ECG BasicsECG Basics

What is an ECG?What is an ECG?

The electrocardiogram (ECG) is a The electrocardiogram (ECG) is a representation of the electrical events of the representation of the electrical events of the cardiac cycle.cardiac cycle.

Each event has a distinctive waveform, the Each event has a distinctive waveform, the study of which can lead to greater insight study of which can lead to greater insight into a patient’s cardiac pathophysiology.into a patient’s cardiac pathophysiology.

Electrocardiogram (ECG/EKG)Electrocardiogram (ECG/EKG)

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ECG LeadsECG Leads

Leads are electrodes which measure the Leads are electrodes which measure the difference in electrical potential.difference in electrical potential.

They can be of two types:They can be of two types:

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

2. One point on the body and a zero electrical potential.2. One point on the body and a zero electrical potential.

EKG LeadsEKG Leads

The standard EKG has 12 leads:The standard EKG has 12 leads: 3 Standard Limb Leads

3 Augmented Limb Leads

6 Precordial Leads

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

Standard Limb LeadsStandard Limb Leads

Types of ECG RecordingsTypes of ECG Recordings

Bipolar leadsBipolar leads record record voltage between voltage between electrodes placed on electrodes placed on wrists & legs (right leg is wrists & legs (right leg is ground)ground)Lead ILead I records between records between right arm & left armright arm & left armLead IILead II: right arm & left leg: right arm & left legLead IIILead III: left arm & left leg: left arm & left leg

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Summary of LeadsSummary of Leads

Limb LeadsLimb Leads Precordial LeadsPrecordial Leads

BipolarBipolar I, II, IIII, II, III(standard limb leads)(standard limb leads)

--

UnipolarUnipolar aVR, aVL, aVF aVR, aVL, aVF (augmented limb leads)(augmented limb leads)

VV11-V-V66

Arrangement of Leads on the EKGArrangement of Leads on the EKG

Precordial LeadsPrecordial Leads

Adapted from: www.numed.co.uk/electrodepl.html

Precordial LeadsPrecordial Leads

ECG Graph Paper

•one small block equals 0.04 s

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

Normal conduction pathway:

SA node -> atrial muscle -> AV node -> bundle of His -> Left and Right Bundle Branches -> Ventricular muscle

3 distinct waves are 3 distinct waves are produced during produced during cardiac cyclecardiac cycle

P waveP wave caused by caused by atrial depolarizationatrial depolarization

QRS complexQRS complex caused by ventricular caused by ventricular depolarizationdepolarization

T waveT wave results from results from ventricular ventricular repolarizationrepolarization

ECG

Elements of the ECG:

• P wave: Depolarization of both atria;• Shape and duration of P may indicate atrial enlargement•Relationship between P and QRS helps distinguish various cardiac arrhythmias

•

Waveforms and Intervals

•QRS complex: Ventricular depolarization

• Larger than P wave because of greater muscle mass of ventricles

• Normal duration = 0.08-0.12 seconds

• Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.

• Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI

•PR interval: from onset of P wave to onset of QRS

• Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)

• Represents atria to ventricular conduction time (through His bundle)

• Prolonged PR interval may indicate a 1st degree heart block

ST segment:• Connects the QRS complex and T wave• Duration of 0.08-0.12 sec (80-120 msec

T wave:

• Represents repolarization

QT Interval• Measured from beginning of QRS to the end of the T wave• Normal QT is usually about 0.40 sec• QT interval varies based on heart rate

Determining the Heart RateDetermining the Heart Rate

Rule of 300Rule of 300

10 Second Rule10 Second Rule

Rule of 300Rule of 300

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

Although fast, this method only works for Although fast, this method only works for regular rhythms.regular rhythms.

What is the heart rate?What is the heart rate?

(300 / 6) = 50 bpm

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What is the heart rate?What is the heart rate?

(300 / ~ 4) = ~ 75 bpm

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What is the heart rate?What is the heart rate?

(300 / 1.5) = 200 bpm

The Rule of 300The Rule of 300

It may be easiest to memorize the following table:It may be easiest to memorize the following table:

# of big # of big boxesboxes

RateRate

11 300300

22 150150

33 100100

44 7575

55 6060

66 5050

10 Second Rule10 Second Rule

As most EKGs record 10 seconds of rhythm per As most EKGs record 10 seconds of rhythm per page, one can simply count the number of beats page, one can simply count the number of beats present on the EKG and multiply by 6 to get the present on the EKG and multiply by 6 to get the number of beats per 60 seconds.number of beats per 60 seconds.

This method works well for irregular rhythms.This method works well for irregular rhythms.

The QRS AxisThe QRS Axis

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

Abnormalities of axis can hint at:Abnormalities of axis can hint at:

Ventricular enlargementVentricular enlargement

Conduction blocks (i.e. hemiblocks)Conduction blocks (i.e. hemiblocks)