This document provides an overview of electrocardiography (ECG) and ECG interpretation. It discusses the cardiac conduction system, the components of the ECG waveform and their timing, placement of ECG leads, and common monitor problems. The conduction system generates and transmits electrical signals through the heart to coordinate contractions. Proper placement of ECG leads is important for interpreting the heart's electrical activity from different views. Continuous monitoring also requires selecting the best leads depending on the diagnostic purpose.

1. By: Ame Mehadi (BSc, MSc)
Basic
ECG Interpretation

3. Outline
1. Review the conduction system
2. ECG waveforms and intervals
3. ECG leads
4. Determining heart rate
5. Simplified approach to ECG reading
6. Some examples for practice

4. The heart’s valves
• Tricuspid — AV valve b/n RA and RV
• Mitral — AV valve b/n LA and LV
• Aortic — semilunar valve b/n LV and Aorta
• Pulmonic — semilunar valve b/n RV and PA

5. Blood flow
• Deoxygenated blood from the body returns to the RA
and then flows to the RV.
• The RV pumps blood into the lungs where it’s
oxygenated.
• Then the blood returns to the LA and flows to the LV.

6. Coronary Arteries and Veins
• RCA— supplies blood to RA, RV and part of the LV.
• LADA— supplies anterior wall of the LV, interventricular septum,
RBB, and left anterior fasciculus of the LBB.
• Circumflex Artery — supplies lateral walls of the LV, LA, and left
posterior fasciculus of the LBB.
• Cardiac Veins — collect blood from the capillaries of the
myocardium.
• Coronary Sinus — returns blood to the RA

7. Cardiac Cycle Dynamics
• Atrial Kick —
• atrial contraction, contributing about 30% of the cardiac output.
• Cardiac Output —
• the amount of blood the heart pumps in 1 minute, calculated by multiplying HR
X SV.
• Stroke Volume (SV) —
• the amount of blood ejected with each ventricular contraction;
• affected by preload, afterload, and contractility.

8. Cardiac Cycle Dynamics ….
• Afterload the amount of pressure the LV
must work against to pump blood into the
aorta.
 Preload — the passive stretching
exerted by blood on the ventricular
muscle at the end of diastole.

9. Innervation of the Heart
• Two branches of the ANS supply the heart:
• Sympathetic NS — increases HR, automaticity, AV conduction, and contractility
through release of NE and epinephrine.
• Parasympathetic NS — vagus nerve stimulation reduces HR and AV conduction
through release of acetylcholine.

10. Transmission of Electrical Impulses
• Generation and transmission of electrical impulses depend on
these cell characteristics:
• Automaticity — a cell’s ability to spontaneously initiate an impulse,
such as found in pacemaker cells
• Excitability — how well a cell responds to an electrical stimulus that
results from ion shifts across the cell membrane.
• Conductivity — the ability of a cell to transmit an electrical impulse to
another cardiac cell.
• Contractility — how well the cell contracts after receiving a stimulus
(after depolarization).

12. Action potential curve

13. Depolarization - Repolarization Cycle
• Cardiac cells undergo the following cycles of depolarization &
repolarization as impulses are transmitted:
• Phase 0: Rapid depolarization — the cell receives an impulse from a nearby
cell & is depolarized.
• Phase 1: Early repolarization — early rapid repolarization occurs.
• Phase 2: Plateau phase — a period of slow repolarization occurs.
• Phase 3: Rapid repolarization — the cell returns to its original state.
• Phase 4: Resting phase — the cell rests & readies itself for another stimulus.

15. • Cardiac Conduction System
• The electrical impulse begins in the SA node and travels through the internodal
tracts and Bachmann’s bundle to the AV node.
• From the AV node, the impulse travels down the bundle of His, along the bundle
branches, and through the Purkinje fibers.
• Intrinsic Firing Rates
• SA node — 60 to 100/minute
• AV junction — 40 to 60/minute
• Purkinje fibers — 20 to 40/minute.

16. The Normal Conduction System

17. • Abnormal impulses
• Retrograde Conduction — impulses that are transmitted backward toward
the atria
• Reentry — when an impulse follows a circular, rather than the normal,
conduction path.
• The heart can’t pump unless an electrical stimulus occurs first.
• Cardiac cells at rest are considered polarized
• Cell membranes separate different concentrations of ions and create a more -ve
charge inside the cell. This is called the resting potential.
• After a stimulus occurs, ions cross the cell membrane and cause an action
potential – cell depolarization.
• When a cell is fully depolarized, it attempts to return to its resting state in a
process called repolarization.
• After depolarization and repolarization occur impulse travels to next pathway
called the conduction system.
• So where is this impulses comes from? From SA node.

