Introduction to ElectrocardiographyDr. Ranjeet Dalvi NSCB Medical College, Jabalpur (MP)

Outline

1. Review of the conduction system2. ECG leads and recording 3. ECG waveforms and intervals4. Normal ECG and its variants5. Interpretation and reporting of an ECG

Electrical Conduction System of Heart

Brief history of ECG• 1842- Italian scientist Carlo Matteucci realizes that

electricity is associated with the heart beat.• 1876- Irish scientist Marey analyzes the electric pattern of

frog’s heart.

• 1895 - William Einthoven , credited for the invention of EKG• 1906 - using the string electrometer EKG, William Einthoven

diagnoses some heart problems• 1924 - the noble prize for physiology or medicine is given to

William Einthoven for his work on EKG• 1938 -AHA and Cardiac society of great Britain defined and

position of chest leads• 1942- Goldberger increased Wilson’s Unipolar lead voltage

by 50% and made Augmented leads.

William Einthoven:

What is an ECG?• An Electrocardiogram (ECG) is the recording(gram)

of the electrical activity (electro) generated by the cells of the heart (cardio) that reaches the body surface.• The electrocardiograph machine is a sophisticated

galvanometer , a sensitive electromagnet, which can detect and record small changes in electromagnetic potential.

Electric field of heart• Heart is situated at center of the electrical field which it

generates.

• The intensity of electrical field diminishes algebraically with the distance from its center.

• With the distance greater than 15 cm from the heart , decrement in the intensity of electrical field is hardly noticeable.

Indications for ECG• Cardiac Arrhythmias• Myocardial ischemia and infarction• Pericarditis• Chamber hypertrophy• Electrolyte disturbances• Drug effects and toxicity

Precautions to be taken for recording an ECG• Explain the procedure to patient,

• Good contact between skin and the electrode,

• Patient and machine should be properly grounded,

• Proper standardization of machine,

• All electronic equipment should be away from the machine.

ECG Paper: Dimensions & standardization

Always Check the standardization and paper speed before analyzing an ECG.

The conventional Electrocardiographic leads • 3 Standard Limb

Leads

• 3 Augmented Limb Leads

• 6 Precordial Leads

Bipolar limb leads & Einthoven’s triangle

Lead I (LA-RA): Oriented to left lateral

wall

Lead II (LL-RA)

Lead III ( LL-LA): Oriented to inferior surface of heart

Precordial LeadsElectro

de Level Location

V1 4th intercostal space Right sternal border

V2 4th intercostal space Left sternal border

V3 Midway between V2 and V4

Along a straight line between the two

V4 5th intercostal space† Left mid-clavicular line

V5 Same horizontal level as V4

Left anterior axillary line

V6 Same horizontal level as V4/V5 Left mid-axillary line

Monitoring Cable Connections

Arrangement of Leads on the ECG Paper

Anatomic Groups

Components of an ECG tracing• Wave: A deflection, either positive or negative, away from the baseline (isoelectric line)

of the ECG tracing• Complex: Several waves• Segment: A straight line between waves or complexes• Interval: A segment and a wave during which a depolarization wave travels from• the atria to the ventricles.• QRS Complex: Three deflections following P wave. Indicates ventricular depolarization

(and contraction).• Q WAVE: First negative deflection• R WAVE: First positive deflection• S WAVE: First negative deflection after R wave• ST Segment: Distance between end of S wave and beginning of T wave. Measures time

between ventricular depolarization and beginning of repolarization.• T Wave: Rounded, upright (positive) wave following QRS complex. Represents

ventricular repolarization.• QT Interval: Measured from beginning of QRS complex to end of T wave. Represents

total ventricular activity.• U Wave: Small, rounded, upright wave following T wave. Most easily seen with a slow

heart rate. Indicates repolarization of Purkinje fibers.

