ECG analysis and application

Previously you have examined cardiac assessment and the analysis and treatment ofarrhythmias. Analysis of the 12 lead ECG is a vital
component of cardiovascular assessment needed for many cardiac conditions. A systematic approach to the assessment of the ECG is
vital and ensures success. This is a large module and will continue to be work in progress throughout the year. Most of this is assumed
knowledge, so use it for revision and choose what is important in regard to your personal learning.
Learning outcomes for this section
Upon successful completion of this section, you should be able to:
describe how the 6 limb leads of an ECG are obtained
identify the sites of attachment of the six precordial leads and indicate over which region of the heart each electrode lies
list the leads which view the major surfaces of the heart
demonstrate a systematic approach to analysis of the 12 lead ECG
demonstrate a basic understanding of the physiology and characteristics of bundle branch blocks
demonstrate a basic understanding of the ECG characteristics of ischemia and infarction
describe the ECG changes associated with pericarditis and myocardial trauma
identify the changes associated with hyper and hypokalemia on an ECG
identify the major characteristics of chamber enlargement.
3/24/2020 Study plan: Week 3 – ECG analysis and application
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Suggested readings
Text reading
Thompson, P 2011, chapter 26 ‘Electrocardiographic monitoring’, in Coronary care manual, 2nd edn, Elsevier
Australia. – Click Here
.
Optional reading
Woods, S, Froelicher, E, Motzer, S & Bridges, E 2010, Cardiac nursing, 6th edn, Lippincott Williams and Wilkins.
Available online Flinders University Library www.flinders.edu.au/library through Ovid.
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The 12 Lead ECG
A 12 Lead electrocardiogram consist of the following:

1. Six limb or extremity leads
   Three standard (bipolar) leads, I, II &III
   Three augmented (unipolar) limb leads aVR, aVL & aVF.
2. Six precordial (unipolar) leads V1-V6.
   So far this Study plan has concentrated on the bipolar and unipolar limb leads. The six precordial unipolar leads are a vital part of ECG
   analysis as they give a view of the heart on its horizontal plane.
   The electrical activity of the heart consists of multiple individual currents with the ECG representing the sum result of these electrical
   impulses from a point on the body surface. If the sum result of the electrical impulse or mean vector is towards the positive ECG pole a
   positive deflection is recorded. A current flowing towards the negative pole records a negative deflection.
   The following diagram illustrates the normal sequence of depolarisation through the heart as recorded by the limb and the precordial
   leads. These give a view of the heart on its frontal and horizontal plane.
   Figure 4.1: (A) Normal sequence of depolarisation through the heart as recorded by the frontal plane leads. This diagram also includes
   the hexaxial reference system or axis wheel. (Adapted from Woods, S, Froelicher, E, Motzer, S & Bridges, E (eds) 2010,
   Cardiac nursing, Lippincott.)
   Figure 4.2: (B) Cross section of the thorax illustrating how the six precordial leads record normal electrical activity in the ventricles. In both
   examples the small arrow (1) shows the initial depolarisation through the septum, followed by the mean direction of
   ventricular free wall depolarisation, larger arrow (2). (Adapted from Woods, S Froelicher, E, Motzer, S & Bridges, E (eds)
   2010, Cardiac nursing, Lippincott.)
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   Activity—Review the following
3. The correct sequence of R wave progression through the precordial leads. Relate this to the normal
   sequence of depolarisation of the heart.
4. The sequence of ventricular depolarisation through the right and left bundle branches and relate this to the
   ECG characteristics in the precordial leads.
5. The configuration of lead AVR? Why does this lead normally record a negative deflection?
   12 Lead Interpretation Part 1: Introduct…
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   Examining the ECG: the process
   Examination of the ECG should follow a defined process. You may find that this process differs from many different processes in your text.
   Find one which suits you and use it with all of you ECG analysis exercises and in clinical practice. Your process should include
   examination of the each of the following:
   Patient identification and calibration (as in week 2)
   Rate and rhythm (as in week 2)
   Intervals: Is the PR and QT intervals normal? Use your ECG text to determine the normal characteristics.
   Waveforms: Are the P QRS and T waves within normal range?
   Cardiac axis. Determine if the axis is normal, left axis deviation, right axis deviation or indeterminate.
   ST segments: Is there any indication of myocardial injury or infarction.
   Other abnormalities: Signs of electrolyte abnormalities hypertrophy.