18. Pace makers of the heart

19. •The SA node
• is located in the upper right
corner of the right atrium
• is the main pacemaker of the
heart,
• generate impulses 60-100
times per minute in adults.

20. Inter-atrial and Inter-nodal Pathways
• Spread electrical impulses from
the SA node across the atria to the
AV node resulting in atrial
depolarization.

21. The AV Node • The impulse travels from the AV node to the AV
junction just below it. Here it slows down
allowing the atria to contract or vent to relax.

22. The AV node
• located in the inferior RA near the ostium of the coronary sinus.
• responsible for delaying the impulses by 0.04 second
• keep the ventricles from contracting too quickly or
• permits ventricular filling during diastole and
• allows atria to contract too quickly.
• Its junctional tissue can fire at a rate of 40 to 60 times per minute.

23. Bundle of His & Bundle Branches
• Once the atria have contracted the
impulse travels through the bundle
of His, then follows the right and left
bundle branches.

24. Purkinje Fibers
• extend from the bundle branches into the
endocardium, deep into the myocardial
tissue causing Ventricular Depolarization
• Also serve as a pacemaker
• discharge impulses at a rate of 20 - 40 times
per minute.

25. What is an EKG?
• The electrocardiogram (EKG) is a surface representation of the
electrical events of the cardiac cycle.
• Each event has a distinctive waveform, the study of which can lead
to greater insight into a patient’s cardiac pathophysiology.

26. ECG Lead Placement

27. Five-leadwire electrode placement

28. 12-lead electrode placement

29. Recording of the ECG:
• Leads used:
• Limb leads are I, II, II.
• Each of the leads are bipolar; i.e., it requires
two sensors on the skin to make a lead.
• If one connects a line b/n two sensors, one has
a vector.
• There will be a positive end at one electrode
and negative at the other.

30. Recording of the ECG:
• Electrodes placed on the skin measure the
direction of electrical current discharged by the
heart.
• That current is then transformed into
waveforms.
• A lead provides a view of the heart’s electrical
activity b/n one +ve pole & one -ve pole.
• The direction of the current affects the
direction in which the waveform points on an
ECG.
• When no electrical activity or too weak to
measure, straight line called an isoelectric
waveform formed.

31. A rhythm Strip
• can be used to monitor cardiac status,
• provides information about the heart’s electrical activity from one or more
leads simultaneously.
• Chest electrodes pick up the heart’s electrical activity for display on the
monitor.
• The monitor also displays HR and other measurements and allows for
printing strips of cardiac rhythms.
• Commonly monitored leads include the bipolar leads I, II, III, V1, V6, MCL1,
and MCL6.
• These leads are similar to the unipolar leads V1 and V6 of the 12-lead ECG.
• MCL1 and MCL6, however, are bipolar leads.

32. A 12-lead ECG:
• records info from 12 different views of the heart and
• provides a complete picture of electrical activity.
• these 12 views are obtained by placing electrodes on the patient’s
limbs and chest.
• The limb leads and the chest, or precordial, leads reflect
information from the different planes of the heart.

33. 12-lead ECG
• The six limb leads provide information about the heart’s frontal
(vertical) plane.
• Bipolar leads (I, II, & III) require a -ve & +ve electrode for monitoring.
• Unipolar leads (aVR, aVL, & aVF) record information from one lead & require
only one electrode.
• The six precordial leads (leads V1 through V6) provide information
about the heart’s horizontal plane.

34. A 12-lead ECG…
Limb Leads
• The six limb leads are:
• Leads I, II, III,
• Augmented Vector Leads
• aVR - augmented vector right,
• aVL - augmented vector left, and
• aVF - augmented vector foot
• Those six limb leads provide information about the heart’s frontal
(vertical) plane.

35. Types of ECG Recordings
• Bipolar leads record voltage b/n electrodes
placed on wrists & legs
• Right leg is ground
• Lead I records b/n right arm & left arm
• Lead II: records b/n right arm & left leg
• Lead III: records b/n left arm & left leg

36. Limb Leads
Leads I, II, & III
• are standard limb leads
• are bipolar (require a negative and positive electrode for
monitoring).
• typically produce +ve deflection on ECG tracings.
• axes those leads (I, II, & III) form a triangle around the heart and
provide a frontal plane view of the heart.

37. Limb Leads
Leads I, II, & III
• Lead I
• produces a positive deflection on ECG tracings and
• is helpful to monitor atrial arrhythmias & hemiblocks.
• Lead II
• tends to produce a -ve, high voltage deflection, resulting in tall P, R, and T waves.
• commonly used for routine monitoring and
• is useful for detecting sinus node and atrial arrhythmias.
• Lead III
• produces a positive deflection.
• its +ve electrode is placed on the LL; the -ve electrode, on the LA.
• Along with lead II, this lead is useful for detecting changes associated with an inferior
wall MI.
• Lead III helps to detect changes associated with inferior wall MI.

38. Augmented leads
• Leads aVR, aVL, and aVF are called augmented leads because the small
waveforms that normally would appear from these unipolar leads are
enhanced by the ECG.
• The “a” stands for “augmented,” and “R, L, and F” stand for the +ve
electrode position of the lead.
• unipolar (record information from one lead).
• In lead aVR, the +ve electrode is placed on the right arm and produces a -ve
deflection because the heart’s electrical activity moves away from the lead.
• In lead aVL, the +ve electrode is on the left arm and produces a -ve
deflection on the ECG.
• In lead aVF, the -ve electrode is on the left leg (despite the name aVF) and
produces a -ve deflection.
• These three limb leads also provide a view of the heart’s frontal plane.

40. A 12-lead ECG…
Precordial leads
• The six precordial or V leads
• are V1, V2, V3, V4, V5, and V6
• provide information about the heart’s
horizontal plane.
• are unipolar, requiring only a single
electrode.
• their opposing pole is the center of
the heart as calculated by the ECG.

41. A 12-lead ECG…
Precordial leads
View of the heart’s horizontal plane
• Lead V1—placed on the right side of the sternum at the 4th ICS.
• Lead V2—placed at the left of the sternum at the 4th ICS.
• Lead V3—goes b/n V2 and V4.
• Lead V4—placed at the fifth ICS at the midclavicular line
Leads V1, V2, V3 and V4 are biphasic, with both +ve and –ve deflections.
• Lead V5—placed at the 5th ICS at the anterior axillary line.
—produces a +ve deflection on the ECG.
• Lead V6—placed level with V4 at the midaxillary line.
—produces +ve deflection on the ECG.

42. Precordial leads
• Lead V1
– Biphasic
– Distinguishes b/n right & left ventricular ectopic beats
– Monitors ventricular arrhythmias, ST-segment changes, and BBB
• Leads V2 and V3
– Biphasic
– Monitors ST-segment elevation
• Lead V4
– Produces a biphasic waveform
– Monitors ST-segment and T-wave changes
• Lead V5
– Produces a +ve deflection on the ECG
– Monitors ST-segment or T-wave changes (when used with lead V4)
• Lead V6
– Produces a +ve deflection on the ECG
– Detects BBB

43. Modified Chest Lead
• MCL1 is similar to lead V1 on the 12-lead ECG
• The Negative electrode on the left upper chest,
• The +ve on the right side of the sternum at the 4th ICS,
• And the ground electrode usually on the right upper chest below the clavicle.
• Waveform has a –ve deflection. As a result, ectopic or abnormal beats deflect
in –ve direction.
• can be used to monitor PVC and to distinguish different types of tachycardia.
• MCL6 may be used as an alternative to MCL1.
• Like MCL1 it monitors ventricular conduction changes.
• The +ve lead in MCL6 is placed in the same location as its equivalent, lead V6.
• The +ve electrode at the 5th ICS at the MAL,
• The -ve electrode below the left shoulder,
• And the ground below the right shoulder.

44. • A three-electrode system has one
+ve electrode, one –ve electrode,
and a ground which is placed on the
chest to prevent electrical
interference from appearing on the
ECG recording.
• The popular five-electrode system
uses an exploratory chest lead to
monitor any six modified chest leads
as well as the standard limb leads.

45. Types of ECGs
• 12–lead ECG
– records electrical activity from 12 views of the heart.
– Single–lead or dual–lead monitoring – provides continuous
cardiac monitoring.

46. Recommended Leads for Continuous ECG Monitoring
Purpose Best Leads
Arrhythmia detection V1 (V6 next best)
RCA ischemia, inferior MI III, aVF
LAD ischemia, anterior MI V3
Circumflex ischemia, lateral MI I, aVL (III, aVF best limb leads)
RV infarction V4R
Axis shifts I and aVF together

48. Monitor Problems
• Artifact, also called waveform interference,
• may be seen with excessive movement (somatic tremor).
• The baseline of the ECG appears wavy or bumpy
• Dry electrodes may also cause this problem due to poor contact.

49. Monitor Problems
• Electrical interference
• is caused by electrical power leakage.
• It may also occur due to interference from other room equipment or
improperly grounded equipment.
• appears on the ECG as a baseline that’s thick and unreadable.
• A wandering baseline undulates,
• meaning that all waveforms are present but the baseline isn’t stationary.
• Mov’t of the chest wall during respiration,
• poor electrode placement, or
• poor electrode contact usually causes this problem.
• Faulty equipment, such as broken lead wires and cables, can also cause
monitoring problems and places the patient at risk for shock.

52. Monitor Problems
• Before you attach electrodes to your patient:-
• make sure he knows you’re monitoring his heart rate and rhythm, not controlling
them.
• Tell him not to become upset if he hears an alarm
• Explain the electrode placement procedure to the patient
• Clip dense hair with clippers or scissors.
• If the patient has oily skin, clean each site with an alcohol pad and let it air-dry.
• After the electrodes are in proper position:-
• Observe the screen.
• Compare and verify the patient’s apical rate with the rate displayed on the
monitor.
• Set alarm limit
• Heart rate alarms are generally set 10 to 20 beats per minute higher and
lower than the patient’s heart rate.

53. Warning !
• Excessively worn equipment can cause improper grounding, putting
the patient at risk for accidental shock.
• Be aware that some types of artifact resemble arrhythmias.
• So remember to treat the patient, not the monitor.

54. ECG Waveform
J point

55. On the ECG Paper
• The horizontal axis of the ECG strip represents time.
• Each small block equals 0.04 seconds, and five small blocks form a
large block, which equals 0.2 seconds
• 6-seconds strip consisting of 30 large blocks is usually used
• Vertical axis measures amplitude in millimeters (mm) or electrical
voltage in millivolts (mV).
• Each small block represents 1mm or 0.1mV; each large block, 5 mm
or 0.5mV.

58. ECG Graph Paper
0.2 sec
0.04 sec
Time
Paper speed 25mm/sec

59. ECG strip
• 1 small horizontal block = 0.04 second
• 5 small horizontal blocks = 1 large block ? 0.2 second
• 5 large horizontal blocks = 1 second
• Normal strip = 30 large horizontal blocks = 6 seconds
• 1 small vertical block = 0.1 mV
• 1 large vertical block = 0.5 mV
• Amplitude (mV) = number of small blocks from baseline to highest
or lowest point

61. Interpreting a Rhythm Strip
• P-wave:-
• represents atrial depolarization — conduction of an electrical impulse through the
atria.
• location —precedes the QRS complex
• amplitude — 2 to 3 mm high
• duration — 0.06 to 0.12 second
• configuration —usually rounded and upright
• deflection —
• positive or upright in leads I, II, aVF, and V2 to V6;
• usually positive but variable in leads III and aVL;
• negative or inverted in lead aVR;
• biphasic or variable in lead V1.

62. • PR interval:-
• tracks the atrial impulse from the atria through the AV node, bundle of
His, and right and left bundle branches.
• location—from the beginning of the P wave to the beginning of the
QRS complex
• duration— 0.12 to 0.20 second.
• When evaluating a PR interval, look especially at its duration
• ST segment:-
• end of ventricular conduction or depolarization and the beginning of
ventricular repolarization.
• the point that marks the end of the QRS complex and the beginning of
the ST segment is known as the J point.
• location — extends from the S wave to the beginning of the T wave.
• deflection — usually isoelectric (neither +ve nor -ve)

63. • QRS complex:-
• represents depolarization of the ventricles
• Beginning of the Q wave to the end of the S wave or from the beginning of the R
wave if the Q wave is absent.
• Location — follows the PR interval
• Amplitude—5 to 30mm high but differs for each lead used
• Duration —0.06 to 0.10seconds, or half of the PR interval.
• Configuration — consists of the Q wave, the R wave and the S wave. You may not
always see all three waves.
• Deflection — positive in leads I, II, III, aVL, aVF, and V4 to V6 and negative in leads
aVR and V1 to V3.
• T wave:-
• represents ventricular recovery or repolarization.
• location — follows the S wave
• amplitude — 0.5 mm in leads I, II, and III and up to 10 mm in the precordial leads.
• configuration — typically round and smooth
• deflection — usually upright in leads I, II, and V3 to V6; inverted in lead aVR;
variable in all other leads.

