>Measured from the beginning of the P wave to the beginning of the QRS complex </li></ul><ul><li>represents SA node depolarization through the AV node </li></ul><ul><ul><li>During the PR segment is when actual atrial kick occurs allowing for 10 – 30% more volume pushed into the ventricles </li></ul></ul><ul><li>has normal duration of 0.12 to 0.20 second </li></ul><ul><ul><li>if longer than 0.20 second, a disease process may be affecting the cardiac conduction pathway </li></ul></ul>
  6. 6. AV Node/Bundle of HIS <ul><li>Atrioventricular (AV) node </li></ul><ul><ul><li>receives the atrial impulse and (after a brief pause to let the ventricles fill from atrial kick) transmits it to the ventricles via the bundle of His </li></ul></ul><ul><li>Bundle of His </li></ul><ul><ul><li>collection of cardiac conduction fibers that splits into the right and left bundle branches </li></ul></ul>
  7. 7. Bundle branches <ul><li>high-speed conducting fibers that run down the intraventricular septum </li></ul><ul><li>transmit cardiac impulse to the Purkinje fibers </li></ul><ul><ul><li>complex network that mingles with ventricular myocardial cells </li></ul></ul><ul><ul><li>rapidly stimulating ventricular muscle resulting in ventricular depolarization </li></ul></ul>
  8. 8. Ventricular depolarization <ul><li>generates the QRS complex—the electrical equivalent of ventricular systole </li></ul><ul><ul><li>Remember that electrical activity precedes mechanical activity and ECG shows only electrical activity. </li></ul></ul><ul><ul><li>You should be able to palpate a pulse with each QRS complex on the ECG monitor. </li></ul></ul><ul><li>QRS complex </li></ul><ul><ul><li>The largest complex on the ECG </li></ul></ul><ul><ul><li>Normal amplitude ranges from 2 to 15mm </li></ul></ul><ul><ul><li>normal duration is 0.06 to 0.10 second </li></ul></ul><ul><ul><li>duration greater than 0.12 seconds usually indicates prolonged ventricular conduction caused by a bundle branch block </li></ul></ul><ul><ul><li>can look different among different patients or among leads in the same patient </li></ul></ul>
  9. 9. QRS Complex <ul><li>Made of 3 deflections </li></ul><ul><li>Q wave </li></ul><ul><ul><li>1 st Negative waveform preceeding the R wave </li></ul></ul><ul><ul><ul><li>May not be present in all patients </li></ul></ul></ul><ul><li>R wave </li></ul><ul><ul><li>1st Positive waveform </li></ul></ul><ul><li>S wave </li></ul><ul><ul><li>2 nd Negative waveform </li></ul></ul>
  10. 10. ST segment <ul><li>Consists of the isoelectric line between the end of the QRS complex and the beginning of the T wave </li></ul><ul><ul><li>End of ventricular depolarization to the beginning of ventricular repolarization </li></ul></ul><ul><li>Reveals information about </li></ul><ul><li>the heart’s oxygen status </li></ul><ul><li>When elevated </li></ul><ul><ul><li>key indicator of myocardial infarction (MI) </li></ul></ul><ul><li>When depressed </li></ul><ul><ul><li>can indicate ischemia or other abnormalities </li></ul></ul>
  11. 11. J Point <ul><li>Point in which the QRS complex ends </li></ul><ul><ul><li>Can be challenging to find due ST segment elevation or depression </li></ul></ul>
  12. 12. T wave <ul><li>Characteristics </li></ul><ul><ul><li>is larger and broader than the P wave </li></ul></ul><ul><ul><ul><li>< 5mm amplitude </li></ul></ul></ul><ul><ul><ul><li>≤ 0.30 seconds in duration </li></ul></ul></ul><ul><ul><li>is rounded </li></ul></ul><ul><ul><li>Asymmetrical (the peak is closer to the end of the wave than to the beginning) </li></ul></ul><ul><ul><li>Positive in Lead II </li></ul></ul><ul><ul><li>represents ventricular repolarization </li></ul></ul>
  13. 13. Refractory Period <ul><li>The time between depolarization & repolarization </li></ul><ul><li>2 parts </li></ul><ul><ul><li>Absolute Refractory Period </li></ul></ul><ul><ul><ul><li>Beginning of QRS to peak of the T; absolutely no stimulation can be received </li></ul></ul></ul><ul><ul><li>Relative Refractory Period </li></ul></ul><ul><ul><ul><li>2 nd half of T wave, some cells have repolarized enough to respond to stimulus </li></ul></ul></ul>
  14. 14. QT Interval <ul><li>Measured from beginning of QRS to end of T wave </li></ul><ul><li>Represents onset of ventricular depolarization through ventricular repolarization </li></ul><ul><li>QT intervals varies </li></ul><ul><ul><li>Sex, age, heart rate </li></ul></ul><ul><li>Should be less than ½ the distance between 2 R waves </li></ul><ul><ul><li>As the heart rate increases the QT interval usually decreases </li></ul></ul><ul><ul><li>As the heart rate decreases the QT interval usually increases </li></ul></ul>
  15. 15. U wave <ul><li>Small, rounded, wave sometimes seen after the T wave </li></ul><ul><ul><li>Amplitude < 2mm </li></ul></ul><ul><li>Neither presence nor absence is considered abnormal </li></ul><ul><li>Large U wave causes </li></ul><ul><ul><li>Hypokalemia </li></ul></ul><ul><ul><li>Cardiomyopathy </li></ul></ul><ul><ul><li>Drug effects </li></ul></ul><ul><ul><li>LV enlargement </li></ul></ul>
