ECG - basics

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n ECG is electrical recording of the heart’s conduction system. 




This is recorded on an ECG paper which has the following things –

1.    Patient’s name

2.    Time of recording

3.    Place of recording

4.    Date of recording

5.    Age of the patient

The ECG paper is basically a graph paper where a single large square is divided into 25 smaller squares thus, making the larger square of size 5mm x 5mm.

The horizontal line represents time period and the vertical line represents the voltage of the heart or the voltage of the conducting system of the heart to be precise. Each small square is of 1mm x 1mm size. The speed of ECG is 25mm/sec, i.e. it travels though the entire 25 smaller boxes in 1 second or in other words, it can be also said that 5 large square per seconds.


Close up view of the squares.

1mm corresponds to 0.04 seconds and 1 large box (5mm x 5mm) is equal to 0.2 seconds. Similarly, one small 1mm square corresponds to 0.1 mV of voltage thus, 1 large box (5mm x 5mm) corresponds to 0.5 mV of current. Hence 2 large boxes will attribute towards 1 mV of voltage.

Measurements of some of the important parameters of the heart –

1.    PR interval – 3 to 5 small boxes = 0.12 – 0.20 seconds

2.    PQ interval – 0.16 seconds

3.    QRS duration – 1.5 to 2.5 small boxes = 0.06 to 0.10 seconds

4.    QT interval – approximately equal to 0.42 seconds

5.    P wave voltage – 0.1 to 0.3 mV

6.    QRS complex voltage – 1.0 to 1.5 mV

7.    T wave voltage – 0.2 to 0.3 mV

Heart rate calculation – The rate of ECG paper is in seconds so on converting 60 x 25mm/sec into minutes we get 1500mm/minutes or in terms of boxes we simply divide it by 5, thus, we have 300 large boxes/ minute. 1 heart beat is 1 cardiac cycle, hence count the number of large squares between 2 RR. Thus, heart rate is calculated by 1500 / no. of small squares between 2 RR, or 300 / large squares between 2 RR. 

 


Normal ECG at II lead.


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