The PQRST Heart Trace |
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An ECG will give a trace of a healthy heart that looks like the graph below. Each trace is a single heartbeat and therefore lasts about 0.8 seconds. The main features of the trace are labelled as P, Q, R, S and T according to convention. The "P" wave corresponds to atrial depolarisation and contraction. The "QRS" complex relates to the depolarisation and contraction of the ventricles, it is much larger than the "P" wave due to the relative muscle masses of the atria and ventricles - and masks the repolarisation and relaxation of the atria. The repolarization and relaxation of the ventricles can be seen in the form of the "T" wave, the repolarisation of the atria being masked by the "QRS" complex. The contraction of cardiac muscle is regulated in a centre located in the right atrium known as the sinus node. Cardiac muscle cells have a special property: they spontaneously depolarize at various rates (i.e. the charge of their membranes changes at a given rate without external stimuli). The resultant coordinated contraction leads to the ECG waveforms that need to be known for the examination. Timing is important – it shows whether the heartbeat is normal of not! You should know the timings:
To measure and display an ECG waveform, the pd between two chosen points on the body surface must be amplified from about 1 mV to about 1 V. The allows it to be displayed on an oscilloscope, a chart recorder or a computer VDU screen. The amplifier circuit must have the following characteristics:
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