  16. 16. Examining a 12-lead ECG <ul><li>Some QRS complexes have upward spike, others have downward deflections. </li></ul><ul><li>Each lead has a positive electrode and a negative electrode </li></ul><ul><li>When the heart’s electrical current is directed toward the positive electrode, the QRS complex spikes up; when directed toward the negative electrode, the QRS complex spikes down. </li></ul>
  17. 17. Vectors <ul><li>electrical energy generated by cardiac cells </li></ul><ul><ul><li>Represented by arrows </li></ul></ul><ul><li>have strength (measured in millivolts) and direction (measured in degrees) </li></ul><ul><li>should follow heart’s conduction pathway--downward and to the left </li></ul><ul><ul><li>Overall energy flows from base (atria) to the apex (ventricles) </li></ul></ul><ul><ul><li>QRS Axis refers to energy flowing down the ventricles </li></ul></ul><ul><ul><ul><li>mainly to the left ventricle due to its larger muscle mass </li></ul></ul></ul>
  18. 18. Frontal Plane Leads <ul><li>Standard Limb Leads </li></ul><ul><li>Augmented Voltage Leads </li></ul><ul><ul><li>measure the mean vector of the heart’s electrical flow ~ QRS Axis ~ in the frontal plane </li></ul></ul>
  19. 19. Limb leads I to III <ul><li>First six leads of the 12-lead ECG are derived from these </li></ul><ul><li>called bipolar or standard leads </li></ul><ul><li>Each has both a positive pole and a negative pole </li></ul><ul><li>Central neutral zone is created </li></ul><ul><ul><li>Machine uses this point as the negative pole for the unipolar leads </li></ul></ul>
  20. 20. Lead I <ul><li>Uses the negative electrode on the right arm looking toward the positive electrode on the left arm </li></ul><ul><li>produces upward deflection of the QRS </li></ul>
  21. 21. Lead II <ul><li>Uses the negative electrode on the right arm looking towards the positive electrode on the left leg, </li></ul><ul><li>shows most upright QRS complexes and most prominent P waves (monitoring lead) </li></ul>
  22. 22. Lead III <ul><li>Uses the negative electrode on the left arm looking towards the positive electrode on the left leg </li></ul><ul><li>produces upward QRS deflection, can be equiphasic </li></ul>
  23. 23. aVR, aVL, aVF <ul><li>Augmented leads (unipolar leads) </li></ul><ul><li>The software of the ECG machine looks toward the positive pole of limb leads to acquire these leads </li></ul><ul><ul><ul><li>While using the central neutral zone set up by the bipolar leads as the negative pole </li></ul></ul></ul>
  24. 24. Lead aVR <ul><li>Uses negative central pole looking toward the positive electrode on the right arm </li></ul><ul><li>Produces a downward deflected QRS because it looks at the heart in the opposite direction of the hearts normal electrical current </li></ul>
  25. 25. Lead aVL <ul><li>Uses negative central pole looking toward the positive electrode on the left arm </li></ul><ul><li>Produces the least upright QRS among the limb leads </li></ul>
  26. 26. Lead aVF <ul><li>Uses negative central pole looking toward the positive electrode on the left leg </li></ul><ul><li>has very upright QRS complexes with prominent P waves </li></ul>
  27. 27. Horizontal Plane Leads <ul><li>Chest or Precordial leads: </li></ul><ul><ul><li>Unipolar leads </li></ul></ul><ul><ul><li>lie across anterior chest </li></ul></ul><ul><ul><ul><li>V1 – V6 </li></ul></ul></ul><ul><ul><li>measure the mean vector of the heart’s electrical flow ~ QRS Axis ~ in the horizontal plane </li></ul></ul>
  28. 28. Precordial / Chest Leads <ul><li>The software of the ECG machine looks toward the positive pole of the chest leads </li></ul><ul><ul><li>While using the central neutral zone set up by the bipolar leads as the negative pole </li></ul></ul>
  29. 29. Chest leads <ul><ul><ul><li>Lead V 1 </li></ul></ul></ul><ul><ul><ul><ul><li>located at right sternal border, fourth intercostal space </li></ul></ul></ul></ul><ul><ul><ul><ul><li>lies above right ventricle and septum </li></ul></ul></ul></ul><ul><ul><ul><li>Lead V 2 </li></ul></ul></ul><ul><ul><ul><ul><li>located at the left side of sternum, fourth intercostal space </li></ul></ul></ul></ul><ul><ul><ul><li>Lead V 3 </li></ul></ul></ul><ul><ul><ul><ul><li>located midway between leads V 2 and V 4 </li></ul></ul></ul></ul>
  30. 30. Chest leads <ul><li>Lead V 4 </li></ul><ul><ul><li>located at the midclavicular line, fifth intercostal space </li></ul></ul><ul><li>Lead V 5 </li></ul><ul><ul><li>located at the anterior axillary line, fifth intercostal space </li></ul></ul><ul><li>Lead V 6 </li></ul><ul><ul><li>located at the midaxillary line, fifth intercostal space, above lateral wall of the left ventricle </li></ul></ul>
  31. 31. R wave progression <ul><li>Part of a normal ECG, R wave progression occurs due to the overwhelming power of the left ventricle. </li></ul><ul><ul><li>QRS complex becomes increasingly upright </li></ul></ul><ul><ul><ul><li>V1 V2 – downward deflected QRS complex </li></ul></ul></ul><ul><ul><ul><li>V3 V4 – equiphasic QRS complex </li></ul></ul></ul><ul><ul><ul><li>V5 V6 – upward deflected QRS complex </li></ul></ul></ul><ul><li>As the leads progress the R wave is more prominent and S wave is gone in V6. </li></ul>

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