Heart Rate and Arterial Pulse

Introduction

The rhythmic beating of the heart determines heart rate and generates the arterial pulse, two vital indicators of cardiovascular function. Variations in autonomic activity influence heart rate, while characteristic pulse patterns provide important clinical clues to conditions such as heart failure and aortic valve disease.

• In healthy individuals, the pulse rate is normally equal to the heart rate.
• Each ventricular contraction ejects blood into the arterial system, producing a palpable arterial pulsation.
• Therefore, the arterial pulse reflects the frequency of ventricular ejections and corresponds closely to the heart rate under normal physiological conditions.

Heart Rate

• The normal heart rate in adults ranges from 60 to 100 beats per minute.
• A heart rate below 60 beats per minute is termed bradycardia, whereas a rate above 100 beats per minute is termed tachycardia.
• Heart rate is generally higher in infants and children and tends to decrease with advancing age.
• Under normal conditions, heart rate reflects the firing frequency of the sinoatrial node, which serves as the heart’s primary natural pacemaker.

Physiological Variations

• Heart rate varies continuously under the influence of physiological factors and autonomic nervous system activity.
• Heart rate is higher in infants and children because of greater metabolic demands and gradually declines with advancing age.
• In healthy adults, females generally have a slightly higher resting heart rate than males.
• A daily rhythm is present, with heart rate usually higher during daytime activity and lower during sleep because of increased parasympathetic influence.
• During inspiration, heart rate increases slightly, whereas during expiration it decreases, producing respiratory sinus arrhythmia, a normal physiological phenomenon.
• An increase in body temperature accelerates heart rate, while a decrease in temperature slows cardiac activity.
• Environmental heat tends to increase heart rate, whereas cold environments generally reduce it.
• Food intake may transiently increase heart rate because of enhanced metabolic activity and increased blood flow to the gastrointestinal tract.
• Changing posture from the supine to the standing position usually increases heart rate due to reduced venous return and activation of the baroreceptor reflex.
• Exercise markedly increases heart rate through sympathetic stimulation and withdrawal of parasympathetic tone.
• Anticipation of physical activity or emotional excitement may also elevate heart rate before exercise begins through activation of higher brain centers.

Regulation of Heart Rate

• Heart rate is a dynamic physiological variable that responds rapidly to internal and external influences.
• Regulation of heart rate occurs predominantly through the autonomic nervous system.
• Parasympathetic stimulation through the vagus nerve decreases heart rate, whereas sympathetic stimulation increases it.
• At rest, heart rate is primarily determined by vagal tone, making parasympathetic activity the dominant regulator.
• Mechanisms controlling heart rate are broadly classified into neural and humoral regulatory pathways.

Neural Control Mechanisms

Autonomic Control

• Neural regulation is the principal mechanism controlling heart rate and operates through autonomic, reflex, and higher-center influences.
• The autonomic nervous system continuously modulates the activity of the sinoatrial node, the heart’s natural pacemaker.
• Under resting conditions, parasympathetic activity predominates over sympathetic activity, resulting in a lower resting heart rate.
• The intrinsic heart rate, observed in a completely denervated heart, is approximately 100–110 beats per minute.
• The normal resting heart rate is lower than the intrinsic rate because of continuous inhibitory vagal influence.
• Parasympathetic fibers supplying the heart arise from medullary cardiovascular centers and travel through the vagus nerves.
• The right vagus nerve primarily influences the sinoatrial node, whereas the left vagus nerve exerts a greater effect on the atrioventricular node.
• Stimulation of the vagus nerve decreases heart rate by releasing acetylcholine, which suppresses pacemaker activity in the sinoatrial node.
• Acetylcholine increases potassium permeability and slows spontaneous depolarization of pacemaker cells, thereby reducing the frequency of impulse generation.
• Nodal tissues contain abundant cholinesterase, the enzyme responsible for rapid breakdown of acetylcholine.
• Because acetylcholine is inactivated quickly, the effects of vagal stimulation are brief and highly adjustable.
• This rapid neurotransmitter turnover enables precise, beat-to-beat regulation of heart rate according to the body’s physiological needs.
• Through these mechanisms, parasympathetic control plays a dominant role in maintaining normal resting cardiac rhythm.