   12 Lead Interpretation Part 2: The 6 St…
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   Determining the electrical axis
   The axis represents a measurement of the sum of the electrical vectors during depolarisation of the heart. Determination of the electrical
   axis shows the mean vector generated by depolarisation of the ventricles as determined by examining the ECG leads of the frontal plane.
   Deviations in the electrical axis from normal or a change in axis may reflect anatomical alterations or significant pathology associated with
   myocardial damage.
   Axis is most commonly represented using the hexaxial reference system on the frontal plane leads represented in diagram (A) above. This
   labels each of the 6 frontal plane leads within a 360° circle beginning with lead I at 0°. The mean QRS vector or normal axis lies between 0
   and +90°.
   There are several methods of determining the mean frontal plane QRS axis each of which require examination of leads I and AVF. The
   exact axis as a percentage value can be plotted on the hexaxial wheel, or a simpler method classifies the axis as normal, left axis
   deviation, right axis deviation or indeterminate as demonstrated in the following diagram.
   Figure 4.3: The four quadrants of the axis wheel. (A) If the QRS in lead I is positive and the QRS in aVF is negative, the axis is in the left
   quadrant. (B) If the QRS is positive in both leads I and aVF, the axis is normal. (C) If the QRS in lead I is negative and the QRS in aVF is
   positive, the axis is in the right quadrant. (D) If the QRS is negative in both leads I and aVF, the axis is indeterminate. (Adapted from
   Woods, S Froelicher, E, Motzer, S & Bridges, E (eds) 2010, Cardiac nursing, Lippincott.)
   It is important that you can determine the axis within the four categories described above. Consult your ECG text to practice a method axis
   determination to achieve this. Consult your mentor or clinical facilitator if you have difficulty with this process.
   Activity
6. What is meant by electrical axis? Use your diagram of Einthoven’s triangle from the previous section to
   determine the direction of the electrical axis in the limb leads.
7. Read the section in your texts regarding determining the electrical axis. What are the characteristics and
   clinical significance of a left or right axis deviation? Relate this to the patients that you are caring for in the
   clinical area.
   12 Lead Interpretation Part 3: R-wave P…
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   The normal ECG
   Examine this ECG and follow the examining process.
   Normal sinus rhythm is present at a rate of 110 beats per minute.
   PR and QRS intervals are normal
   The QRS complexes and R wave progression is normal
   The T waves are normal
   There are no abnormal Q waves
   The Cardiac Axis is normal
   The ST segment is at baseline in all leads.
   There are no other abnormalities.
   This ECG can be used for comparison as abnormalities are discussed throughout this module.
   Figure 4.4: The normal ECG
   (Adapted from Hampton, J 2014, The ECG made easy, Churchill Livingstone.)
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   Q wave and ST segment abnormalities
   ECG analysis involves careful analysis of the Q wave and ST segment for indicators of myocardial pathology.
   The Q wave represents depolarisation of the interventricular septum. The Q wave is always a negative deflection, and may be present in
   some leads of the 12 lead ECG. In all leads accept leads III and AVR a Q wave is considered abnormal or pathological if it is greater than
   0.04 second in duration and more than one third the height of the following R wave.
   Pathological Q waves are highly suggestive of myocardial damage. Dead tissue is electrically inactive, therefore an electrode placed over
   such an area will see through it (like a window) to detect the electrical force generated by the opposite wall, i.e. current flow moving away
   from it as the cells are depolarised from the endocardium to epicardium. It is important to be aware of the characteristics of and be able to
   distinguish between normal and pathological Q waves.
   Figure 4.5: (A) Lead III in a healthy patient. (B) The same lead in the same patient 2 weeks after undergoing an inferior myocardial
   infarction. Note the deep Q wave.
   (Adapted from Woods, S, Froelicher, E, Motzer, S & Bridges, E (eds) 2010, Cardiac nursing, Lippincott.)
   The ST segment of the ECG is the line following the QRS complex connecting the QRS to the T wave. This represents the repolarisation
   segment of the ECG and is highly indicative of myocardial injury ischemia and infarction which can delay the repolarisation process.
   Analysis of the ST segment necessitates identification of the J point, or the connection between the end of the QRS and the beginning of
   the ST segment. A normal ST segment is flat with a variation of a minimum of 0.5-1.0 mm from the isoelectric line.
   Figure 4.6: ST segment elevation associated with myocardial infarction
   (Adapted from Woods, S, Froelicher, E, Motzer, S & Bridges, E (eds) 2010, Cardiac nursing, Lippincott.)
