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Aims

  • To familiarize the student with the technique of recording the 12 lead ECG and with some of the basic information that can be obtained from it.
  • To examine the changes which occur in the ECG when a patient suffers a myocardial infarction. NOTE: It is not expected that students will become competent at ECG interpretation at this stage.

Background

ELECTRODES

  • One of the main sources of technical problems in recording the ECG is the poor electrical conductivity of the surface layers of the skin. Ideally (from the operator's point of view!) needles should be used to bypass this resistance. In practice, we try to minimize it by applying a highly conductive paste or gel between the skin and the metal electrode to which the cable is then attached. Electrodes can be placed anywhere on the limb but recording quality is usually better over the muscle than over the bone. Body hair can also prevent the disposable electrodes forming firm contact with the skin.

LEADS

  • Terminology can be confusing here so read this section carefully.
  • The term "lead" can be used to apply simply to the conductive cables carrying signals from electrode to amplifier, but in electrocardiography the word has another, very specific meaning. It refers to a pair of electrodes and the signal recorded by the amplifier which amounts to a record of the differences in instantaneous electrical potential between the 2 electrodes of that pair during the cardiac cycle.
  • There are of course, an infinite number of possible pairs of electrodes but in clinical practice we normally use 12 standardized pairs derived from three limb electrodes (R and L arms and L leg) and six chest (or precordial, or "V") electrodes. The electrode normally attached to the R leg is not used to record the ECG but as a ground lead to reduce "noise". Note also that limbs act simply as linear conductors, so that an electrode placed anywhere from R shoulder to R wrist for example will show the same shape trace. We will use wrists and lower legs for convenience.
  • What the "leads" actually record is current flow as a result of these instantaneous potential differences (and by Ohm's law, current is directly proportional to the potential difference).
  • Direction of Current Flow: Currents, of course, are vectors i.e. they have direction as well as magnitude and ECG machines are set up normally so that conventional current flowing towards an electrode produces an upward deflection on the chart recorder, current flowing away produces a downward deflection and where no net current flows, the record in that lead will show a flat line trace.
  • Unipolar vs Bipolar Leads: While all leads in fact have 2 electrodes, it has become customary to divide lead pairs into those where both electrodes are "active" i.e. they respond to the underlying electrical activity of the heart, and those where one electrode is "indifferent" (does not change its potential during the cardiac cycle) and merely acts as a baseline against which the other electrode of the pair is measured. The first type are called "Bipolar Leads" (standard leads I, II and III; Fig 1) and the second type "Unipolar".
  • The indifferent electrode is usually obtained by combining the outputs of the electrodes on the R and L arms and the L leg into a single electrode (Fig 2). It can fairly easily be proved that such an electrode will not vary significantly in potential during the cardiac cycle (Kirchoff's Law). This can then be paired with an "active" or "exploring" electrode.

ECGPrac1.png

  • Fig. 1. The standard bipolar leads. When the electrical axis is directed to the left or inferiorly (ie towards the positive end of each lead), an upward deflection occurs. (Taken from Rhoades RA & Tanner GA (1995). Medical Physiology. Boston: Little, Brown and Company. Fig 13-7.)

ECGPrac2.png

  • Fig 2. The indifferent electrode. In the typical hookup for the “indifferent” electrode, the three limbs are combined to give the reference voltage (zero). In this case the active ”exploring” electrode is on the chest, making a precordial or chest lead (V). (Taken from Rhoades RA & Tanner GA (1995). Medical Physiology. Boston: Little, Brown and Company. Fig 13-15.)
  • The 3 commonly used unipolar limb leads are illustrated in Fig 3. Note that these so-called "unipolar" leads aVR, aVL and aVF do not actually use the indifferent electrode just described. When originally invented they did use this electrode and were labelled leads VR, VL. and VF. It was found empirically, (and can be justified theoretically) that unplugging the limb under exploration by the active electrode from the indifferent electrode does not change the shape of the recording but does amplify its size by 50%; hence the prefix "a" for "augmented". The remaining 6 leads of the standard ECG are the precordial, or chest or V leads, V1 - V6 (Fig 4).

ECGPrac3.png

  • Fig 3. The augmented unipolar limb leads. Note that the indifferent electrode consists of the two limbs which are not used by the exploring electrode. (Taken from Boron WF & Boulpaep EL. (2005). Medical Physiology (updated ed.) Philadephia: Elsevier Saunders. Part of Fig 20-7.)

ECGPrac4.png

  • Fig 4. Positions of the active electrodes in the precordial leads (for exact positions see text below). Note the indifferent electrode consists of outputs from the three limbs as shown in *Fig 2. (Taken from Hampton JR (2003). The ECG Made Easy (6th ed.). Edinburgh: Churchill Livingstone. Fig 1.8 and 1.9.)
  • The exact positions for the active electrode for the 6 precordial leads are:
    • V1 4th intercostal space (ICS) just to R of Sternum.
    • V2 4th ICS, just to L of Sternum.
    • V3 Halfway between V2 and V4.
    • V4 5th LICS in midclavicular line.
    • V5 Same horizontal line as V4, in anterior axillary line.
    • V6 As V4, in mid-axillary line.
  • Note:
    • i) The sternal angle is level with the 2nd rib.
    • ii) Each ICS lies below the corresponding rib.
    • iii) V5 and V6 are not in the 5th ICS but are at same horizontal level as V4.
  • Summary table for ECG leads

ECGPrac11.png


  • If the 6 leads oriented in the frontal plane are illustrated as vectors, and translated so they all pass through a common origin in the centre of the chest (Fig 5) it can be seen that they form a hexaxial reference grid and can be used to locate the "mean electrical axis" of the underlying ventricles (see later).
  • Similarly the 6 precordial leads can be used to locate a horizontal plane cardiac vector (we will not consider this further).

ECGPrac5.png


  • Fig 5. Constitution of the hexaxial reference system. (A). Einthoven’s triangle, the sides of which represent the three standard limb leads. (B). The triaxial reference system composed of the three sides of Einthoven’s triangle rearranged so that they bisect one another. (C). Lines of derivation of the three unipolar (aV) limb leads. (D). The hexaxial reference system composed of the lines of derivation of the six limb leads (B + C) arranged so that they bisect each other.

COMPONENTS OF THE ECG

  • The components of the ECG are illustrated below (Fig. 6).
  • Fig 6. Components of the ECG. Taken from Barrett KE, Barman SM, Boitano S & Brooks HL (2010). Ganong’s Review of Medical Physiology (23rd ed.). New York: McGraw Hill Medical. Fig 30-5.

ECGPrac6.png


Legend to Fig.6

  • P wave - Due to atrial depolarization. Normally < 0.12 sec long and < 0.25 mV in amplitude (2.5 mm at normal gain).
  • QRS complex - Due to spread of activation through ventricles. The larger L ventricle contributes most. The changing directions of current spread produce the complexity of this waveform. Normal duration < 0.12 sec.
  • T wave - Due to ventricular repolarization. Usually upright in leads in which the QRS is predominantly upright.
  • U wave - Origin uncertain (can be due repolarization of Purkinje fibres or papillary muscle). Small. Follows T wave.
  • PR interval - Start of P to start of QRS. Represents atrial conduction time plus AV-nodal conduction time. Prolonged in AV-node disease. Normally 0.12-0.20 sec.
  • QT interval - Start of QRS to end of T wave. Represents approximate action potential duration in ventricles. Normal values vary with heart rate and sex, (see table in appendix).
  • ST segment- Observe the ST segment. This is usually close to the baseline ("isoelectric line"). In ischaemic heart disease it frequently deviates up or down and this is of great clinical use. (In some healthy young adults, up to 2.5 mm of ST elevation in the V leads may be normally present).

THE QRS COMPLEX

  • The various components of the QRS complex have different names

ECGPrac7.png

  • Fig. 7. Naming parts of the QRS complex.
    • (Taken from Hampton JR (2003). The ECG Made Easy (6th ed.). Edinburgh: Churchill Livingstone. Fig 1.3.)
  • Note in Fig 7
    • 1. If the first deflection is downward (negative), it is a Q wave.
    • 2. The first upright deflection is an R wave, whether or not it is preceded by a Q.
    • 3. A negative deflection following an R wave is an S wave.

