3. Introduction
• Electrocardiogram (ECG) retains its central role – even though
there continues to be new technologies developed for the
diagnostic evaluation of patients with cardiovascular disease
• The most important test for interpretation of the cardiac
rhythm, conduction system abnormalities and for the
detection of myocardial ischemia
• Of great value in the evaluation of other types of cardiac
abnormalities
including
valvular
heart
disease,
cardiomyopathy, pericarditis, and hypertensive disease
• Finally, the ECG can be used to monitor drug treatment
(specifically antiarrhythmic therapy) and to detect metabolic
disturbances
4. A systematic approach to interpretation of the
ECG is important in order to avoid overlooking
important abnormalities
5. Introduction
• Graphical record of electric potentials generated by
myocytes during each cardiac cycle
• Detected on the body surface using electrodes
attached to extremities and chest wall
• Amplified by the electrocardiograph machine and
displayed on special graph paper
6. Electrodes and Leads
• Electrodes – Sites at which an electrical
potential is measured
• ECG leads – Record
the difference in
potentials between two electrodes
8. Leads
• Record differences in electrical potentials
• Six extremity (limb) leads and six chest (precordial)
leads
• Two types
– Bipolar – between two surface electrodes
• Limb Leads I, II and III are bipolar leads
– Unipolar – between a surface electrode and the central
terminal of Wilson
• All leads except the bipolar limb
(aVR, aVL, aVF, V1, V2, V3, V4, V5, and V6)
leads
are
unipolar
9. Leads Orientation – Limb Leads
• Lead I records potential difference
between left arm (the +ve pole) and right
arm (-ve pole)
• Lead II records between left leg (+ve) and
right arm (-ve)
• Lead III records between left leg (+ve) and
left arm (-ve)
• Lead aVR records right arm potentials
• Lead aVL records left arm potentials
• Lead aVF records left leg (foot) potentials
10. Leads Orientation – Precordial leads
• The six chest/precordial leads – represent the voltage
difference between the central terminal and electrodes
placed in the following positions
–
–
–
–
–
–
V1 — 4th intercostal space (ICS), just to the right of the sternum
V2 — 4th ICS, just to the left of the sternum
V3 — midway between V2 and V4
V4 — 5th ICS in the mid-clavicular line
V5 — anterior axillary line, same level as V4
V6 — mid-axillary line, same level as V4 and V5
11. Leads
• 12-lead ECG provides spatial information
about the heart's electrical activity in 3
approximately orthogonal directions:
– Right ⇔ Left
– Superior ⇔ Inferior
– Anterior ⇔ Posterior
12. I and AVL
V3 & v4
V1 & v2
V5 & v6
II, III and AVF
Where the positive electrode is
positioned, determines what part
of the heart is seen!
13. Electrophysiology
• If an electrode is placed so that wave of
depolarization spreads toward the recording
electrode, the ECG records a positive (upward)
deflection.
• If wave of depolarization spreads away from
recording electrode, a negative (downward)
deflection occurs.
21. Genesis of Normal ECG
• P wave – wave preceded by QRS complex
• Q wave – first downward deflection after P wave
– Signals start of ventricular depolarization
• R wave is positive deflection after Q wave
• S wave is negative deflection preceded by Q or R
waves
• T wave follows QRS
23. ECG Interpretation – Stepwise Approach
A systematic approach to interpreting an ECG is essential for correct
diagnosis
Step 1: Rate
Is the rate between 60 and 100? Rates less than 60 is bradycardia and greater
than 100 is tachycardia
Step 2: Rhythm
Are P waves present?
Is there a P wave before every QRS complex and a QRS complex after every P
wave?
Are the P waves and QRS complexes regular?
Is the PR interval constant?
Step 3: Axis
Is there left or right axis deviation?
24. ECG Interpretation – Stepwise Approach
Step 4: Intervals
What is the PR interval?
– Short PR interval is suggestive of Wolff-Parkinson-White (WPW)
syndrome
– Long PR intervals are usually seen in first degree AV block
What is the QRS interval?
– Long QRS intervals represent a bundle branch block, ventricular
preexcitation, ventricular pacing, or ventricular tachycardia
What is the QT interval?
– Short and long QT intervals may be present
25. ECG Interpretation – Stepwise Approach
Step 5: P wave
– What is the shape and axis of the P wave?
– The P wave morphology should be examined to determine
if the rhythm is sinus or from another atrial location
– Amplitude and duration should also be analyzed to
determine left and right atrial enlargement
26. ECG Interpretation – Stepwise Approach
Step 6: QRS complex
Is the QRS wide?
– If so, examination of the morphology can determine if
there is left or right bundle branch block
– Increased voltage may indicate left or right ventricular
hypertrophy
– Are Q waves present? – suggestive of infarction
27. ECG Interpretation – Stepwise Approach
Step 7: ST segment-T wave
– Is there ST elevation or depression?
