Learn the ECG

 

Our heart has its own electrical system, consisting of specialized nervous tissue able to autonomously generate electrical stimuli that do contract the heart muscle.

The part that generates the heart's electrical impulse is the sinoatrial node (SA), which has an intrinsic rate of 60 - 100 beats per minute (bpm) and is located in the upper wall of the right atrium, at the junction with the superior vena cava. The atrioventricular node is located in the lower wall of the right atrium to the base of the interatrial septum.  The AV node picks up the signal sent by the SA node and transmits it through the atrioventricular (AV) bundle. The AV bundle is a strand of conductive tissue that runs through the interatrial septum and into the interventricular septum. The AV bundle splits into left and right branches in the interventricular septum and continues running through the septum until they reach the apex of the heart. 

Purkinje fibers that carry the signal to the walls of the ventricles, stimulating the cardiac muscle cells to contract in a coordinated manner to efficiently pump blood out of the heart.

The heart's electrical activity can be recorded and then analyzed studied and interpreted with the execution of the electrocardiogram (ECG).

Standardization ECG:

1 small square = 0.04 sec = 40 ms = 1mm

1 square (5 small squares) = 0.2 sec. = 200 ms. = 5mm

Paper speed = 25 mm / sec.

10mm height = 1 mV

 

The ECG consists of waves, intervals, segments.

Waves and their meaning:

P wave = atrial depolarization (systole)

QRS complex = ventricular depolarization

T wave = ventricular repolarization (diastole)

 

Segments are the spaces between two waves, they are always on the baseline of the ECG which is called "isoelectric line." The PR segment (or PQ segment) is that space between the end of the P wave and the beginning of the QRS complex. The ST segment is the space between the end of the QRS and the beginning of the T wave.

The intervals include both the waves and the segments, for example the PR interval (or PQ) runs from the beginning of the P wave at the beginning of the QRS, while the QT interval from the beginning of the QRS wave to the end T.

 

The P wave has a duration of approximately 100 ms (2.5 small squares)

The PR interval (from the beginning of the P wave at the beginning of the QRS) has a duration of 120-200 ms (3-5 small squares)

The QRS interval lasts 100 ms (2.5 small squares)

The QT interval (from beginning of QRS to the end of the T wave) lasts from 320 to 400 ms (8-10 small squares)

 

To execute a 12-lead ECG, the patient should be lying supine and relaxed to prevent muscle tremors. The electrodes (leads) are placed according to the international color standard.

 

Limb leads:

Red = right arm (usually on wrist)

Black = right leg (or left the case of limb amputee)

Yellow = left arm

Green = left leg (or right if the limb amputee)

 

The precordial leads:

V1 (red) = fourth intercostal space (from the top) Right parasternal

V2 (yellow) = fourth left intercostal space parasternal

V3 (green) = halfway between V2 and V4

V4 (brown) = 5th left intercostal space, midclavicular

V5 (black) = 5th intercostal space, anterior axillary

V6 (purple) = 5th intercostal space mid axillary

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