Physiology of Cardiac Conduction

In an adult with a healthy heart, the heart rate is usually about 72 beats per minute.

The excitatory and electrical conduction system of the heart is responsible for the contraction and relaxation of the heart muscle. The sinoatrial node (SA node) is the pacemaker where the electrical impulse is generated. This node is located along the posterior wall of the right atrium right beneath the opening of the superior vena cava. It is crescent shaped and about 3 mm wide and 1 cm long.

The impulse travels from the SA node through the internodal pathways to the atrioventricular node (AV node). The AV node is responsible for conduction of the impulse from the atria to the ventricles. The impulse is delayed slightly at this point to allow complete emptying of the atria before the ventricles contract.The impulse continues through the AV bundle and down the left and right bundle branches of the Purkinje fibers. The Purkinje fibers conduct the impulse to all parts of the ventricles, causing contraction (Guyton, 1982).

Abnormal heart rhythms occur for several reasons.

  1. The vagal stimulation of the parasympathetic nervous system can cause a decrease in the rate at the SA node and can also decrease the excitability of the AV junction fibers. This causes a slowing of the heart rate, and in severe cases a complete blockage of the impulse through the AV junction.
  2. Sympathetic stimulation also effects cardiac rhythm and conduction. It increases the rate at the SA node and increases the rate of conduction and excitability throughout the heart. It also increases the force of myocardial contraction. Subsequently, the overall workload on the heart is increased.
  3. A small area of the heart can become more excitable than normal, which causes abnormal heart beats called ectopy. Ectopic foci are usually caused by an irritable area in the heart. This irritability can be caused by ischemia, stimulants such as nicotine and caffeine, lack of sleep or anxiety (Guyton, 1982).
  4. Aberrant conduction pathways can cause arrhythmias by providing an alternate route for the wave of depolarization. Normally, atrial and ventricular tissues are isolated by nonconductive fat and connective tissue. Aberrant pathways form a short circuit bridge across the nonconductive tissue. Hence, tissue can be depolarized prematurely or out of sync.
  5. Inherited or acquired channelopathy can predispose an individual to dangerous arrhythmias, notably torsades de pointes. Faulty ion channels may result in prolonged repolarization, lengthening the QT interval. The heart is most vulnerable during repolarization. Channelopathies can be due to "leaky" or slow closing sodium channels, ineffective potassium channels or drugs and toxins which can affect the transport of ions across these channels.

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