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The Heart and ECG (electrocardiogam)

The heart is a muscular pump with 4 chambers; 2 at the top (atria- right and left) and 2 at the bottom (ventricles-right and left). 

Electrical impulses, which occur naturally, pass through these chambers in turn from top to bottom and this results in the collection and ejection of blood from the heart around the body.


Blood comes into the atria, they help to pass blood into the ventricles, the ventricles squeeze blood out again.

This stimulation, from the natural electrical impulse, happens repeatedly at regular intervals but always atria first followed by the ventricles second. Each time this happens it produces one cardiac cycle or heart beat.


The electrical activity, which passes through the heart, during this process is picked up on the surface of the skin by electrodes and this produces the different parts of the ECG.

It can also be felt as a pulse, commonly on the wrist and neck.

The following is a brief, simple and basic explanation of the features seen on a normal ECG pattern.

It does not cover any of the normal variants which commonly occur for a number of reasons.

Any concerns regarding your own ECG should always be brought to the attention of a health professional.


As mentioned previously, the heart has a natural pacemaker which sends signals through the heart in an ordered sequence to initiate a cardiac cycle or heart beat. This normally occurs  from top to bottom.


The signal causes contraction of the atria first, followed by the ventricles and then a relaxation phase of the ventricles before the cycle repeats.

This is called normal sinus rhythm (NSR), it occurs regularly and produces an ECG complex similar to the one shown.


NSR at rest= 60-100 bpm

Sinus tachycardia = greater than 100 bpm

Sinus bradycardia= less than 60 bpm


All of the above variations in rate can occur in a healthy heart, depending on the physiological situation, such as rest, exercise, sleep, stress etc.

1. Normal sinus rhythm, rest.

2. Sinus Tachycardia, normal during exercise or stress.

3. Sinus Bradycardia, normal during sleep

Atrial Fibrillation - this occurs due to an electrical glitch in the atria causes a rapid "quivering" motion rather than contraction of these chambers. Not all of these signals is followed by ventricular contraction and so the result is an irregular heat beat and an inefficient pumping action of the heart. 

This inefficiency means that blood moves more slowly through the heart making clots formation more likely. Added to this, the rapid atria activity means that these chambers become enlarged over time becoming a possible location for clots to sit. 


Atrial fibrillation.jpg
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