Subsequent excursions above the line are labelled successively R’, R’’ etc; similarly later negative excursions are labelled S’, S’’ and so on.

ELECTRICAL AXIS (Frontal Plane)

  • The mean electrical axis or cardiac vector in the frontal plane is frequently of clinical value and is routinely estimated when "reading" an ECG. In practice, an accuracy of +/- 10° is adequate.
  • The axis is given as the angle (θ) where the mean vector of the cardiac depolarization intersects with a line parallel to the floor (patient upright). By convention, 0° is on the left of this line, angles below the line are +ve, angles above the line are -ve.

ECGPrac8.png


  • Fig. 8 illustrates the concept of a mean vector derived from the sum over the whole of systole of all the instantaneous vectors produced as the action potential activates the heart in the time sequence illustrated. The vector loop analysis shown in panel C is rarely used clinically but the mean vector shown in panel B is easy to calculate from the QRS complexes of any 2 frontal plane leads by a process akin to triangulation. Many people choose to use 2 leads at right angles to each other (e.g. I & aVF) to simplify the geometry. We will use this approach (see later)

ECGPrac9.png

  • Fig 8. The vector loop. (A) Diagram of successive instantaneous axes (or vectors) as the ventricular muscle mass is depolarized. (B) Regrouping of the instantaneous vectors in A as though they all originated from a single “centre”; the thick arrow represents the mean vector, being the resultant of the 10 instantaneous vectors. If the heads of the vector arrows are joined by a continuous line the vector loop is formed (C).
  • The "normal" range is approximately 0° to +90°, but moderate deviations beyond this usually have little significance. Axes between -30° and -90° are referred to as "left axis deviation" and those between +120° and +180° as "right axis deviation". Axes between -90° and -180° are rare.
  • NOTE: there are many acceptable ways of determining the axis.

Pathology and the cardiac axis

  • <-30 degrees = left axis deviation. This can be due to anterior hemiblock. To understand anterior hemiblock, note that there are Rt and Lt bundles of His in the interventricular septum. The Lt bundle of His has an anterior and posterior branch. Anterior hemiblock means all Lt ventricular depolarisation occurs through the posterior bundle, and this can cause left axis deviation
  • >+120 degrees = right axis deviation (more right ventricular muscle moves the net axis to the right)
  • Note that LV hypertrophy tends to produce a normal cardiac axis
  • The cardiac axis can vary by 10 degrees during respiration (sinus arrhythmia)
  • While normal is 0 to +90, we don't designate 'disease' until <-30 and >+120,

TASKS FOR THE CLASS

  • A. Record a 12 lead ECG from one member of each group.
  • Your bench will have the following equipment:
    • - Desktop computer which has the CT200C Real Time Software already installed
    • - CT200C Electrocardiograph
    • - USB cable – this connects the CT200C to the computer via USB ports
    • - Patient cable – connects the CT200C to the patient. One end of this cable connects into the patient cable port on the CT200C via screw locks. The other end consists of 10 cables which are labelled RA, LA, RL, LL, V1, V2, V3, V4, V5, V6. Each of these 10 cables ends in a banana plug.
    • - 10 alligator clips. These fit on top of the banana plugs on the patient cable (and may already be in position).
    • - disposable stick-on electrodes
    • - alcohol swaps (for skin preparation)
    • - adhesive tape (to help keep clips and cables in position p12articularly for the chest leads; only use if required)
  • 1. Attach electrodes firmly to each site. Prepare the skin with alcohol swabs and allow to dry. Connect limb electrodes a short distance above the ankle or wrist, over muscle. Chest electrodes should be oriented with tags pointing towards the feet. Only use disposable electrodes once - reuse will result in poor quality recordings.
  • 2. Connect cables. Attach each cable to the corresponding electrode, making sure that each electrode retains good contact with the skin. Ensure that the metal part of each alligator clip is in contact with the conductive side of the electrode tag (i.e. the undersurface when the electrode is stuck onto the skin). Use tape if required to keep the electrode and cables in place.
  • 3. Open the Real Time Software. Double click on the RT200 icon on the computer desk top. You may get an error message “Failed to update system registry. Please try using REGEDIT,” If so, click OK. A window should open with “Untitled RT200” in the title bar. This should contain the ECG grid paper and the names of the leads. Observe the tool bar. You should be able to see a Record button, the rhythm lead should be set at II, Filter should be on 100 Hz, Speed 25 mm/sec, and Sensitivity 10 mm/MV.
  • 4. 12 lead recording. With your subject relaxed and lying still, click on the Record button in the tool bar. You should see traces for all 12 leads as well as the rhythm strip along the bottom of the page. Once you have a stable reading and the rhythm strip has gone at least the length of the screen push Record again to stop the recording.
  • 5. Save the recording. Either click the “File” option on the menu bar, followed by Save or Save As, or use the save button on the tool bar menu. To save the file you will need to include patient identification details. To maintain subject anonymity use your Group number for the four compulsory fields (ID#, Surname, Given name, and Doctor) and click on whether your subject is male or female. Please do not use the subject’s real name. You may change the DOB, weight, height and BP if you wish. Save the file in the ECG directory.

