The Normal Electrocardiogram

Craig W. Clarkson, Ph.D.

The ECG/EKG •  The ECG is a recording of the heart’s electrical

activity that is produced by depolarization and repolarization of heart muscle.

•  The ECG is measured at the surface of the body as a series of variations in voltage.

•  The ECG is a useful tool for diagnosing many different types of cardiac disorders (e.g. myocardial ischemia & infarction, hypertrophy, congenital abnormalities, drug overdose, and cardiac arrhythmias).

The Surface ECG is a non-invasive way to investigate the electrical behavior of the heart. Bipolar pairs of electrodes can be placed on the body surface to detect the voltage changes produced during a normal cardiac cycle. Pairs of bipolar electrodes placed in a fixed orientation are referred to as “leads” (by convention).

Bipolar Recording

Each cardiac cell behaves like an electrical bipole, producing a sequence of voltage changes over time that can be detected by pairs of electrodes placed on the body surface.

Lead I: RA to LA Lead II: RA to LL Lead III: LA to LL

mnemonic: Bipolar Lead No. indicates the # of “L’s” in the placement sites (e.g. Lead III: Left arm to Left Leg.)

(defined by W. Einthoven) The three standard bipolar leads

+ + + + + + + + + -

+ 0

+ + + + + + + + +

Voltmeter

- +

Polarized cell at rest

- - - - - - - - - - - - - - - -

ECG:

zero ΔV

Extracellular Voltage Recording 101a

+ + + + + + -

+ 0

+ + + + + +

Voltmeter

- +

Cell depolarizing - - -

- - -

- deflection: depolarization is moving away from the + electrode

+ deflection: depolarization is moving towards the + electrode

ECG:

+ ΔV

Extracellular Voltage Recording 101b

+ + + + -

+ 0

+ + + +

Voltmeter

- +

Cell depolarizing - - -

- - -

-

-

ECG:

Extracellular Voltage Recording 101c

+ + -

+ 0

+ +

Voltmeter

- +

Cell depolarizing - - - -

- - - - - - -

- - - ECG:

Extracellular Voltage Recording 101d

-

+ 0

Voltmeter

- +

Cell depolarized

- - - - - - - -

- - - - - - - -

ECG:

Extracellular Voltage Recording 101e

-

+ 0

Voltmeter

- +

Cell repolarizing

- - - - - -

- - - - - -

+ +

+ +

- deflection positive electrode is negative

ECG:

Note: repolarization is typically much slower than depolarization, resulting in a deflection that is wider and of lower magnitude

Extracellular Voltage Recording 101f

-

+ 0

Voltmeter

- +

Cell repolarizing

- - - -

+ + + +

ECG:

+ + + +

- - - -

Extracellular Voltage Recording 101g

-

+ 0

Voltmeter

- +

Cell repolarizing

- + + + + + +

ECG:

- - - -

+ + + + + +

-

- -

Extracellular Voltage Recording 101h

-

+ 0

Voltmeter

- +

Cell repolarized + +

+ +

+ +

+ +

+ + + + +

+ + + +

ECG:

Extracellular Voltage Recording 101i

Note: This is the expected QRS & T wave pattern expected for “ventricular beats” (e.g. PVCs) – QRS & T waves occur in opposite directions.

Reflection on ECG Deflections: Voltage Change ECG Deflection

•  Depolarization toward the + electrode: é

•  Depolarization away from the + electrode: ê

•  Repolarization toward the + electrode: ê

•  Repolarization away from the + electrode: é

PR interval ST segment

QT interval

P wave

QRS complex

T wave

Segment – a straight line connecting 2 waves Interval – at least one wave plus a connecting segment

Terminology:

Why is the Human T Wave Upright!?

Test of a Hypothesis (Epi vs Endo APD)

Frog Experiments (Ashman & Hull)

Endo

Epi

Predicted Human Electrophysiology

RA LA +-

Evolution of the ECG in Lead I (NSR)

RA LA +-

Septal fascicle of LBB (activates left septum 1st)

Lead I - Normal Sinus Rhythm

RA LA +-

Ventricular depolarization spreads from endocardial surface to epicardial surface

Lead I - Normal Sinus Rhythm

RA LA +-

A few small areas of the ventricles are activated at a late stage & generate the S wave

Lead I - Normal Sinus Rhythm

RA LA +-

- - -

+

Finally the ventricular muscle repolarizes, starting with the epicardium, spreading to the endocardium - producing an upright T wave.

Lead I - Normal Sinus Rhythm

Endo- cardium

Epi- cardium

Endocardium depolarizes first Epicardium repolarizes first

Endo- vs Epi-cardial Action Potential Durations

Artwork by Dr. Kreisman

A

B C

+

QRS Amplitude

A

B

C

The Angle of Orientation & QRS Amplitude

“essentially flat” “largest”

180o

-90o

0o

+90o

The limb leads view the heart from a different angle of orientation. Leads I-III - frontal plane of body.

0o

✚

lead I

Lead II

+60o

✚ ✚

lead III

+120o

✚

Limb Leads

Note that the QRS & T waves appear differently in different leads (due to different angles of orientation)

The Normal 12 Lead ECG

Lead III

Lead I

Lead II

4 mm Lead I

4 mm

Lead II 4 mm

4 mm

Einthoven’s triangle

--

-+

+

+

I I I I I I I I I I I I I I I I I I I I I I I

Lead III 30oC

0

0

0

The Mean QRS Axis

110o

-30o

Left Axis Deviation

Right Axis Deviation

0o

+30o II +60o

+90o

I

Left axis deviation: • Left Ventricular Hypertrophy* • Inferior wall MI • Left anterior fascicular block

Right axis deviation: • Right Ventricular Hypertrophy • Acute right heart strain • Left posterior fascicular block

III

Axis Deviations

*LVH: Patients w/ HT

QT

RR interval

160

190

220

200 600400 800

Dog

Normal ECG Intervals •  PR: 0.12-0.20 sec (3-5 small boxes)

–  decreased in junctional rhythm –  increased in 1st degree AV block

•  QRS: ≤ 0.1 sec (≤ 2.5 small boxes) –  increased in BBB, PVB, severe hyperkalemia,

drug effect (e.g. quinidine) •  QT: QTc = QT/√RR ≤ 0.44 sec

–  decreased in hypercalcemia, tachycardia –  increased in hypocalcemia, hypokalemia,

hypomagnesemia, congential long QT, drugs (increased risk of Sudden Cardiac Death)

T-wave inversion (ischemia / infarct / hypertrophy)

Isoelectric ST segment ST segment depression (ischemia/ angina)

Pronounced Q wave (transmural infarct)

Q

ECG - Effects of Pathophysiology •  Repolarization is commonly affected by pathophysiology

Q

Transmural MI - The Movie

STEMI

The End