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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