ECG basics

ECG BASICSDONE BY :

MOHAMMED A QAZZAZ

• Review of the conduction system• ECG waveforms and intervals• ECG leads• Determining heart rhythm / rate• Determining QRS axis• Normal waves / intervals

Outline

The Normal Conduction System

The electrocardiogram (ECG) is a 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.

What is an ECG?

Runs at a paper speed of 25 mm/sec

• Each small block of ECG paper is 1 mm2

• At a paper speed of 25 mm/s, one small block equals 0.04 s

• Five small blocks make up 1 large block which translates into 0.20 s (200 msec)

• Hence, there are 5 large blocks per second

• Voltage: 1 mm = 0.1 mV between each individual block vertically

ECG Graph Paper

Normal ECG

What types of pathology can we identify and study from ECGs?

• Arrhythmias

• Myocardial ischemia and infarction

• Pericarditis

• Chamber hypertrophy

• Electrolyte disturbances (i.e. hyperkalemia, hypokalemia)

• Drug toxicity (i.e. digoxin and drugs which prolong the QT interval)

Waveforms and Intervals

Waveforms and Intervals

• 3 distinct waves are produced during cardiac cycle

• P wave caused by atrial depolarization

13-63

ECG

• QRS complex caused by ventricular depolarization

• T wave results from ventricular repolarization

13-63

Leads are electrodes which measure the difference in electrical potential between either:

1. Two different points on the body (bipolar leads)

2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads)

ECG Leads

ECG

The standard ECG has 12 leads:

3 Standard Limb Leads

3 Augmented Limb Leads

6 Precordial Leads

The axis of a particular lead represents the viewpoint from which it looks at the heart.

ECG Leads

ECG Leads

Standard Limb Leads

ECG

Augmented Limb Leads

All Limb Leads

Precordial Leads

ECGPrecordial Leads

Summary of Leads

Limb Leads Precordial Leads

Bipolar I, II, III(standard limb leads)

-

Unipolar aVR, aVL, aVF (augmented limb leads)

V1-V6

Arrangement of Leads on the ECG

Anatomic Groups(Septum)

ECG(Anterior Wall)

Anatomic Groups

ECG(Lateral Wall)

Anatomic Groups

ECG

(Inferior Wall)

Anatomic Groups

summary

Determining the Heart Rhythm

Normal Sinus Rhythm

Each P wave is followed by a QRS

oP wave rate 60 - 100 bpm with <10% variation

o rate <60 = sinus bradycardia o rate >100 = sinus tachycardia o variation >10% = sinus arrhythmia

Determining the Heart Rate

• Rule of 300

• 10 Second Rule

Rule of 300

Take the number of “big boxes” between neighboring QRS complexes, and divide 300 into this number. The result will be approximately equal to the rate

Although fast, this method only works for regular rhythms.

ECG

(300 / 6) = 50 bpm

ECG

(300 / ~ 4) = ~ 75 bpm

ECG

(300 / 1.5) = 200 bpm

The Rule of 300

It may be easiest to memorize the following table:

# of big boxes

Rate

1 300

2 150

3 100

4 75

5 60

6 50

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.

This method works well for irregular rhythms.

What is the heart rate?

33 x 6 = 198 bpm

The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/

The QRS Axis

The QRS axis represents the net overall direction of the heart’s electrical activity.

Abnormalities of axis can hint at:

Ventricular enlargement

Conduction blocks (i.e. hemiblocks)

The QRS Axis

By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°.

-30° to -90° is referred to as a left axis deviation (LAD)

+90° to +180° is referred to as a right axis deviation (RAD)

Determining the Axis

• The Quadrant Approach

• The Equiphasic Approach

Determining the Axis

Predominantly Positive

Predominantly Negative

Equiphasic

The Quadrant Approach2. In the event that LAD

is present, examine lead II to determine if this deviation is pathologic.

If the QRS in II is predominantly positive, the LAD is non-pathologic (in other words, the axis is normal). If it is predominantly negative, it is pathologic.

Quadrant Approach: Example

Negative in I, positive in aVF RAD

The Alan E. Lindsay ECG Learning Center http://medstat.med.utah.edu/kw/ecg/

Quadrant Approach: Example 2

Positive in I, negative in aVF Predominantly positive in II

Normal Axis (non-pathologic LAD)

The Alan E. Lindsay ECG Learning Center http://medstat.med.utah.edu/kw/ecg/

The Equiphasic Approach

1. Determine which lead contains the most equiphasic QRS complex. The fact that the QRS complex in this lead is equally positive and negative indicates that the net electrical vector (i.e. overall QRS axis) is perpendicular to the axis of this particular lead.

2. Examine the QRS complex in whichever lead lies 90° away from the lead identified in step 1. If the QRS complex in this second lead is predominantly positive, than the axis of this lead is approximately the same as the net QRS axis. If the QRS complex is predominantly negative, than the net QRS axis lies 180° from the axis of this lead.

ECG

Equiphasic Approach: Example 1

Equiphasic in aVF Predominantly positive in I QRS axis ≈ 0°


Equiphasic Approach: Example 2

Equiphasic in II Predominantly negative in aVL QRS axis ≈ +150°


Normal waves / intervals

Normal P Waves height < 2.5 mm in lead II (higher = ? P-pulmonale)

width < 0.11 s in lead II (wider = ? P-mitrale)

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

Short PR interval (Wolff-Parkinson-White syndrome / Lown-Ganong-Levine syndrome)

Long PR interval (first degree heart block / 'trifasicular' block)

ECG

Normal

P - pulmonale

ECG

Long P-R

P - mitrale


Normal QRS complex < 0.12 s duration (3 small squares) No pathological Q waves

Pathologic “Q”: - > 0.04 sec (small box) - > 25% of “R” amplitude

Wide QRS (right or left bundle branch block, ventricular rhythm, hyperkalemia)

o No evidence of left or right ventricular hypertrophy

ECG

LVH

Normal

ECG

Wide Complex

Pathologic “Q” wave

Normal waves / intervalsNormal QT interval:

– Males: < 450 ms.– Females: < 470 ms.

o Calculate the corrected QT interval (QTc) by dividing the QT interval by the square root of the preceeding R - R interval.

o Long QT interval is a risk factor for VT / Torsades de Pointes.

o Long QT interval (MI, myocarditis, diffuse myocardial

disease / hypocalcaemia / hypothyrodism / intracerebral haemorrhage / drugs (sotalol, amiodarone) / hereditary (Romano Ward syndrome (autosomal dominant) / Jervill Lange Nielson syndrome (autosomal recessive)

ECG


Normal ST segment no elevation or depression

ST elevation: acute MI / left bundle branch block, normal variants (e.g. athletic heart) acute pericarditis

ST depression: myocardial ischaemia, digoxin effect / ventricular hypertrophy / acute posterior MI / right bundle branch block

ECG

Normal ST

ST elevation

ST depression


Normal T wave: variable morphology & amplitude / usually same direction as the QRS except in V1-2 leads.

In the normal ECG the T wave is always upright in leads I, II, V3-6, and always inverted in lead aVR.

Tall T: hyperkalemia / hyperacute myocardial infarction.

Small, flattened or inverted T waves: ischaemia / LVH / drugs (e.g. digoxin) / pericarditis / PE / RBBB / electrolyte disturbance.

Normal U wave: usually < 1/3 T wave amplitude & same direction in the same lead / prominent at slow heart rates.

o Origin of the U wave is thought to be related to after depolarizations which interrupt or follow repolarization

ECG

The U Wave

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ECG basics