Electrocardiogram—Heart Muscle Activity

August Waller recorded human electrocardiogram (EGG) in 1887 with the capillary electrometer (a voltage-reading device), but the QRS complex was highly distorted 

In 1801, Volta demonstrated his battery in Paris before Napoleon, who made Volta a count and senator of the Kingdom of Lombardy. 

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Neural signals and muscular contraction both involve the flow of electric current, so can be used to monitor the activities of these tissues inside the body. The electrocardiogram (EGG) measures heart muscle activity, the electromyogram (EMG) measures skeletal muscle activity, and the electroencephalograph (EEG) measures brain activity. These measures are usually taken with electrodes placed somewhere near, and on either side of, the source of the signal. They are usually voltage measurements made without significant current flow. 

The electrocardiogram is a skin surface measurement of the electrical activity of the heart muscle (known as myocardium). With each heart beat ions move through the heart muscle and gradients of charge are formed. Taking a differential measurement of the electrical potential on the body surface at different locations generates different ECG vectors. These vectors are known as leads. The ECG leads are formed from three electrodes placed on the body, with an optional fourth electrode used as a reference (MIT, 2013). 

Classically the electrocardiogram (ECG) was believed to be a graphic tracing of the electric current produced by the myoneural activity associated with heart muscle excitation. The normal ECG, it was believed, showed deflections resulting from atrial and ventricular activity. The first signal, P, is due to atrial excitation with the QRS deflections arising from ventricular activity. T waves are believed to be due to ventricular recovery (repolarization) while the U waves are seen in the normal ECG and are accentuated in hypokalaemia (low potassium levels in the blood). 

Electrical activity in the heart can be picked up by electrodes placed on the skin and recorded as the familiar electrocardiogram (ECG). The ECG is a record of the sum of all action potentials in the heart as it contracts. Action potentials are generated by depolarization followed by repolarization of the cardiac muscle cell membrane. Depolarization is initiated by an influx of sodium ions into the cardiac muscle cells, followed by an influx of calcium ions. Repolarization is brought about by efflux of potassium ions. The phases of a cardiac action potential are shown in Eigure 4.3 where the depolarization is the change in resting membrane potential of cardiac muscle cells from —90 mV to 4-20 mV. This is due to influx of sodium ions followed by influx of calcium ions. Contraction of the myocardium follows depolarization. The refractory period is the time interval when a second contraction cannot occur and repolarization is the recovery of the resting potential due to efflux of potassium ions. After this the cycle repeats itself. 

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