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12 Lead Basics

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Understanding the waveform essentials that make up the cardiac cycle

Understanding the waveform essentials that make up the cardiac cycle

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

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