Electrocardiogram ECG Notes

Electrocardiogram (ECG) Definitions

Electrocardiography

• Electrocardiography is the technique by which the electrical activities of the heart are studied.
• The spread of excitation through the myocardium produces local electrical potential. This causes the flow of small currents through the body which acts as a volume conductor.
• These small currents can be picked up from the surface of the body by using suitable electrodes and recorded in the form of electrocardiogram.
• This technique was discovered by Dutch physiologist, Einthoven Willem who is considered the father of ECG.

Read And Learn More: Medical Physiology Notes

Electrocardiograph: Electrocardiograph is the instrument (ECG machine) by which the electrical activities of the heart are recorded.

Electrocardiogram (ECG)

• Electrocardiogram is the record or graphical registration of electrical activities of the heart, which occur prior to the onset of mechanical activities.
• It is the summed electrical activity of all the cardiac muscle fibers recorded from the surface of the body. ECG is recorded in 12 leads.

Uses of ECG

ECG is useful in determining and diagnosing the following:

1. Heart rate
2. Heart rhythm
3. Abnormal electrical conduction
4. Poor blood flow to heart muscle (ischemia)
5. Heart attack
6. Coronary artery disease
7. Hypertrophy of heart chambers.

Electrocardiographic (ECG) Grid

• The paper that is used for recording ECG is called ECG paper. The electrocardiograph or ECG machine amplifies the electrical signals produced from the heart and records these signals on a moving ECG paper.
• EGC grid refers to the markings (lines) on ECG paper. The ECG paper has horizontal and vertical lines at regular intervals of 1 mm. Every 5th line (5 mm) is thickened.

Electrocardiographic (ECG) Grid Duration

• The duration of different ECG waves is denoted by the vertical lines.
• The interval between two thick lines (5 mm) = 0.2 seconds. The interval between two thin lines (1 mm) = 0.04 seconds.

Amplitude: The amplitude of ECG waves is denoted by horizontal lines. The interval between two thick lines (5 mm) — 0.5 mV. The interval between two thin lines (1 mm) = 0.1 mV.

Speed Of The Paper

• The movement of paper can be adjusted at two speeds, 25 mm/second and 50 mm/second.
• Usually, the speed of the paper during recording is fixed at 25 mm/ second.
• If the heart rate is very high, the speed of the paper is changed to 50 mm/second.

ECG Leads

• ECG is recorded by placing a series of electrodes on the surface of the body.
• These electrodes are called ECG leads and are connected to the ECG machine.
• The electrodes are fixed on the limbs. Usually, the right arm, left arm and left leg are chosen.
• The heart is said to be in the center of an imaginary equilateral triangle drawn by connecting the roots of these three limbs. This triangle is called Einthoven’s triangle.
• The electrical potential generated from the heart appears simultaneously on the roots of these three limbs (refer to the next chapter for more details).

ECG is recorded in 12 leads which are generally classified into two categories.

1. Bipolar leads
2. Unipolar leads

Bipolar Leads

• Bipolar leads are otherwise known as standard limb leads.
• Two limbs are connected to obtain these leads and both the electrodes are active recording electrodes, i.e.
• one electrode is positive and the other one is negative.

There are three standard limb leads:

1. Limb lead 1
2. Limb lead 2
3. Limb lead 3

Lead 1: Lead 1 is obtained by connecting the right arm and left arm. The right arm is connected to the negative terminal of the instrument and the left arm is connected to the positive terminal.

Lead 2: Lead 2 is obtained by connecting the right arm and left leg. The right arm is connected to the negative terminal of the instrument and the left leg is connected to the positive terminal.

Lead 3: Lead 3 is obtained by connecting the left arm and left leg. The left arm is connected to the negative terminal of the instrument and the left leg is connected to the positive terminal.

Unipolar Leads

• Here, one electrode is an active electrode and the other one is an indifferent electrode.
• The active electrode is positive and the indifferent electrode is serving as a composite negative electrode.

The unipolar leads are of two types:

1. Unipolar limb leads
2. Unipolar chest leads.

1. Unipolar Limb Leads

• These leads are also called augmented limb leads. The active electrode is connected to one of the limbs.
• The indifferent electrode is obtained by connecting the other two limbs through a resistance.

Unipolar limb leads are of three types:

1. aVR lead
2. aVL lead
3. aVF lead

1. aVR lead: The active electrode is from the right arm. The indifferent electrode is by connecting left arm and left leg.

aVL lead: The active electrode is from the left arm. The indifferent electrode is obtained by connecting the right arm and left leg.

aVF lead: The active electrode is from the left leg (foot). The indifferent electrode is obtained by connecting the two upper limbs.

2. Unipolar Chest Leads

• Chest leads are also called precordial leads. The indifferent electrode is obtained by connecting the three limbs – a left arm, a left leg, and a right arm through a resistance of 5000 ohms. The active electrode is placed on six points over the chest.
• This electrode is known as the chest electrode and the six points over the chest are called V, V2, V3, V4, V5, and V6. V indicates vector, which is explained in the next chapter. Position of chest leads:

V1: Over 4th intercostal space near the right sternal margin
V2: Over 4th intercostal space near the left sternal margin
V3 : In between V2 and V4
V4: Over left 5th intercostal space on the mid-clavicular line
V5: Over left 5th intercostal space on the anterior axillary line
V6: Over left 5th intercostal space on the mid-axillary line.