Sympathetic Control

• Sympathetic fibers regulating heart rate arise from the lower cervical and upper thoracic segments of the spinal cord.
• Stimulation of these fibers increases the firing rate of the sinoatrial node, resulting in an increased heart rate.
• The neurotransmitter norepinephrine released from sympathetic nerve endings acts mainly on beta-1 adrenergic receptors in pacemaker tissue.
• Unlike acetylcholine, norepinephrine is not rapidly degraded by a specific enzyme at the neuroeffector junction.
• Most released norepinephrine is reabsorbed into nerve terminals, while a portion enters the circulation.
• Sympathetic effects are mediated through the cyclic adenosine monophosphate signaling pathway, which develops more slowly than parasympathetic responses.
• Consequently, sympathetic stimulation produces a more prolonged increase in heart rate compared with vagal stimulation.

Reflex Control

Reflex mechanisms help regulate heart rate as part of the integrated control of cardiovascular function and arterial pressure.

Baroreceptor Reflex

• The baroreceptor reflex is one of the most important short-term regulators of heart rate.
• Baroreceptors located in the carotid sinus and aortic arch are stimulated when arterial blood pressure rises.
• Sensory impulses travel through the glossopharyngeal and vagus nerves to the nucleus tractus solitarius in the medulla.
• Activation of this center suppresses sympathetic outflow from cardiovascular control centers.
• Reduced sympathetic activity decreases stimulation of the sinoatrial node, leading to a lower heart rate.
• Simultaneously, vagal activity increases, producing bradycardia.
• When arterial pressure falls, baroreceptor firing decreases, sympathetic activity rises, and heart rate increases to help maintain adequate blood pressure and tissue perfusion.

Chemoreceptor Reflex

• Chemoreceptor reflexes are activated by changes in blood chemistry, particularly decreased oxygen levels, increased carbon dioxide levels, and increased hydrogen ion concentration.
• Peripheral chemoreceptors located in the carotid and aortic bodies detect these changes and transmit signals to cardiovascular centers in the medulla.
• Activation of chemoreceptors generally produces bradycardia, although associated respiratory and sympathetic responses may modify the final heart rate.

Bainbridge Reflex

• The Bainbridge reflex is triggered by increased venous return and stretching of receptors located at the venoatrial junctions.
• This reflex increases heart rate and helps accommodate an increased blood volume, such as after rapid intravenous fluid administration or blood transfusion.
• The Bainbridge reflex counteracts excessive slowing of the heart and functionally opposes the baroreceptor reflex under certain conditions.

Cushing’s Reflex

• Cushing reflex occurs when reduced cerebral perfusion strongly stimulates medullary cardiovascular centers.
• The resulting rise in arterial pressure is followed by reflex bradycardia through activation of baroreceptors.

Control by Higher Centers

• Higher brain centers influence heart rate through their connections with cardiovascular regulatory centers in the brainstem.
• Stimulation of the motor cortex, frontal cortex, or thalamic regions can increase heart rate.
• Emotional states such as anxiety, fear, excitement, and stress increase heart rate through activation of the limbic system and enhanced sympathetic activity.

Humoral Control Mechanisms

Hormonal Control

• Humoral mechanisms regulate heart rate through circulating hormones and changes in blood chemistry.
• Catecholamines and thyroxine increase heart rate by enhancing pacemaker activity and sympathetic effects on the heart.

Chemical Control

• Mild hypoxia generally increases heart rate, partly through stimulation of catecholamine release.
• Severe hypercapnia and acidosis tend to depress cardiac pacemaker activity and may reduce heart rate.

Arterial Pulse

Physiological Aspects

Definition

• Arterial pulse is the rhythmic expansion and recoil of an artery produced by pressure waves generated during each ventricular systole.
• These pressure waves travel along the arterial walls and can be palpated at accessible arteries.