   Figure 4.7: Different types of ST segment depression highly indicative of myocardial ischemia. This example shows (A) down-sloping (B)
   up-sloping (C) horizontal ST depression
   (Adapted from Woods, S, Froelicher, E, Motzer, S & Bridges, E (eds) 2010, Cardiac nursing, Lippincott.)
   Activity
8. Read your ECG text to become familiar with the characteristics of normal and abnormal Q waves and ST
   segments.
9. Distinguish between the characteristics of normal and pathological Q wave.
10. Describe the characteristics for ST elevation and depression in the both the precoridial and limb leads.
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    The indicative leads in altered state ECGs
    Figure 4.8: Localising myocardial ischemia, injury, or infarction using the 12-lead ECG. The different areas of the heart are pattern-coded.
    Standard 12-lead ECG format is illustrated at upper right with leads pattern-coded to correspond to the area of the heart that each lead
    faces.
    (Adapted from Woods, S, Froelicher, E, Motzer, S & Bridges, E (eds) 2010, Cardiac nursing, Lippincott.)
    Now you have examined the characteristics of all the leads in the ECG you can begin to determine their relationship to the relevant parts
    of the heart. This is particularly useful when identifying an area of the myocardium which is affected by an alteration in blood supply. The
    following diagram identifies the anatomical lead groupings of the heart.
    ECG activity 1
11. Check your answers when you reach the end of this section.
12. Take a look at the ECG below.
13. Can you identify the leads which show ST segment elevation?
14. Which surface of the heart are these leads viewing?
15. What coronary artery supplies the area of the ECG that is altered?
16. Use your ECG text to determine what coronary arteries supply each of the patterned areas in Figure 4.8.
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    ECG—ischemia patterns
    Myocardial ischemia results from an imbalance between oxygen supply and demand. The ECG changes associated with myocardial
    ischemia are associated with changes in the depolarisation process, and therefore are often reflected by an inverted T or depressed ST
    segment. However T wave inversion may also associated with other conditions such as bundle branch block, and it can be normal in some
    people. A diagnosis of myocardial ischaemia is often made based on the presence of chest pain and ECG changes. These changes may
    be transient, and may occur during chest pain or exercise testing.
    Figure 4.10: ECG patterns and myocardial ischaemia. Refer to the grey area for the identified change.
    (Adapted from Huszar 2002, Basic dysrhythmias: interpretation & management, 3rd edn.)
    Figure 4.11: ECG patterns associated with myocardial ischemia.
    (Adapted from Woods, S, Froelicher, E, Motzer, S & Bridges, E (eds) 2010, Cardiac nursing, Lippincott.)
    ECG activity 2
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    ECG—injury and infarction
    Myocardial injury is the next stage beyond ischaemia. Injured myocardial cells are still alive, but are vulnerable to progress to infarction
    (cell death). Diagnosis is made in by analysis of the ECG as well as specific and sensitive cardiac biochemical markers, such as troponin
    which detect myocardial damage.
    The underlying pathophysiology of myocardial infarction is associated with that of a ruptured plaque and subsequent thrombus formation
    which may partially or completely occlude a coronary vessel. The injury pattern that is recorded on the ECG as a result are classified as
    ST elevation myocardial infarction, (STEMI) non ST elevation myocardial infarction NSTEMI depending on the ECG pattern and
    measurement of cardiac enzymes.
    The term myocardial infarction (MI) is used loosely to encompass both myocardial injury and necrosis. Not all patients who have a MI
    actually develop tissue necrosis. Early reperfusion intervention can reverse the injury and prevent muscle death.
    The ST segment changes are caused by changes produced by injured myocardial cells. Hence the leads which are facing the injured area
    will reflect a raised or depressed ST segment. The degree of ST segment elevation or depression is variable, an increased or decreased
    of more than one millimetre above the isoelectric line in two or more leads is considered as abnormal. ST changes usually occurs within
    minutes of the onset of infarction.
    Certain characteristics of the Q, ST segment and T wave can provide some clues to the age of the infarction:
    Prominent Q wave (normal ST segment)—old MI
    Q wave + ST elevation (with or without T wave inversion)—acute MI
    Q wave + inverted T wave—indeterminate age
    Reciprocal changes
    This is reflected as ST depression in the reciprocal leads, or those opposite the zone of injury. This is a common phenomenon seen in
    ECG leads facing the opposite side of the area of injury, e.g. ST depression will be seen in the anterior lateral leads in the presence of an
    acute inferior MI.