When the electrical signal of your heart goes to contract the ventricles, to squeeze out blood, it divides into two sections first. One goes to the right ventricle and one to the left. The right section doesn't divide any further, but the left side does. Imagine these are wires which connect to different areas of the heart. The left divides into three areas of the left ventricle. One goes to the middle wall, one goes round to the back and one goes down the front. A hemiblock means that one of these is blocked, the wire is broken. Imagine it like this. You have a large area of heart muscle and you want this to all activate (contract) at the same time. So, the more wires you have, going to different areas, the more likely you are to achieve this. If you have just one wire going to the back of the left side, then the pulse would take longer to feed the back, travel across the surface to the front and travel to the middle. The rear would contract before the front. Also the right ventricle would contract the same time as the rear left, but before the left front. Any variation like this is called an axis deviation but the heart is ok up to a certain amount. Above the threshold, problems can start to occur. The hemiblock you ask about is by far the most common. In the case of this hemiblock is the axis deviation is within limits, then nothing needs to be done and this is often the case. This problem can also occur with normal or diseased hearts and so it not an indicator of anything more sinister. If the axis deviation is above limits you would probably know about it by the way you feel and a pacemaker can sort the problem out. With 2 wires working out of the 3, the signal is likely to be travelling around the left side quickly enough. I hope this helps but if you need it in more basic terms or have any other questions then don't hesitate to ask.
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