1.13.1 Heart Rate Definition

Heart Rate Definition is the frequency at which the heart contracts, expressed as the number of complete cardiac cycles occurring within one minute. Heart rate is determined by the rate of spontaneous electrical depolarization generated by the sinoatrial node, the heart's primary pacemaker, and is continuously modulated by the autonomic nervous system, circulating hormones, and local physiological conditions to match the body's changing metabolic demands.

Origin of Heart Rate

Heart rate arises from the intrinsic electrical activity of specialized pacemaker cells within the heart.

Sinoatrial Node Pacemaker Activity

The sinoatrial node, located in the wall of the right atrium, generates rhythmic electrical impulses through spontaneous, gradual depolarization of its pacemaker cells; the frequency of these spontaneous impulses sets the intrinsic heart rate.

Propagation Through the Conduction System

Each impulse generated at the sinoatrial node spreads through the atria, passes through the atrioventricular node, and travels through the bundle of His and Purkinje fibers to activate the ventricles, coordinating one complete heartbeat per impulse.

Regulation of Heart Rate

Although the sinoatrial node possesses its own intrinsic firing rate, actual heart rate is continuously adjusted by external influences.

Autonomic Nervous Regulation

Sympathetic nervous stimulation increases the rate of sinoatrial node depolarization, raising heart rate, while parasympathetic (vagal) stimulation decreases the rate of depolarization, lowering heart rate; the balance between these two influences determines the prevailing heart rate at any given moment.

Hormonal and Local Influences

Circulating catecholamines, such as epinephrine, can further increase heart rate, while factors such as body temperature and blood chemistry can also modulate the intrinsic firing rate of the sinoatrial node.

Quantitative Expression

Heart rate is measured as the number of beats occurring per minute (bpm), a value that can be determined directly by counting pulse or heart sounds, or derived from the interval between successive heartbeats.

Relationship to Cardiac Output

Heart rate combines with stroke volume to determine total cardiac output, making it one of the two principal variables governing the heart's overall pumping performance.

Diagrammatic Summary

Clinical Relevance

Heart rate is one of the most fundamental and frequently measured vital signs, and abnormalities in its regulation, whether excessively slow (bradycardia) or excessively fast (tachycardia), can reflect underlying disturbances in autonomic balance, conduction system function, or broader cardiovascular and systemic disease.