Waves Of Normal Electrocardiogram

• A normal ECG consists of waves, complexes, intervals and segments. The waves of ECG recorded by Limb Lead II are considered typical waves.
• Normal electrocardiogram has the following waves namely P, Q, R, S, and T.
• Einthoven had named the waves of ECG starting from the middle of the English alphabet (P) instead of starting from the beginning (A).

The major complexes in ECG are:

1. T wave, the atrial complex
2. ‘CiRS’ complex, the initial ventricular complex
3. T wave, the final ventricular complex.
4. ‘QRST’, the ventricular complex

‘P’ WAVE: It is a positive wave and the first wave in ECG. It is also called an atrial complex.

‘P’ WAVE Cause

• ‘P’ wave is produced due to the depolarization of atrial musculature.
• It spreads from the SA node to all parts of the atrial musculature. Atrial repolarization is not recorded as a separate wave in ECG because it merges with the QRS complex.

‘P’ WAVE Duration

0.1 second.

‘P’ WAVE Amplitude

1-0.12 mV.

‘P’ WAVE Morphology

• ‘P’ wave is normally positive (upright) in leads I, II, aVF, V4, V5, and V6. It is normally negative (inverted) in aVR.
• It is variable in the remaining leads, i.e. it may be positive, negative, biphasic, or flat.

‘P’ WAVE Clinical Significance

Variation in the duration, amplitude, and morphology of ‘P’ wave help in the diagnosis of several cat disc- problems such as:

1. Right atrial hypertrophy: ‘P’ wave is tall (more than 2.5 mm) in lead II. It is usually pointed
2. Left atrial dilatation or hypertrophy: It is tall and broad-based or M shaped
3. Atrial extrasystole: Small and shapeless ‘P’ wave followed by a small compensatory pause
4. Hyperkalemia: ‘P’ wave is absent or small
5. Atrial fibrillation: ‘P’ wave is absent
6. Middle AV nodal rhythm: V wave is absent
7. Sinoatrial block: ‘P’ wave is inverted or absent
8. Atrial paroxysmal tachycardia: ‘P’ wave is inverted
9. Lower AV nodal rhythm: V’ wave appears after QRS complex.

‘Qrs’Complex

• It is also called the initial ventricular complex. The ‘Q’ wave is a small negative wave.
• It is continued as the tall ‘R’ wave, which is a positive wave. The ‘R’ wave is followed by a small negative wave, the ‘S’ wave.

‘Qrs’Complex Cause

• ‘QRS’ complex is obtained because of depolarization of ventricular musculature.
• ‘Q’ wave is due to the depolarization of the basal portion of the interventricular septum.
• ‘R’ wave is due to the depolarization of the apical portion of the interventricular septum and the apical portion of the ventricular muscle.
• And, the ‘S’ wave is due to the depolarization of the basal portion of the ventricular muscle near the atrioventricular sign.

‘Qrs’Complex Duration

0.08-0.10 seconds.

‘Qrs’Complex Amplitude

• ‘Q’wave: 0.1-0.2 mV.
• ‘R’ wave: 1 mV.
• ‘S’ wave: 0.4 mV.

‘Qrs’Complex Morphology

‘Q’ wave is normally small with an amplitude of 4 mm or less. It is less than 25% of the ‘R’ wave amplitude in leads 1, II, aVL, V5, and V6. In the remaining leads, its amplitude is < 0.2 mm.

From chest leads V1 to V6 ‘R’ wave becomes gradually larger. It is smaller in V6 than in V5. The ‘S’ wave is large in V1 and larger in V2. It gradually becomes smaller from V3 to V6.

‘Qrs’Complex Clinical Significance

Variation in the duration, amplitude, and morphology of ‘QRS’ complex helps in the diagnosis of several cardiac problems such as:

1. Bundle branch block: QRS is prolonged or deformed.
2. Hyperkalemia: QRS is prolonged.

‘T’ Wave: It is the final ventricular complex and is a positive wave.

‘T’ Wave Cause: T wave is due to the repolarization of ventricular musculature.

‘T’ Wave Duration: 0.2 second.

‘T’ Wave Amplitude: 0.3.mV.

‘T’ Wave Morphology

• The T’ wave is normally positive in leads 1, 2, and V5, and V6.
• It is normally inverted in lead aVR. It is variable in the other leads, i.e. it is positive, negative or flat.

‘T’ Wave Clinical Significance

Variation in the duration, amplitude, and morphology of ‘T’ wave helps in the diagnosis of several cardiac problems such as:

1. Acute myocardial ischemia: Hyperacute T wave develops. Hyperacute T wave refers to a tall and broad-based T wave with slight asymmetry
2. Old age, hyperventilation, anxiety, myocardial infarction, left ventricular hypertrophy, and pericarditis: T wave is small, flat, or inverted
3. Hypokalemia: T wave is small, flat, or inverted.
4. Hyperkalemia: T wave is tall and tented

‘U’ WAVE

‘U’ wave is not always seen. It is also an insignificant wave in ECG. It is supposed to be due to the repolarization of the papillary muscle.

