Electrical Activity of the Heart and ECGs

At rest, the heart muscle is polarized. This means that the cells are negatively charged on the inside and positively charged on the outside, due to different concentrations of ions on the inside and outside. These ions include sodium and potassium (most important), calcium and chloride.

The heart must depolarize before it can contract, which means ths insides of the cells must become positively charged and the outsides must become negatively charged. This depolarization is caused by an increase in the conductance of sodium across the membrane. The sodium brings with it calcium, to activate the contractile proteins. When the impulse spreads through transverse tubules (T-tubules) from the sarcolemma to calcium storage sites in the sarcoplasmic reticulum (membrane tubes near the sarcomeres, which cause contraction), calcium ions are released and cause contraction. Once the contraction is complete, the calcium is pumped back into the storage sites. During this phase, repolarization, the cells return to their resting action potentials. Potassium conductivity increases and the movement of sodium and calcium into the cell slows down.

An electocardiogram (ECG) measures the differences in membrane potentials on the heart's surface by determining the difference between 2 electrodes placed on the body's surface.

The SA node creates an impulse that carries a wave of depolarization over both atria. This is the p-wave. It immediately precedes atrial contraction and allows for simultaneous contraction of the left and right atria.

Once the impulse reaches the AV node, there is a slight delay to allow for blood to pass through the AV valve. Once the AV node is stimulated, the ventricles depolarize, creating the QRS complex.

The impulse then enters the AV Bundle, left and right bundle branches and purkinje fibers, which allow for rapid stimulation of the ventricular muscles. The contraction of the ventricles occur just after the appearance of the QRS complex. Atrial repolarization occurs at this time, but is usually hidden by the QRS complex on ECGs.

The T-wave represents the repolarization of the ventricles, just after ventricular contraction. The ST segment appears between the S and T wave, and detects deficiencies in coronary artery blood flow.

A refractory period occurs after contraction of cardiac muscle. During this time, the muscle is incapable of a full contraction, and an attempt at depolarization during this interval results in a reduced force of cardiac contraction.

Training's effects on ECG
Appearance of sinus bradycardia- Trained athletes have reduced resting heart rates. This is probably due to reduced stimulation from the sympathetic nervous system and increased stimulation from the parasympathetic nervous system. Also, a lower intrinsic heart rate may contribute to the lower RHR.

Other disturbances that may be visible in the ECGs of athletes include: AV conduction delays, wandering atrial pacemaker, ST segment elevation, T-wave inversion, substitute of the AV node as primary pacemakes. These are also common in those with ischemic heart disease. Athletes tend to have an increased stimulation of parasympathetic influences from the vagus nerve, which can disappear during exercise since parasympathetic stimulation decreases and sympathetic stimulation increases during exercise.