Wave forms of ECGSegment Normal Duration

P waves • height < 2.5 mm in lead II• width < 0.11 s in lead II

PR interval 0.12 to 0.20 s (3 - 5 small squares)

QRS complex 0.08 - 0.12 sec (2 - 3 small squares)

QT interval 0.35 - 0.43 sec

Calculating Heart Rate

• For regular rhythms: Rate = 300 / number of large squares in between each consecutive R wave.

• For very fast rhythms: Rate = 1500 / number of small squares in between each consecutive R wave.

• For slow or irregular rhythms: Rate = number of complexes on the rhythm strip x 6 (this gives the average rate over a ten-second period).

Axis of ECG

Method for Axis determination• Look at the leads I + aVF & Lead II

LEAD I LEAD aVF QUADRANT AXIS

Positive Positive Left lower quadrant

Normal (0 to +90 degrees)

Positive Negative Left upper quadrant

Possible LAD (0 to -90 degrees)Is lead II positive?Yes -> Normal (0 to -30 degrees)No -> LAD (-30 to -90 degrees)

Negative Positive Right lower quadrant

RAD (+90 to 180 degrees)

Negative Negative Right upper quadrant

Extreme Axis Deviation (-90 to 180 degrees)

Causes of Axis Deviation• Left Axis Deviation LVH, LBBB, Inf MI ,Ventricular pacing /ectopy, WPW syn(Rt lat), Sept primum ASD, Lt Ant Fasc Block-diagnosis of exclusion, Horizontally orientated heart – short, squat patient• Extreme Axis Deviation

Ventricular rhythms – e.g.VT, AIVR, ventricular ectopy Hyperkalaemia Severe right ventricular hypertrophy• Right Axis Deviation:

RVH, Acute RV strain, e.g. due to pulmonary embolism Lat. WMI, COPD, Hyperkalaemia, Dextrocardia, Ventricular ectopy, WPW syn (Lt lat.) Secundum ASD ,Normal paediatric ECG Lt Post FB – diagnosis of exclusion, Vertically orientated heart – tall, thin patient

Method for Axis determination

QT Interval• QT interval is the interval from beginning of QRS complex to the end

of T wave.• Represents the total duration of ventricular electrical activity-

depolarization + repolarization.• Measured commonly in leads I, II or leads aVL, V5 or V6. • End of T wave - defined by the intersection of the baseline with the

tangent drawn at the maximum downslope of T wave.

• Corrected QT:• QT inertval shortens with tachycardia & lengthens with bradycardia.

( due to shortening or lengthening of myocardial refractory period.) Normal value 390± 40 msBazett’s Formula :

Chamberlin's 10 rules of Normal ECG

Rule 1

Rule 2

The width of the QRS complex should not exceed 110 ms, less than 3 little squares

Rule 3

The QRS complex should be dominantly upright in leads I and II

Rule 4

QRS and T waves tend to have the same general direction in the limb leads

Rule 5

All waves are negative in lead aVR

Rule 6

The R wave must grow from V1 to at least V4

The S wave must grow from V1 to at least V3

and disappear in V6

Rule 7

The ST segment should start isoelectric except in V1 and V2 where it may be elevated.

Rule 8

The P waves should be upright in I, II, and V2 to V6

Rule 9

There should be no Q wave or only a small q less than 0.04 seconds in width in I, II, V2 to V6

Rule 10

The T wave must be upright in I, II, V2 to V6

Analyzing An ECG Rhythm

Analyzing An ECG Rhythm

How to check for Wrong lead placement

1. Right arm/left arm reversal include the following: Inverted P-QRS-T waves in lead I; “Normal" appearance of lead aVR (i.e., upright P-QRS-T in lead

aVR as opposed to the expected inversion of these waveforms in a normal tracing);

QRS vector in lead I does not match that of lead V6.* An inverted P-QRS-T in lead I will result in an unexpected QRS

axis deviation (either rightward or extreme axis deviation, depending upon the major vector of the QRS complex in lead aVF)

Right arm/left arm reversal (Cont.)