– Are the T waves inverted?
– Abnormalities of the ST segment or T wave may represent myocardial
ischemia or infarction
28. ECG Interpretation – Stepwise Approach
Step 8: Overall interpretation
– Only after the prior steps have been completed should an overall
interpretation and possible diagnoses be determined
– This ensures assimilation of all information in the ECG and that no
detail will be overlooked
29. Step 1 - Heart Rate
Heart rate
– Both atrial and ventricular rates - if different
– Normal heart rate 60-100 beats per minute
>100 beats/min = tachycardia
<60 beats/min = Bradycardia
Methods of measurement
– 1500 divided by No. of small boxes in Inter R-R interval
– 300 divided by No. of big boxes in Inter R-R interval
– What if heart rate irregular?
31. Step 2 – Rhythm Analysis
Interpreting the rhythm of the ECG is sometimes difficult
Locate the P wave
– The most important and first step in rhythm interpretation is the
identification of P waves and an analysis of their morphology
– Are P waves visible? Each lead needs to be examined for P waves
– What is the rate of the P waves (i.e., the PP interval)? If the rate is less
than 60, then a bradycardia and if the atrial or P wave rate is over
100, then there is tachycardia
– What is the morphology and axis of the P waves? The normal sinus P
wave is generally upright in leads I, II, aVF, and V4-V6. It will be
negative in lead aVR
32. Step 2 – Rhythm Analysis
Establish the relationship between P waves and the QRS
complex
• To determine the relationship between the P waves and
the QRS complexes, addressing the following questions:
– Are the P waves associated with QRS complexes in a 1:1
fashion?
• If not, are there more or less P waves than QRS complexes
and what are the atrial and ventricular rates?
– Do the P waves precede each QRS complex?
– What is the PR interval, and is this interval fixed?
33. Step 2 – Rhythm Analysis
Analyze the QRS morphology
• If the QRS complexes are of normal duration (<0.12 sec) and
morphology, then the rhythm is supraventricular
• It is essential to analyze the QRS in all 12 leads to be sure that
it is normal
• If the QRS is wide (i.e., >0.12 sec), then the rhythm is either
supraventricular with aberrant conduction, pre-excitation, or
ventricular pacing, or it is of ventricular origin – take the
widest QRS in the 12 lead ECG
34. Step 2 – Rhythm Analysis
• State the basic rhythm e.g., "normal sinus rhythm", "atrial
fibrillation", etc.
• Identify
additional
rhythm
e.g., "PVC's", "PAC's", etc.
events
if
present
• Remember that arrhythmias may originate in the atria, AV
junction, and ventricles
38. Step 4 - Intervals
PR Interval
• PR interval includes the P wave as well as the PR segment
• Measured from the beginning of the P wave to the first part of the QRS
complex (which may be Q wave or R wave)
• Includes time for atrial depolarization (the P wave) and conduction
through the AV node and the His-Purkinje system (which constitute the PR
segment)
• The length of the PR interval changes with heart rate, but is normally 0.12
to 0.20 sec (three to five small boxes)
39. Step 4 - Intervals
QT interval
• Consists of the QRS complex, the ST segment, and T wave
• Primarily a measure of ventricular repolarization
• Depends upon the heart rate
– Shorter at faster heart rates and longer when the rate is slower
– Need to be corrected for heart rate (QTc)
40. Step 5 – P wave
• The P wave represents atrial depolarization
• The normal sinus P wave demonstrates right followed by left
atrial depolarization and is an initial low amplitude positive
deflection preceding the QRS complex
• The duration is generally <0.12 sec (three small boxes) and the
amplitude <0.25 mv (2.5 small boxes)
• Since right atrial depolarization precedes that of the left
atrium (as the sinus node is in the high right atrium), the P
wave is often notched in the limb leads and usually biphasic in
lead V1
– The initial positive deflection in V1 is due to right atrial
depolarization that is directed anteriorly, while the second
negative deflection represents left atrial depolarization that is
directed posteriorly.
41. Step 6 – QRS complex
QRS complex
• Represents the time for ventricular depolarization
• If the initial deflection is negative, it is termed a Q wave
• The first positive deflection of the QRS complex is called the R
wave – represents depolarization of the left ventricular
myocardium
• The negative deflection following the R wave is the S wave –
represents terminal depolarization of the high lateral wall
• If there is a second positive deflection it is known as an R’
42. • Lower case letters (q, r, or s) are used for relatively small amplitude
waves of less than 0.5 mV (less than 5 mm with standard
calibration)
• An entirely negative QRS complex is called a QS wave
• The entire QRS duration normally lasts for 0.06 to 0.10 seconds (1½
to 2½ small boxes) and is not influenced by heart rate
• The R wave should progress in size across the precordial leads V1V6. Normally there is a small R wave in lead V1 with a deep S wave.
