EKG 12 Leads

Internal Medicine2010

Electrocardiography

• A recording of the electrical activity of the heart over time

• Gold standard for diagnosis of cardiac arrhythmias

• Helps detect electrolyte disturbances

• Allows for detection of conduction abnormalities

• Screening tool for ischemic heart disease (stress tests)

• Helpful with non-cardiac diseases

ECG Leads

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

– 3 Augmented Limb Leads

– 6 Precordial Leads

ECG Limb Leads

• 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)

Recording of the ECG

• 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 between two sensors, one has

a vector.• There will be a positive end at one

electrode and negative at the other.• The positioning for leads I, II, and III

were first given by Einthoven (Einthoven’s triangle).

ECG Limb Leads

Precordial Leads

Summary of Leads

Limb Leads Precordial Leads

BipolarI, II, III

(standard limb leads)

-

UnipolaraVR, aVL, aVF (augmented limb leads)

V1-V6

Arrangement of Leads on the EKG

Anatomic Groups

Anatomic Groups

Anatomic Groups

Anatomic Groups

Anatomic Groups

ECG Graph Paper

• 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

• 5 large blocks per second• Voltage: 1 mm = 0.1 mV between

each individual block vertically

The Normal Conduction System

ECG Tracing

Guide in ECG Reading

1 Rhythm2 Rate: Atrial and Ventricular3 Axis4 P wave: morphology and

duration5 P – R interval6 QRS complex: morphology and

duration7 ST segment8 T wave9 U wave10 Q – T interval

• Regular or Irregular• Accessing whether the PP intervals and RR intervals

are regularly spaced

• If the rhythm is irregular, determine if:– Occasionally irregular– Regularly irregular (there is the pattern of

irregularity)– Irregularly irregular (no pattern of irregularity)

Determining the Rhythm

Determining the Heart Rate

• Rule of 300–Take the number of “big boxes” between neighboring QRS complexes, and divide this into 300. The result will be approximately equal to the rate–Although fast, this method only works for regular rhythms.

•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.

The QRS Axis

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

• 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)

The QRS Axis

The QRS Axis The Quadrant Approach1. Examine the QRS complex in leads I and aVF to determine if

they are predominantly positive or predominantly negative. The combination should place the axis into one of the 4 quadrants below.

The QRS Axis 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.

P-wave• Depolarization of both atria• Relationship between P and QRS helps

distinguish various cardiac arrhythmias• Shape and duration of P may

indicate atrial enlargement

ECG Tracing

P wave morphology

4

I

V1

AVR

Upright P

Biphasic P

Inverted P

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

ECG Tracing

P – R interval• Normal = 0.12 – 0.22

• Short = < 0.12

• Prolonged = > 0.22

QRS complex• Ventricular depolarization• Larger than P wave because of

greater muscle mass of ventricles• Normal duration = 0.08-

0.12 seconds

ECG Tracing

QRS complex

• 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

6

I

V3

AVR

Positive QRS

Biphasic QRS

Negative QRS

QRS morphology and duration

ST segment

• Connects the QRS complex and T wave

• Duration of 0.08-0.12 sec (80-120 msec)

ECG Tracing

7

I

V3

V1

Normal

Depressed

Elevated

S – T Segment

T waves

• Represents repolarization or recovery of ventricles

• Interval from beginning of QRS to apex of T is referred to as the absolute refractory period

ECG Tracing

8

IAVR

Upright T Inverted T

T wave morphology

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

ECG Tracing

Chamber Enlargement

• Right Atrial Enlargement (RAE)– Tall P waves in II, III, AVF > 2.5mm

• Left Atrial Enlargement (LAE)– P wave in I > 0.11 secs– Terminal Negativity of P wave in VI = >

1mm• Bi-atrial Enlargement

– RAE + LAE

Sinus Bradycardia

• Heart is slower than 60 beats per minute• RR interval is longer• P wave followed by QRS complex in 1:1

ratio• PR interval slightly prolonged

Sinus Tachycardia

• Sinus rhythm is faster than 100 beats per minute

• RR interval is shorter, less than 0.6 seconds• P wave followed by QRS complex in 1:1

ratio

Atrial Flutter

• atria contract at 200-350 beats per minute • F waves are larger than normal P waves and

they have a saw-toothed waveform• ventricular rate is usually regular but slower

than the atrial rate• fixed ratio of flutter waves to QRS

complexes can be observed, for instance 2:1, 3:1 or 4:1

Atrial Flutter

Atrial Fibrillation• occurs when the atria depolarize repeatedly

and in an irregular uncontrolled manner usually at at atrial rate greater than 350 beats per minute

• no concerted contraction of the atria• No P-waves are observed

• QRS complexes have normal shape, due to normal ventricular conduction. However the RR intervals vary from beat to beat. The ventricular rate may increase to greater than 150 beats per minute if uncontrolled.

Atrial Fibrillation

Ventricular Tachycardia• occurs when electrical impulses

originating either from the ventricles cause rapid ventricular depolarization

(140-250 beats per minute)

• QRS complexes are wide and bizarre• P-waves may be inverted/may be

present but not associated with QRS complexes (AV dissociation)

• RR intervals are usually regular

Ventricular Tachycardia

Ventricular Fibrillation• occurs when parts of the ventricles

depolarize repeatedly in an erratic, uncoordinated manner

• random, apparently unrelated waves• no recognizable QRS complex• almost invariably fatal because the

uncoordinated contractions of ventricular myocardium

• electrical defibrillation restores normal regular rhythm

Ventricular Fibrillation

Wolff-Parkinson-White Syndrome• presence of an accessory atrioventicular

pathway located between the wall of the right or left atria and the ventricles, known as the Bundle of Kent - allows the impulse to bypass the AV node and activate the ventricles prematurely

• initial slur to the QRS complex, known as a delta wave may be observed

• QRS complexes are wide, > 0.11 sec

• PR is shortened, to less than 0.12 sec

Wolff-Parkinson-White Syndrome

Right Bundle Branch Block• Criteria for right bundle branch block (RBBB)

[1] – QRS >0,12 sec – Slurred S wave in lead I and V6 – RSR'-pattern in V1 where R' > R

Left Bundle Branch Block• QRS duration is 120 msec or greater• poor R wave progression in V 1 thru V4• T wave vector is in the opposite direction to

the QRS vector (T waves are inverted in I, V5 and V6)

• conduction abnormality often appears as "rabbit ears" (rsR pattern) on the left side of the chest (V4,5,6)