Cardiac pacemaker ventricles, within the atrial septum. If the AV node also fails, Purkinje fibers are occasionally capable of acting as the default or "escape" pacemaker. The reason Purkinje cells do not normally control the heart rate is that they generate action potentials at a lower frequency than the AV or SA nodes. Cardiac pacemaker The contraction of cardiac muscle (heart muscle) in all animals is initiated by electrical impulses known as action potentials. The rate at which these impulses fire controls the rate of cardiac contraction, that is, the heart rate. The cells that create these rhythmic impulses, setting the pace
Heart rate turbulence the heart. Especially after a heart attack, sympathetic nervous activity tends to be increased. A healthy HRT indicates the presence of a healthy amount of parasympathetic activity, countering sympathetic activity. To take a wider view, however, it may be that a healthy HRT is also an indication of a healthy brain, and is the reason small HRT also predicts a likelihood of death from non- cardiac causes as well as from cardiac causes. Heart rate turbulence Heart rate turbulence (HRT) is the return to equilibrium of heart rate after a premature ventricular contraction (PVC). It consists of a brief speed-up