Normal electrical pattern of a heart beat

Cardiac Arhythmias are also called cardiac DYSRHYTHMIAS and can cause an irregular heartbeat. The heart beats in a coordinated fashion because it has a CONDUCTION SYSTEM, although the myocytes (heart cells) beat on their own without it, and the heart will beat slowly (about half speed) anyway. The above diagram represents what is seen on an ECG during a normal heart beat. It should occur regularly in SINUS RHYTHM (there is slight variation with breathing). The P bit is the contraction of the atria, the Q-R-S bit is the contraction of the ventricles. and the T bit is the ventricles recovering. The U wave may not be seen. It is probably a bit like the T-wave (ie it is a repolarization).

A lot can be told from an ECG – entire books have been written on it – and not just different rhythms. If the blood supply to the heart is reduced, (ischaemia or ischemia), the flat line at the end of S and beginning of T is raised; if part of the heart muscle dies (INFARCTION, that is, a heart attack) then the T wave swings round and dips down. Because an ECG looks at the heart from multiple viewpoints (hence all the leads), these changes may occur in some views but not others; hence an ECG has multiple traces.

Cardiac arhythmias come in all sorts. The simplest is that the heart may speed up (TACHYCARDIA is the medicalese) or slow down (BRADYCARDIA). The atria and the ventricles may become electrically separated (HEART BLOCK) but the commonest is ATRIAL FIBRILLATION (AF).

The ECG in AF shows no P waves before the QRS complex. The atria ‘flutter’ rather than pump (hence the alternative name of ATRIAL FLUTTER). It doesn’t especially bother a normal heart as the ventricles fill and work relatively normally, but it’s important because blood may clot on the inside walls of the atria. This, in itself, does not matter. It only matters because bits of the clot (EMBOLI) may break off, go into the circulation, and lodge in the brain, where they cause a STROKE (of which more next week).

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