The R wave amplitude should increase in size until V4-V6, due to
more left ventricular forces being seen, while the S wave becomes
less deep. This is termed R wave progression across the precordium
43. Step 7 – ST Segment
ST segment
•
Occurs after ventricular depolarization has ended and before repolarization
has begun
•
Time of electrocardiographic silence
•
Initial part of the ST segment is termed the J point
•
The ST segment is usually isoelectric and has a slight upward concavity
•
Depressed (below the isoelectric line) in Myocardial ischemia and infarction
•
Elevated (above the isoelectric line) acute myocardial infarction and
pericarditis
44. Step 7 –T wave
• T wave represents the period of ventricular repolarization
• The rate of repolarization is slower than depolarization, the T
wave is broad, has a slow upstroke, and rapidly returns to the
isoelectric line following its peak (i.e., slow upstroke, rapid
downstroke)
– Thus, the T wave is asymmetric and the amplitude is variable
– In addition, the T wave is usually smooth up and down
– If there is any irregularity on the T wave
(bump, notch, rippled, nipple, etc) a superimposed P wave
should be considered
45. ECG Interpretation
• Conclusion of the above analyses
• Interpret the ECG as "Normal" or "Abnormal“
• Occasionally the term "borderline" is used if unsure about the
significance of certain findings or for minor changes
• List all abnormalities
46. Summary
•
•
•
•
•
•
•
•
A systematic approach to interpretation of the ECG is
critically important
Rate — Is the rate between 60 and 100?
Rhythm — Is it normal sinus or other?
Axis — Is there axis deviation?
Intervals — Are all intervals normal?
P wave — What is its height, width, and axis?
QRS complex — Are there pathologic Q waves, bundle
branch block, or chamber hypertrophy?
ST-T waves — Is it isoelectric, elevated, or depressed
relative to the TP segment?
Overall interpretation — What is the diagnosis?
49. ECG Changes : Injury
• ST segment elevation of greater than 1mm in at least 2
contiguous leads
• Heightened or peaked T waves
• Directly related to portions of myocardium rendered
electrically inactive
Baseline
50. ECG Changes : Infarct
Infarct
ECG Changes:
• Significant Q-wave where none previously existed
– Why?
• Impulse traveling away from the positive lead
• Necrotic tissue is electrically dead
• No Q-wave in Subendocardial infarcts
– Why?
• Not full thickness dead tissue
• But will see a ST depression
• Often a precursor to full thickness MI
• Criteria
– Depth of Q wave should be 25% the height of the R wave
– Width of Q wave is 0.04 secs
– Diminished height of the R wave
51. Evolving MI and Hallmark of AMI
Q wave
ST Elevation
1 year
T wave
inversion
53. Anterior Myocardial Infarction
Myocardial Infarction
Anterior
– Anterior infarct with ST
elevation
– Left Anterior Descending
Artery (LAD)
– V1 and V2 may also indicate
septal involvement which
extends from front to the back
of the heart along the septum
59. Posterior Myocardial Infarction
– Posterior Infarct with ST
Depressions and/ tall R wave in
V1 and V2
– RCA and/or LCX Artery
– Rarely by itself usually in
combo
63. Non ST Elevation Myocardial Infarction
• They are not transmural
• Look for diffuse or
localized changes and non
Q wave abnormalities
– T-wave inversions
– ST segment depression
ECG GRID — The ECG is a plot of voltage on the vertical axis against time on the horizontal axis. The electrodes are connected to a galvanometer that records a potential difference. The needle (or pen) of the ECG is deflected a given distance depending upon the voltage measured.
The ECG waves are recorded on special graph paper that is divided into 1 mm 2 grid-like boxes. The ECG paper speed is ordinarily 25 mm/sec. As a result, each 1 mm (small) horizontal box corresponds to 0.04 second (40 ms), with heavier lines forming larger boxes that include five small boxes and hence represent 0.20 sec (200 ms) intervals. On occasion, the paper speed is increased to 50 mm/sec to better define waveforms. In this situation there are only six leads per sheet of paper. Each large box is therefore only 0.10 sec and each small box is only 0.02 sec. In addition, the heart rate appears to be one-half of what is recorded at 25 mm/sec paper speed, and all of the ECG intervals are twice as long as normal.Vertically, the ECG graph measures the height (amplitude) of a given wave or deflection, as 10 mm (10 small boxes) equals 1 mV with standard calibration. On occasion, particularly when the waveforms are small, double standard is used (20 mm equals 1 mv). When the wave forms are very large, half standard may be used (5 mm equals 1 mv). Paper speed and voltage are usually printed on the bottom of the ECG.
The normal electrocardiogram is composed of several different waveforms that represent electrical events during each cardiac cycle in various parts of the hear. ECG waves are labeled alphabetically starting with the P wave, followed by the QRS complex and the ST-T-U complex (ST segment, T wave, and U wave). The J point is the junction between the end of the QRS and the beginning of the ST segment.
