Master Electrocardiogram – Part 1: ECG basics

• ECG is the graphical representation of the electrical activity of the heart. It shows us where the electrical impulses start and how they propagate or travel through the various layers of the heart. ECG doesn’t show us how well the heart is pumping the blood. We will learn about the ECG basics in this blog post.
• ECG is a tool. Clinical history and Examination are a must.
• You will become a pro in ECGs only with practice. It is all about pattern recognition
• Most abnormalities in the ECGs can be explained by reasoning
• ECGs become easy to understand if you start the journey from first principles.
• Electrical changes (Depolarisation) result in muscle contraction. 

• Rhythm of the heart: Electrical activation usually begins in the SA node in the right atrium. This pattern of electrical activation is called sinus rhythm. So, the word rhythm can indicate the part of the heart initiating the electrical activity. Depending on this, the heart’s contraction can be regular or irregular.
• The atrial muscle mass is less. Hence, the deflection on an ECG paper caused by the electrical activity of the atria is small. Ventricular mass is more than atria, so ventricular electrical activity causes a larger deflection on ECG paper.

ECG paper

• Each small box is 1mm, the large box is 5mm
• Standard paper speed is 25mm/sec (5 large boxes in a second, 300 large boxes in a minute).
• So each large square represents 0.2 seconds or 200milli seconds
• Voltage calibration is set such that 1mV causes 1cm of upward deflection

Parts of ECG:

• Depolarisation of atria – P wave
• Depolarisation of ventricles: QRS complex
  • The first deflection downward is called the Q wave
  • Upward deflection is called an R wave. It doesn’t matter if it is preceded by Q wave or not
  • Any deflection below the baseline following an R wave is called an S wave. It doesn’t matter if there is a preceding Q wave or not.
  • QRS complex duration indicates how long it takes for the electrical impulse to travel through the ventricular mass. It doesn’t mean the contraction of the ventricles. 
  • The duration of the QRS complex is usually 3 small boxes or less (120ms)
  • If there is any conduction abnormality, the QRS complex becomes wider, indicating it is taking slightly more time for the electrical impulse to travel through the ventricles.
• Repolarisation of ventricles: T wave ( return of ventricular mass to resting electrical state)
• PR interval: Starting of the P wave to the beginning of the QRS complex.
  • Normal: 120-200 milliseconds, i.e.3-5 small squares
  • What if the PR interval is short? It can mean two things: the atrial repolarisation has begun further down, close to the AV node, or abnormally fast conduction from the atria to the ventricles.
• QT interval
• ST segment

ECG leads

We can refer to lead as a view of the heart for easy understanding. One lead is one view of the electrical activity of the heart. It is different from electrodes. Sometimes, one lead (for example, Lead 1) requires two electrodes.

For a full view of the heart, 12 lead ECG is routinely done – 12 electrical views of the heart

• There are four limb electrodes. One electrode is attached to each limb. These 4 limb electrodes provide us with 6 leads or 6 different views of the heart in the frontal/ vertical plane

• Similarly, 6 electrodes are attached over the chest wall, resulting in 6 leads/views of the heart in the horizontal plane.

Placement of electrodes

RA	On the right arm, avoiding thick muscle.
LA	In the same location where RA was placed, but on the left arm.
RL	On the right leg, lower end of inner aspect of calf muscle.  (Avoid bony prominences)
LL	In the same location where RL was placed, but on the left leg.
V1	In the fourth intercostal space (between ribs 4 and 5) just to the right of the sternum (breastbone)
V2	In the fourth intercostal space (between ribs 4 and 5) just to the left of the sternum.
V3	Between leads V2 and V4.
V4	In the fifth intercostal space (between ribs 5 and 6) in the mid-clavicular line.
V5	Horizontally even with V4, in the left anterior axillary line.
V6	Horizontally even with V4 and V5 in the mid-axillary line.

(Note: in women with large breasts, V4-6 leads should be placed under the breast surface as close to the 5th IS as possible)

Leads

To understand leads, try to understand Einthoven’s triangle. It is an imaginary triangle formed by joining the electrodes: RA, LA, LL

Shape of QRS Complex

The ECG machine is such that 

• If a wave of depolarisation travels in the direction of a lead, it is recorded as a positive deflection on the ECG paper.
  • Positive deflection in the QRS complex is indicated by the fact that the R wave is greater than the S wave
• If the wave of depolarisation is traveling in the opposite direction of the lead, it is recorded as a negative deflection on the ECG paper.
  • The S wave is greater than the R wave, indicating negative deflection
• When the depolarisation wave is moving at the right angle to the lead, the R wave and S waves are of equal size

• Cardiac axis: The average direction of spread of the wave of depolarization through the ventricles is called the cardiac axis. In a normal 11’0 clcok – 5’o clock axis,positive deflection is noted in lead 1,2,3

• If the right ventricle becomes hypertrophies, right ventricle has more effect on qrs complex than left ventricle and the cardiac axis shifts to the right. The deflection in lead 1 will become negative as the wave of depolarisation is spreading away from it, and positive deflection is seen in lead 3 as the depolarisation wave is now predominantly along lead 3.

• Similarly, in left axis deviation, there is a negative deflection in Lead 3. Left axis deviation is not significant until there is a negative deflection in both lead 2 and 3.

The shape of QRS complexes in chest leads is influenced by two key points:

1. The septum between the ventricles is depolarised before the ventricles, and the depolarisation wave spreads across the septum from left to right.
2. In the normal heart, there is more muscle in the wall of the left ventricle than in that of the right ventricle – so the left ventricle exerts more influence on the ECG pattern than the right ventricle.

Leads V1, V2 look at the right ventricle, leads V3, V4 look at the septum and Leads V5, v6 look at the left ventricle. (refer fig below)

1. So when a wave of electrical activity reaches ventricles, septum is depolarised first from left to right. In the right ventricular lead (v1) positive deflection (R wave) is seen, whereas in left ventricular lead V6, negative deflection (q wave) is seen
2. As the ventricular mass gets depolarised (dominated by the left ventricle), negative deflection(s wave) is seen in the right ventricular lead, and positive deflection (R wave ) is seen in the left ventricular lead
3. After depolarisation, the electrical activity returns to baseline

So, the chest leads are progressing from V1 (downward) to V6 (upward). The transition point where R wave and S waves are equal indicates the interventricular septum

Implication: If the right ventricle is enlarged and occupies more of the precordium, the transition point moves from V3/v4 to V5/v6. As seen below, the heart can be thought of as rotated clockwise, a characteristic of chronic lung disease.

Reporting an ECG:

1. Rhythm and rate
2. Conduction intervals 
3. cardiac axis 
4. a description of the QRS complexes 
5. a description of the ST segments and T waves.

Next Post: