The electrocardiogram

ECG or EKG The ECG is a measurement of the sum total

of electrical activity generated by the heart measured from the surface of the body

An electrical record of the heart’s activity

It is one of the most valuable diagnostic tools for the recognition of a large variety of cardiac disorders

Characteristics of the normal electrocardiogram

The normal electrocardiogram is composed of:

P wave: is caused by electrical potentials generated when the atria depolarize before atrial contraction begins

QRS complex: is caused by potentials generated when the ventricles depolarize before contractionThe P wave and the components of the QRS complex are depolarization waves

T wave: is caused by potentials generated as the ventricles recover from the state of depolarization. the T wave is known as a repolarization wave

The electrocardiogram is composed of both depolarization and repolarization waves.

The atrial repolarization wave, known as the atrial T wave, is usually obscured by the much larger QRS complex. For this reason, an atrial T wave seldom is observed in the electrocardiogram

The “PQRST”

P wave - Atrial depolarization

• T wave - Ventricular repolarization

• QRS - Ventricular depolarization

Depolarization Waves Versus Repolarization Waves

In figure (A) depolarization, The first half of the fiber has already depolarized, while the remaining half is still polarized

The left electrode on the outside of the fiber is in an area of negativity, and the right electrode is in an area of positivity, this causes the meter to record positive

When depolarization has reached half way mark the record risen to maximum positive value

Depolarization Waves Versus Repolarization Waves

In figure (B) depolarization has extended over the entire muscle fiber, and the recording to the right has returned to the zero baseline because both electrodes are now in areas of equal negativity. The completed wave is a depolarization wave because it results from spread of depolarization along the muscle fiber membrane

Depolarization Waves Versus Repolarization Waves

In figure (C) shows halfway repolarization of the same muscle fiber, with positivity returning to the outside of the fiber. At this point, the left electrode is in an area of positivity, and the right electrode is in an area of negativity

Consequently, the recording, as shown to the right, becomes negative

Depolarization Waves Versus Repolarization Waves

In figure (D) the muscle fiber has completely repolarized, and both electrodes are now in areas of positivity, so that no potential difference is recorded between them

This completed negative wave is a repolarization wave because it results from spread of repolarization along the muscle fiber membrane

Relation of ventricle action potential to the QRS and T waves in the electrocardiogram

No potential is recorded in the electrocardiogram when the ventricular muscle is either completely polarized or completely depolarized

Only when the muscle is partly polarized and partly depolarized does current flow from one part of the ventricles to another part, and therefore current also flows to the surface of the body to produce the electrocardiogram

The time of the onset of the P wave to the onset of the QRS complex is termed as PR interval. It represent the conduction time from the atrial to the ventricle

The time from the beginning of the Q wave to the end of the S wave is called the QRS interval. It indicates the time taken by the impulse to separate to the two ventricles

The time from the beginning of the Q wave to the end of T wave is called the QT interval. It represent the total electrical activity of ventricles

The line between the QRS complex and T wave is called ST segment. It represent the time between completion of depolarization and onset of repolarization

The time interval from the apex of one R wave to the next R wave is called R-R interval

R-R interval is related to the heart rate or rate of ventricular contraction

The time interval from the beginning of one P wave to the beginning of the next P wave is called P-P interval

Vertical Axis = Voltage

Vertical axis represents voltage on the EKG

One small box (1 mm) represents 0.10 mV

Horizontal Axis = Time

1 small (1 mm) box = 0.04 seconds (40 ms)

1 large (5 mm) box = 0.20 seconds (200 ms)

5 large (5 mm) boxes = 1 second (1000 ms)

15 large (5 mm) boxes = 3 seconds and is marked on EKG paper

The ECG Paper

Horizontally One small box - 0.04 s One large box - 0.20 s

Vertically One large box - 0.5 mV

The ECG Paper

Every 3 seconds (15 large boxes) is marked by a vertical line.

This helps when calculating the heart rate.

NOTE: the following strips are not marked but all are 6 seconds long.

3 sec

3 sec

Rhythm Analysis

Step 1: Calculate rate. Step 2: Determine regularity. Step 3: Assess the P waves. Step 4: Determine PR interval. Step 5: Determine QRS duration.

Step 1: Calculate Rate

Option 1 Count the # of R waves in a 6 second

rhythm strip, then multiply by 10.

Interpretation?

9 x 10 = 90 bpm

3 sec

3 sec

Step 1: Calculate Rate

Option 2 Find a R wave that lands on a bold line. Count the # of large boxes to the next R

wave. If the second R wave is 1 large box away the rate is 300, 2 boxes - 150, 3 boxes - 100, 4 boxes - 75, etc. (cont)

R wave

Step 1: Calculate Rate

Option 2

Interpretation?

300

150

100

75

60

50

Approx. 1 box less than 100 = 95 bpm

What is the heart rate?

Step 2 : Determine Regularity

Regular: If the difference between the longest R-R interval in the ECG and the shortest R-R interval is less than 0.12 second

Irregular: If the difference between the longest R-R interval in the ECG and the shortest R-R interval is greater than 0.12 second

Step 2: Determine regularity

Look at the R-R distances (using a caliper or markings on a pen or paper).

Interpretation? Regular

R R

Step 3: Assess the P waves

Are there P waves? Do the P waves all look alike? Do the P waves occur at a regular

rate? Is there one P wave before each

QRS?Interpretation?

Normal P waves with 1 P wave for every QRS

Step 4: Determine PR interval

Normal: 0.12 - 0.20 seconds. (3 - 5 boxes)

Interpretation?

0.12 seconds

Step 5: QRS duration

Normal: 0.04 - 0.12 seconds. (1 - 3 boxes)

Interpretation?

0.08 seconds

Rhythm Summary

Rate 90-95 bpm Regularity regular P waves normal PR interval 0.12 s QRS duration 0.08 sInterpretation?

Normal Sinus Rhythm