Clinical Importance

• The radial pulse is most commonly examined in clinical practice because it is easily accessible and reliable.
• Pulse assessment is an essential component of evaluating the body’s vital signs, along with blood pressure, respiratory rate, and body temperature.
• Examination of the arterial pulse provides valuable information about cardiac function, circulatory status, and vascular health.
• Pulse characteristics may help identify abnormalities in heart rhythm, stroke volume, and arterial compliance.

Normal Pulse Rate

• Under normal conditions, the pulse rate is equal to the heart rate because each ventricular contraction generates a detectable arterial pulse wave.
• The normal pulse rate in adults ranges from 60 to 100 beats per minute.
• A difference between heart rate and pulse rate is termed a pulse deficit, indicating that some cardiac contractions fail to produce an effective peripheral pulse.
• Pulse deficit is absent in healthy individuals, where pulse rate and heart rate correspond exactly.
• A pulse rate above 100 beats per minute is termed tachycardia, whereas a rate below 60 beats per minute is termed bradycardia.
• Pulse rate is generally higher in children and lower in older adults.

Normal Pulse Tracing

• A normal arterial pulse tracing consists of two waves and one notch, reflecting events during the cardiac cycle.
• The percussion wave is the primary upward wave and is produced by the rapid ejection of blood from the left ventricle during ventricular systole.
• The dicrotic wave appears during ventricular diastole and results from elastic recoil of the arterial wall and brief retrograde blood flow after closure of the aortic valve.
• The dicrotic notch marks closure of the aortic valve and indicates the end of ventricular systole.

Causes of Tachycardia

• Physiological causes of tachycardia include exercise, emotional excitement, anger, food intake, pregnancy, infancy, childhood, and exposure to high environmental temperatures.
• Pathological tachycardia may occur in fever, anemia, thyrotoxicosis, high-output states such as arteriovenous fistula, heart failure, and various cardiac tachyarrhythmias.

Causes of Bradycardia

• Physiological causes of bradycardia include athletic training, fear, grief, advanced age, and prolonged meditative practices.
• Pathological bradycardia may occur in hypothyroidism, raised intracranial pressure, obstructive jaundice, atrioventricular conduction blocks, and during treatment with drugs such as digitalis.

Common Abnormal Pulses

• A pulse is considered normal when its rate, rhythm, volume, and character show no detectable abnormalities.
• Several abnormal pulse patterns provide important clues to underlying cardiovascular disorders.

Anacrotic Pulse

• An anacrotic pulse is characterized by a slow-rising, low-amplitude pulse with a secondary wave on the ascending limb of the pulse tracing.
• This pulse pattern is typically associated with severe aortic stenosis, where left ventricular outflow is obstructed.

Dicrotic Pulse

• A dicrotic pulse is characterized by an exaggerated dicrotic wave, producing the sensation of two palpable pulsations during one cardiac cycle.
• Dicrotic pulse usually occurs in conditions associated with reduced cardiac output and decreased peripheral vascular resistance.
• It is commonly observed in febrile illnesses, septic states, and severe heart failure.
• Recognition of abnormal pulse characters assists in bedside cardiovascular assessment and diagnosis.

Water-Hammer Pulse

• The water-hammer pulse, also called a collapsing pulse, is characterized by a brisk rise and rapid collapse of the arterial pulse.
• This pulse is classically associated with severe aortic regurgitation, although it may occur in other high-output states.
• On palpation, the pulse feels forceful, high in amplitude, and disappears abruptly during diastole.
• The rapid upstroke results from a markedly increased left ventricular stroke volume during systole.
• In aortic regurgitation, blood enters the left ventricle from both the left atrium and the incompetent aortic valve during diastole.
• The increased end-diastolic volume augments ventricular contraction through the Frank–Starling mechanism, producing a large stroke volume.
• The rapid collapse of the pulse occurs because blood flows back into the left ventricle during diastole.
• A second contributing factor is the rapid runoff of blood into the peripheral circulation, particularly when systemic vascular resistance is reduced.
• Consequently, pulse pressure becomes markedly widened and may exceed 100 millimeters of mercury in severe cases.
• Conditions associated with a collapsing pulse include aortic regurgitation, patent ductus arteriosus, large arteriovenous fistula, ventricular septal defect with significant shunting, and hyperdynamic circulatory states such as thyrotoxicosis.
• Recognition of this pulse at the bedside suggests increased stroke volume combined with rapid diastolic runoff.