12leadECG.jpg

ECGPrac13.png

ECGPrac14.png


  • B. Observe and note the effects of the following technical problems:
    • For technical problems 1- 4, you will record from just a single lead rather than all 12 leads.
    • Click on the 12 lead/Monitor icon in the tool bar to toggle between 12 lead and single lead viewing. (This icon looks like a “stick man” and is the 5th icon from the left of the tool bar). **When you are on single lead, the Record button will change to Monitor. You can select any lead for recording – Lead II is a good option for most subjects. When you are ready, click Monitor to start recording.
    • 1. Comparison of the 100 Hz filter and the 40 Hz filter.
    • After recording for several seconds with the 100 Hz filter in place, click Monitor to stop the recording. Change to the 40 Hz filter and repeat your recordings.
    • Effect:
      • More noise is noted on the 100Hz filter than the 40Hz filter. The 100Hz filter is designed to remove all signals at a frequency >100Hz, while the 40Hz filter removes all signal at a frequency >40Hz. Most electrical interference and noise due to muscle movement is between 50 and 60Hz
    • 2. Effects of muscle movement with the 100 Hz filter.
    • After recording with the 100 Hz filter in place for several seconds, and without stopping the recording, ask the subject to make rhythmic movements with their right hand. Observe the effect. Click Monitor to stop recording.
    • Effect:
      • Irregular shape of trace, high amount of noise. Hence keeping still and using the appropriate filter is important for the ECG recording.
    • 3. Effects of muscle movement with the 40 Hz filter.
    • After recording with the 40 Hz filter in place for several seconds, and without stopping the recording, ask the subject it make rhythmic movements with their right hand. Observe the effect. Click Monitor to stop recording.
    • Effect:
      • Not much difference to normal
    • Which filter do you think would be more appropriate to use for an Exercise ECG?
      • 40Hz. This will filter muscular noise (so we'll see only heart activity). Lower cutoff frequency for filter hence less sensitive to noise. Heart activity is relatively slow, so information is not lost.
    • 4. Removal of the ground electrode.
    • Restore the 100 Hz filter. After recording for several seconds, and without stopping the recording, remove the right leg (ground) alligator clip from the electrode. Observe the effect.
    • Click it back in place. You may wish to also carry out this manoeuvre with concurrent muscle movement. Click monitor to stop the recording.
    • Effect:
      • Lots more noise. V1 depolarisation is greatly enlarged and inverted. Very irregular trace. Even worse with ground removed AND muscle movement. Hence there is more interference with it off and it is important to plug in the ground electrode for ECG recording
  • 5. Incorrect cable connections.
    • Reverse the RA and LA cables. Use the 12 Lead/Monitor icon to toggle back to 12 lead viewing. Repeat your 12 lead recording as in Part A above. Make sure you save the recording under a different name eg “reverse connections”.
    • Effect:
      • Lead 1 is inverted (since 1 goes straight across the heart). Leads 2 and 3 are swapped with each other. When machine reads lead 2, instead of reading RA and LF electrodes, it actually reads LA and LF (which is normally lead 3).
    • Are the precordial leads affected? Why/why not?
      • aVR --> aVL. aVL --> aVR. aVF unchanged. Precordial leads V1-V6 not affected as it compares chest to indifferent electrode. Leads 1 and 2 sum to make indifferent electrode, so precordial leads not affected by Kirchoff's law
  • C. Measurements on the ECG done by your group.

