Thursday, January 12, 2012
This can be illustrated by consideration of two unipolar chest leads, one (e.g. V1) lying over the surface of the right ventricle and the other (e.g.V5) lying over the surface of the left ventricle. In the both these leads the initial wave (small r in V1 and small q in V5) is due to the same activation of the septum from left to right, and the subsequent wave (S in V1 and R in V5) is due to the spread of the same impulse through the left ventricle. A lead taken with an electrode which faces the cavities of the ventricles (e.g. a VR) shows a negative wave (deep Q) because the impulse is moving away from the electrode. There is considerable individual variation in the pattern of the QRS complexes of the 12 lead ECG in normal people. This is partly related to variation in the anatomical position of the heart; for example, a long narrow heart gives a different ECG from a broad, transverse heart.