Normal ECG 

Intro:  A normal ECG tracing will have several predictable and reproducible components corresponding to electromechanical events in the cardiac cycle. That is important to understand how dose the mechanical part of the heart work and to compline it with the electrical perspective. At the following part of this article, I will reflect the most important mechanocardiac aspects in order to remember it and realise the normal ECG. Then I will figur out the non-normal ECG in a singular topic to understand the pathological concept of the heart. 

The Electrical activity of the heart: 

From the SA node which is the pacemaker of the heart producing 70 to 80 bets /min, it is located on the right atrium superolateral and has the ability to auto-depolarize. 

The AV node: is to delay the impulse comes from the SA node that the atria have time to empty their contents into the ventricles before they contract. it located in the posterior part of the septal wall of right atrium. 

The Hiss bundle and Purkinje fibers are represented by fibers that conduct excitation from the AV node to the ventricles. 

Isoelectric baseline: 

. Flat tracing free of positive or negative deflection in between waves and complexes.

. Represents period of electrical inactivity tin the cardiac cycle. 

the cardiac electrical cycle begins in the SA node of the right atrium. 

Waves: 

1. P wave: represents atrial depolarization Positive deflection in inferior and lateral leads. 

the atria infused with blood from vena cava in left and pulmonary veins in the left atrium. 

Electrical impuls of depolarization spreads throughout the atria via intranodal pathways and arrives the AV node on AV septum. 

2. T wave: represents ventricle repolarization and has a positive deflection. 

The intervals: 

PR interval: form the beginning of P wave to the initial defection of QRS complex.

It is representing the time needed for the electrical impulse to travel from the SA to AV. 

QT interval: From the beginning of the QRS complex to the end of T wave 

Represents ventricular depolarization, contraction and repolarization. 

RR interval: the Time between 2 successive QRS complexes used to calculate heart rate. 

300 \ large boxes between RR or 1500 \ small boxes. 

Segments: 

PQ segment: isoelectric segment between p wave and deflection of QRS complex. 

ST segment: isoelectric segment between the S wave and deflection of T wave 

TP segment: the isoelectric bassline between T wave and the initial deflection of the P wave. 

QRS complex: 

it is representing ventricular depolarization and composed of 3 waves: 

Q wave: negativ deflection 

R wave positive deflection 

S wave negativ deflection 

• The cardiac electrical cycle begins spontaneously in the SA node of the right atrium:

  • ECG: termination of TP segment, onset of the P wave 

  • Mechanical: atria infused with blood from passive venous filling:

    • Vena cava: fills the right atrium

    • Pulmonary veins: fill the left atrium

• Electrical impulse of depolarization spreads throughout the atria via the internodal pathways and arrives at the AV node located in the AV septum:

  • ECG: completion of the P wave:

    • The atria repolarize electrically during the ventricular portions of the cardiac electrical cycle.

    • Atrial repolarization is obscured by the QRS complex on ECG tracing.

  • Mechanical: atrial contraction, ventricular relaxation:

    • Tricuspid/mitral valves open

    • Pulmonic/aortic valves close

    • Ventricles fill with blood

• Electrical activity is slowed considerably by specialized conductive cells in the central portions of the AV node:

  • ECG: PQ segment (isoelectric baseline)

  • Mechanical: ventricles fill with blood from atrial contraction, atria relax and passively fill with blood

• Electrical activity resumes as the cardiac impulse arrives at the rapidly conducting pathways in the interventricular septum (atrioventricular bundle or His bundle):

  • ECG: onset of Q wave

  • Mechanical: ventricular septum contracts, passive atrial filling continues:

    • Tricuspid/mitral valves shut, chordae tendineae taut

    • Papillary muscles isometrically contract to maintain the integrity of tricuspid/mitral apparatus

• Electrical activity spreads toward the apex of the heart along the right and left bundle branches traversing the thickest portions of the myocardium:

  • ECG: R wave

  • Mechanical: continuation of ventricular free wall contraction, passive atrial filling:

    • Tricuspid/mitral valves “balloon” back into the atria

    • Pulmonic/aortic valves open

• Electrical activity terminates in the Purkinje fibers, penetrating the deepest portions of the myocardium near the endocardium:

  • ECG: S wave

  • Mechanical: completion of ventricular contraction, the continuation of passive atrial filling 

• Cardiac electrical activity plateaus briefly:

  • ECG: ST segment

  • Mechanical: ventricles begin to relax, passive atrial filling continues:

    • Tricuspid/mitral valves closed

    • Pulmonic/aortic valves closed

• Ventricular repolarization:

  • ECG: T wave and TP segment

  • Mechanical: ventricular relaxation, passive atrial filling continues:

    • Pressure gradient exists between filling atria and emptied ventricles

    • Ventricles begin to passively fill

    • Tricuspid/mitral valves partially open

• Another cardiac cycle begins.

How can we read a normal ECG?

The Axis: 

The electrical axis of the heart is a crucial concept in interpreting electrocardiograms (ECGs). It represents the overall direction of the electrical activity in the heart during a cardiac cycle. The axis is typically measured in degrees, and it helps in diagnosing various cardiac conditions.

To determine the electrical axis on an ECG, you need to look at the QRS complex, specifically the R wave, in leads I and aVF. Here's a step-by-step guide:

• Identify Leads I and aVF:

  • Lead I is the horizontal line on the ECG, and aVF is the inferior lead.

• Determine the QRS Complex in Leads I and aVF:

  • Look at the R waves in both leads.

• Analyze the QRS Complex:

  • If the R wave is taller in lead I, the axis is deviated to the left.

  • If the R wave is taller in lead aVF, the axis is deviated to the right.

• Calculate the Axis:

  • If the R waves are equal in height, the axis is normal.

  • If the axis is deviated, you can estimate the degree of deviation based on standard criteria.

• Normal Axis Range:

  • The normal axis is roughly between -30 degrees and +90 degrees.

Determining the degree of deviation in the electrical axis on an electrocardiogram (ECG) involves assessing the amplitude of the R waves in specific leads. Here's a brief explanation:

Left Axis Deviation:

• If the R wave in lead I is taller than in lead aVF, it indicates left axis deviation.

• To estimate the degree of deviation, observe the QRS complex in lead I.

• If lead I shows a predominantly positive QRS complex, the axis is likely mildly deviated to the left.

• If lead I shows a strongly positive QRS complex, the left axis deviation is more pronounced.

Right Axis Deviation:

• If the R wave in lead aVF is taller than in lead I, it suggests right axis deviation.

• To assess the degree of deviation, examine the QRS complex in lead aVF.

• A predominantly positive QRS complex in lead aVF indicates mild right axis deviation.

• A strongly positive QRS complex in lead aVF suggests more significant right axis deviation.

Normal Axis:

• If the R waves in leads I and aVF are roughly equal, the axis is considered normal.