2. Right arm/right leg reversal • A near-"flat line" appearance in one of the three standard

limb leads (in the case of right arm/right leg reversal, this occurs in lead II).• This change results from the placement of the right leg

(ground) lead—which the electrocardiographic processor recognizes as having no potential difference from the left leg—in the right arm position.

3. Left Arm/Left Leg Reversal• Reversing the arm and leg electrodes on the left side

changes the findings in all limb electrodes except lead aVR—thus, lead aVR maintains its "normally abnormal" appearance (i.e., upside-down P-QRS-T complex in most cases), • lead II becomes lead I; lead aVF becomes lead aVL; and lead

III is inverted. • Alternately stated, the lateral limb leads (I and aVL) become

inferior, and two of the inferior leads (II and aVF) become lateral.>• The inversion of lead III, although detectable when

scrutinizing a comparison ECG, is not blatantly abnormal, because the principal QRS complex vector ordinarily may be positive or negative in this lead.

So how does one detect left arm/left leg reversal? • a P wave that was smaller in lead II than in lead I, or an upward

terminal phase in a biphasic, lead III P wave (i.e., P-wave deflection that is down/up, rather than up/down as would be expected in this relatively right-sided lead) correctly predicted reversal of arm and leg electrodes on the left side in 90% of cases. • This is the key clue to detecting left arm/left leg reversal

without a comparison tracing. • One should also look for unexpected shifts in the QRS axis (and

P-wave axis); although this is not specific for limb electrode reversal, it is one cause to be considered when an axis change is evident during comparison of old and new ECGs.

Precordial Electrode Misplacement• Improper positioning of the precordial electrodes may

result in a pseudo infarction pattern.

• If the clinical history is not consistent with myocardial infarction during the time between new and old/baseline tracings, electrode misplacement must be considered in addition to silent myocardial infarction.

• Repeating the tracing with meticulous attention to precordial electrode positioning will usually eradicate the problem if it is due to improper positioning.

Artifact on the ECG• It can be

A. Non-physiologic artifact: is due to equipment problems or interference from neighbouring electrical devices,

B. Physiologic artifact: results from muscle activity or skin interference. At times, physiologic interference is easy to discern, but it can mimic true dysrhythmia and lead to unnecessary testing & therapy.

• Artifact is likely if the baseline is unstable before or after the dysrhythmia in question, or if a normal QRS complex appears immediately after cessation of the apparent dysrhythmia at an interval that is too short to be physiologic.

How to report an ECG?• Name…. Age…• W/B… Registration number…• Date…• Rate & Rhythm…..• P wave...• PR interval….• QRS Complex…..• Width • Axis• Configuration

• T-wave….• U wave….• Comments….• Advice…

Signature

Take home message• Proper technique of ECG recording is equally important as

proper interpretation of an ECG.• Do not rely completely on interpretation done by the

machine.• Check for the misplaced leads if history & clinical findings

do not correlate with ECG findings, repeat ECG if necessary.• Always follow a sequence while interpreting an ECG.• ‘’ECG-Normal’’ Statements are not sufficient on ECG

reports.

References:• 1. Narsimhan , Francis, Schamroth; LeoSchamroth An

Introduction to Electrocardiography; 8th Edition; Wiley India. 2013

• 2. G. Waner, D. Strauss; Marriot’s Practical Electrocardiography; 12th edition; Wolters Kluwer Health. 2014.

• 3.K.M. Lewis, K. Handal ; Sensible ECG Analysis ; Delmar Publishers, Canada; 2000.

• 4. Harrison’s Principles Of Internal Medicine, 19th Edition, 2015.• 5. Richard A. Harrigan et al ;Electrocardiographic Electrode

Misplacement, Misconnection, and Artefact:; J Emerg Med. 2012;43(6):1038-1044

• 6. Littman L, Monroe MH. Electrocardiographic artifact (letter). N Engl J Med 2000;342:590–1.

• 7. www.lifeinfastlane.com/ecgbasics