‘U’ WAVE Clinical Significance

• The appearance of the ‘U’ wave in ECG indicates some clinical conditions such as:
• Hypercalcemia, thyrotoxicosis, and hypokalemia: ‘U’ wave appears. It is very prominent in hypokalemia.

2. Myocardial ischemia: Inverted ‘U’ wave appears.

intervals and segments OF ECG

‘P-R’ Interval

• It is the interval between the onset of the ‘P’ wave and the onset of the wave.
• The ‘P-R’ interval signifies the atrial depolarization and conduction of impulses through the AV node.
• It shows the duration of conduction of the impulses from the SA node to the ventricles through the atrial muscle and AV node.
• V wave represents the atrial depolarization. The short isoelectric (zero voltage) period after the end of the ‘P’ wave represents the time taken for the passages of depolarization within the AV node.

‘P-R’ Interval Duration

• The normal duration is 0.18 seconds and varies between
  0.12 and 0.2 seconds.
• If it is more than 0.2 seconds, that signifies the delay in the conduction of impulse from the SA node to the ventricles. Usually, the delay occurs in the AV node.
• So it is called the AV nodal delay.

‘P-R’ Interval Clinical Significance

Variation in the duration of ‘P-R’ intervals helps in the diagnosis of several cardiac problems such as:

1. ‘P-R’ interval is prolonged in bradycardia and first-degree heart block
2. It is shortened in tachycardia, Wolf-Parkinson-White syndrome, Lawn-Ganong-Levine syndrome, Duchenne muscular dystrophy, and type II glycogen storage disease.

‘Q-T’Interval

1. It is the interval between the onset of the ‘Q’ wave and the end of ‘T’ wave.
2. ‘Q-T’ interval indicates the ventricular depolarization and ventricular repolarization, i.e. it signifies the electrical activity in ventricles.

‘Q-T’Interval Duration

From 0.4-0.42 seconds.

‘Q-T’Interval Clinical Significance

1. ‘Q-T’ interval is prolonged in long ‘Q-T’ syndrome, myocardial infarction, myocarditis, hypocalcemia, and hypothyroidism
2. ‘Q-T’ interval is shortened in short ‘Q-T’ syndrome and hypercalcemia.

‘S-T’SEGMENT

The time interval between the end of the ‘S’ wave and the onset of the T wave is called the ‘S-T’ segment. It is an isoelectric period.

J Point

The point where the’S-T’ segment starts is called the ‘J’ point.

It is the junction between the QRS complex and the’S-T’ segment.

Duration of ‘S-T’ Segment

0.08.second.

‘S-T’SEGMENT Clinical Significance

• Variation in duration of the ‘S-T’ segment and its deviation from the isoelectric base indicate the pathological conditions such as:
• Elevation of the ‘S-T’ segment occurs in anterior or inferior myocardial infarction, left bundle branch block, and acute pericarditis. In athletes ‘S-T’ segment is usually elevated
• Depression of the ‘S-T’ segment occurs in acute myocardial ischemia, posterior myocardial infarction, ventricular hypertrophy, and hypokalemia
• The ‘S-T’ segment is prolonged in hypocalcemia
• ‘S-T’ segment is shortened in hypercalcemia.

‘R-R’ INTERVAL

‘R-R’ interval is the time interval between two consecutive ‘R’ waves.

‘R-R’ INTERVAL Significance

• ‘R-R’ interval signifies the duration of one cardiac cycle. Duration The normal duration of the ‘R-R’ interval is 0.8 seconds.
• Significance of Measuring ‘R-R’ Interval

Measurement of the ‘R-R’ interval helps to calculate:

1. Heart rate
2. Heart rate variability

Heart Rate

Heart rate is calculated by measuring the number of ‘R’ waves per unit time.

calculation of heart rate

• The vertical lines are used to calculate the time. The interval between two thick lines is 0.2 sec.
• The tone duration for 30 thick lines is 6 seconds. The number of ‘R’ waves (QRS complexes) in 6 seconds (30 thick lines) is counted and multiplied by 10 to obtain the heart rate.
• For the sake of convenience, the ECG paper has a special time marking at every 3 seconds.
• So it is easy to find the time duration of 6 seconds.

Heart Rate Variability

• Heart rate variability (HRV) refers to the beat-to-beat variations. Under resting conditions, the ECG of healthy individuals exhibits some periodic variation in ‘R-R’ intervals.
• This rhythmic phenomenon is known as respiratory sinus arrhythmia (RSA) since it fluctuates with the phases of respiration.
• The ‘R-R’ interval decreases during inspiration and increases during expiration.

Significance of HRV

HRV decreases in many clinical conditions such as cardiovascular dysfunctions (hypertension), diabetes mellitus, psychiatric problems (panic and anxiety), etc.