Hi friends!!!
In our previous blogs, we have discussed about Electrocardiogram and 12 Lead ECG Placement. Please check out for the link below.
In today's blog we are going to discuss about ECG leads placement.
The correct interpretation of ECG waveform is very much useful in proper diagnosis of heart conditions. Therefore it is important for every healthcare professionals to know about interpretation of ECG waveform.
Let's get into the topic.
HOW TO READ ECG WAVEFORM?
The normal ECG waveform consists of P wave, QRS
complex and T wave. The knowledge of different characteristics of waveform can
be helpful in interpretation of the waveform. The characteristics to look out
in ECG waveform is amplitude, duration and deflection.
The normal heart rate is between 60-100 bpm. The heart rate in case of Tachycardia will be >100 bpm. The heart rate in case of bradycardia will be <60 bpm.
P WAVE
The P wave is the sum of electrical signals from two
atria. The amplitude is 2-2.5 mm. The deflection is from +in, I, II, AVF, V2-V6.
The duration of P wave is from 0.06-0.12 sec. In case of atrial fibrillation P
wave is absent.
PR INTERVAL
The PR interval is from the beginning of P wave to the start of Q wave. It is the time interval from the start of atrial depolarization to the start of ventricular depolarization. It gives information on normality of the signal conduction from atria to ventricles. The PR interval is usually 120-200 ms long.
PR SEGMENT
The PR segment is the time taken for electrical impulse to travel from sinus node to ventricles, through AV node. The PR segment happens because of the delay made by atrioventricular node, which slows down the electrical depolarization before it proceeds to the ventricles.
QRS COMPLEX
The largest portion of ECG waveform is QRS complex
which is from start of the start of the Q wave to the end of the S wave. The
QRS complex is due to the depolarization of ventricles.
The amplitude of QRS complex is 5-30mm. The deflection
is by +in, I, II, III, AVL, AVF, V4-V6. The QRS duration longer than 110
milliseconds indicates the presence of conduction delay.
T WAVE
The T wave is the final portion of the ECG waveform.
The T wave is due to the start of ventricle repolarization. The amplitude of T
wave is approximately 0.5mm. The deflection is from I, II, V3-V6. The duration
of T wave can be 0.1-0.25 seconds.
ST SEGMENT
The ST segment is from the end of the S wave to
the beginning of the T wave. The duration of the ST interval can be approximately
0.08 seconds.
QT INTERVAL
The QT interval represents how fast the ventricles
repolarize. The QT interval decreases when the heart rate increases. The
duration of the QT interval is approximately at the average of 0.4 seconds.
HOW EACH LEADS VIEW THE
HEART?
Each of 12 ECG leads can give information about
electrical signal passing through the heart from different planes. The limb
leads like I, II, III, AVR, AVL and AVF views heart in vertically, which is
used to measure electrical axis. The direction of travel of depolarization signal
through the heart is called electrical axis.
The basic concept of ECG recording is that when depolarization
signal travels towards the recording lead, the graph is plotted positive and
upwards, when depolarization signal travels away from the recording lead, the
graph is plotted negative and downwards. Here electrical axis is usually
downwards and towards the left.
Each lead can measure the electrical signal of the
heart at different directions. If we see the above diagram, the lead I is used
to view heart at 0 degree and it is taken as reference point. Based on this
reference point, the direction of other leads towards the heart is calculated.
For example: The leads II measure electrical signal of heart at 60 degree.
The chest leads like V1, V2, V3, V4, V5 and V6 views
heart horizontally.
HOW TO DETERMINE HEART RATE
USING ECG WAVEFORM?
RR interval is the distance between R point of one QRS
complex to the R point of other QRS complex in a ECG graph. If two R points are
separated by one large box in ECG strip, then heart rate will be 300 bpm. If
two R points are separated by two large boxes in ECG strip, then heart rate will
be 150 bpm. Likewise the heart rate is calculated.
For example, let’s consider a single strip of ECG
graph is measured for a time period of 10 seconds. Now count the number of QRS
complexes in a ECG strip and multiple by 6. If an ECG graph has 10 QRS complex
(10 QRS complex for 10 seconds and therefore in order to calculate heart rate
for 1 minute) multiply 10*6. That is the heart rate per minute is 60 bpm. If
there are 9 QRS complex in a ECG strip, then multiply 9*6. That is 54 bpm.
Now I hope you have acquired some knowledge about reading an ECG waveform. We will discuss in detail about types of ECG machine in our future blogs.
NOTE: Dear friends!!!... Please do comment a topic related to Biomedical, so that we can discuss it in future blogs.
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