You can either do the following measurements after you have printed out your recordings, or you can make them on the screen.

  • To make measurements on the screen, zoom in on the lead of interest by double clicking over the trace. (This will work for all 12 leads except the rhythm strip in the 12 lead recording. You cannot zoom in on the single lead recording “Monitor” mode”). Then use the calliper icons to make the measurements. Note: you can click on “sensitivity” and “speed” to change the scales if you wish, but this is generally not required.

ECGPrac15.png

  • To make these measurements on the paper copy, note that at the usual paper speed (25 mm/sec), 1mm (one small square) = 0.04 sec, 5mm (one large square) = 0.20 sec.

To determine the heart rate: Try these 2 methods:

  • (1) Determine the R-R interval in seconds. Note that the heart rate is obtained by dividing 60 by this interval.
  • (2) Divide 300 by the number of large squares between consecutive beats.

ECGPrac16.png

  • It is better to measure the heart rate over a period several beats instead of using just two beats. Why?
    • This is because measuring over several beats will negate the variation in heart rate due to sinus arrhythmia (variation with breathing).
  • To determine the intervals:
  • Refer to Fig. 6 to ensure that you measure the intervals correctly.
  • Where there is variation from lead to lead always record the longest interval.

ECGPrac17.png

  • To determine the axis:
  • 1. Look at the QRS complexes in your own leads I and aVF

ECGPrac18.png

  • 2. Calculate the algebraic sum of the R & S deflections in mm, (i.e. R wave height minus S wave depth) for both leads.

ECGPrac19.png

  • 3. Draw lines of the calculated lengths in the appropriate direction for each lead.
  • 4. Join the origin of the line for lead I to the end of the line for lead aVF.

ECGPrac20.png

  • 5. Calculate the angle θ.

ECGPrac21.png

  • After you have made all your measurements, you might like to compare your results with those generated by the software itself. To do this, open your 12 lead recording and click on the “Cardionics Interpretation” icon. You will see the Cardionics Interpretations License *Agreement. Click “I agree”.
  • How do your results compare? (Note: you were not asked to determine the P axis).

*Do you agree with the Analysis?

  • D. Examine the traces showing myocardial infarction.
    • What deviations from normal are seen?
      • See separate section on "myocardial infarction" below
  • E. Further exercises.
    • 1. Lead II usually has the largest QRS voltage of leads I, II, and III. Why is that?
      • Lead II is at angle of 60 degrees, which is in approximately the same direction as the cardiac axis in most people (in the direction of ventricular depolarisation)
    • 2. Which leads show U waves best? Why might that be?
      • U waves are produced by Purkinje fibre repolarisation or papillary muscle repolarisation. It is best seen by V2, V3 and V4 (anterior view of the heart). They are a very small effect.
    • 3. Why so many leads?
      • To see all different parts of the heart and detect and localise pathologies. Right ventricle viewed by V1, V2. Interventricular septum = V3, V4. Left ventricle = V5, V6.
    • 4. Which lead has the P, QRS + T waves all inverted?
      • aVR. Positive electrode for aVR is on the right arm. Net movement of everything in the heart is to the left.
    • 5. How does the shape of the QRS vary from lead V1 to lead V6?
      • Signal moves from predominantly downwards (V1) to predominantly upwards (V6). This is because of the locations of the different electrodes relative to the electrical activities in the heart - V1 is in the 4th intercostal space (most signals moving away from it), and V6 is in the axilla (most signals moving towards it). V2-V5 are various gradations between these two extremes. V6 depolarisation isn't necessarily the biggest, just it has more common positive deflection.

Myocardial infarction

Appendix

ECGPrac10.png