64. • QT interval:-
• measures ventricular depolarization and repolarization.
• location — extends from the beginning of the QRS complex to the end of the T
wave
• duration — varies according to age, sex, & heart rate;
• usually lasts from 0.36 to 0.44 second; shouldn’t be greater than half the
distance between consecutive R waves when the rhythm is regular.
• The U wave:-
• represents the recovery period of the Purkinje or ventricular conduction fibers.
• location —follows the T wave
• configuration — typically upright and rounded
• deflection — upright.
• The U wave may not appear on an ECG.
• A prominent U wave may be due to hypercalcemia, hypokalemia, or digoxin toxicity.

66. 8-step method
• Step 1: Determine the rhythm
• use either the paper-and-pencil method or the caliper method.
• atrial rhythm, measure the P-P intervals—the intervals b/n consecutive P
waves.
• ventricular rhythm, measure the intervals b/n two consecutive R waves.
• if an R wave isn’t present, use the Q wave of consecutive QRS complexes.
• keep in mind that variations of up to 0.04 seconds are considered
normal.

67. Step 2: Determine the rate
• Always check a pulse to correlate it with the heart
rate on the ECG.
• 10-times method
• 1,500 method
• Sequence method- 300, 150, 100, 75, 60, and 50.

68. Count Down Method

69. Large Boxes Method
• performed by counting the number of large boxes b/n the peak of
two sequential:
• P waves for atrial rates, and
• R waves for ventricular rates.
• Divide the number of large boxes counted into 300.
• E.g., five large boxes b/n the peaks of two sequential R waves would give a
HR of 60 bpm
(300/5 = 60 bpm).

70. Small Boxes Method
• is performed by counting the number of small boxes b/n the peak of
two sequential
• P waves for atrial rates, and
• R waves for ventricular rates.
• Divide the number of small boxes counted into 1,500.
• E.g., 15 small boxes b/n the peaks of two sequential R waves would give HR of
100 bpm
(1,500/15 =100 bpm).

71. 10 Second Rule
• As most EKGs record 10 seconds of rhythm per page, one can simply
count the number of beats present on the EKG and multiply by 6 to get
the number of beats per 60 seconds.
• Count number of QRS and multiply by 6.
• This method works well for irregular rhythms.
What is the heart rate?
33 x 6 = 198 bpm

72. • Step 3: Evaluate the P wave
• Are P waves present? Do they all have normal configurations? Do they
all have a similar size and shape?
• Is there one P wave for every QRS complex?
• Step 4: Determine the duration of the PR interval
• count the small squares b/n the start of the P wave and the start of the
QRS complex.
• Is the duration a normal 0.12 to 0.20 seconds? Is the PR interval
constant?
• Step 5: Determine the duration of the QRS complex
• From the end of the PR interval to the end of the S wave
• Is the duration a normal 0.06 to 0.10 second? Are all QRS complexes the same
size and shape?
• Does a QRS complex appear after every P wave? We're up to step

73. • Step 6: Evaluate the T waves
• Are T waves present? Do they all have a normal shape?
• Do they all have a normal amplitude?
• Do they all have the same amplitude?
• Do the T waves have the same deflection as the QRS complexes?
• Step 7: Determine the duration of the QT interval
• Between the beginning of the QRS complex and the end of the T wave
• Is the duration a normal 0.36 to 0.44 second?
• Step 8: Evaluate any other components
• Check for ectopic beats and other abnormalities.
• Also check the ST segment for abnormalities, and look for the presence of a U wave.
• origin of the rhythm (for example, sinus node, atria, AV node, or ventricles)
• Rhythm abnormalities (for example, flutter, fibrillation, heart block, escape rhythm, or
other arrhythmias).

74. Practice rhythm reading by using 8-step method in your home
Reference and further reading
ECG interpretation made incredibly easy, 5th edition

75. Unipolar Bipolar
Precordial Leads V1, V2, V3, V4, V5, V6 --
Limb Leads aVR, aVF, aVL I, II, III

79. Interpreting the recording
• ECG tracings from multichannel machines will all look the same.
• The printout will show the patient’s name and identification number.
• At the top of the printout, you’ll see the patient’s heart rate and wave durations,
measured in seconds.
• Some machines can also record ST-segment elevation and depression.
• The name of the lead appears next to each 6-second strip.
• If not already present on the printout, be sure to write the date, time,
practitioner’s name, and special circumstances.
• For example, you might record an episode of chest pain, abnormal electrolyte
level, related drug treatment, abnormal placement of the electrodes, or the
presence of an artificial pacemaker and whether a magnet was used while the
ECG was obtained.
• Remember, ECGs are legal documents.
• They belong on the patient’s chart and must be saved for future reference and
comparison with baseline strips.