Pulsus Bisferiens

• Pulsus bisferiens is characterized by two systolic peaks felt during a single cardiac cycle.
• It is best appreciated in the carotid artery.
• This pulse pattern is typically observed in combined aortic stenosis and aortic regurgitation.
• It may also occur in hypertrophic obstructive cardiomyopathy.
• The first peak results from initial ventricular ejection, whereas the second peak reflects altered left ventricular outflow dynamics.

Pulsus Paradoxus

• Pulsus paradoxus refers to an exaggerated inspiratory decline in systolic arterial pressure.
• Normally, systolic blood pressure falls by less than 10 millimeters of mercury during inspiration.
• Pulsus paradoxus is present when this inspiratory fall exceeds 10 millimeters of mercury.
• During inspiration, the peripheral pulse becomes markedly weaker and may disappear temporarily in severe cases.
• Important causes include cardiac tamponade, constrictive pericarditis, severe bronchial asthma, and acute exacerbations of chronic obstructive pulmonary disease.
• Detection of pulsus paradoxus is clinically valuable because it may indicate serious cardiopulmonary compromise requiring urgent evaluation.

Causes

• Pulsus paradoxus is most commonly associated with constrictive pericarditis and significant pericardial effusion, particularly cardiac tamponade.
• Less frequent causes include severe bronchial asthma, emphysema, massive pleural effusion, intrathoracic masses, and advanced right ventricular failure.
• The hallmark of pulsus paradoxus is an exaggerated inspiratory fall in systolic arterial pressure exceeding 10 millimeters of mercury.

Mechanisms

• During normal inspiration, intrathoracic pressure becomes more negative, increasing venous return to the right side of the heart.
• Simultaneously, increased pulmonary vascular capacitance transiently reduces blood return from the lungs to the left atrium.
• Consequently, left ventricular filling and stroke volume decrease slightly during inspiration.
• In pathological states, this physiological reduction becomes markedly exaggerated, leading to pulsus paradoxus.
• In constrictive pericarditis and pericardial effusion, the rigid or fluid-filled pericardial sac restricts cardiac chamber expansion during diastole.
• As right ventricular filling increases during inspiration, the interventricular septum shifts toward the left ventricle because total cardiac volume cannot expand adequately.
• This ventricular interdependence further reduces left ventricular filling and stroke volume.
• In severe bronchial asthma or emphysema, forceful inspiratory efforts generate highly negative intrathoracic pressures.
• These pressure changes increase pooling of blood within the pulmonary circulation and reduce left ventricular preload.
• Hyperinflated lungs may also impede left ventricular filling, thereby accentuating pulsus paradoxus.
• In advanced right ventricular failure, altered pulmonary hemodynamics and impaired ventricular function reduce effective left ventricular filling during inspiration, contributing to a pronounced inspiratory decline in pulse volume.

Pulsus Alternans

• Pulsus alternans is characterized by a regular rhythm with alternating strong and weak pulse amplitudes.
• The heart rate remains regular, but successive beats differ in force because of alternating stroke volumes.
• This pulse abnormality is often difficult to detect by simple palpation of a peripheral artery.
• Confirmation is usually obtained during blood pressure measurement.
• While cuff pressure is gradually lowered, only the stronger beats are initially audible.
• With further reduction in cuff pressure, the weaker beats also become audible, causing a sudden doubling in the number of heard beats.
• A difference of approximately 5 to 20 millimeters of mercury may exist between the systolic pressures of alternating beats.
• The most common cause of pulsus alternans is severe left ventricular failure.
• It may also occur in severe myocardial dysfunction, including myocarditis and toxic injury to the myocardium.
• The physiological basis of pulsus alternans is impaired myocardial contractility.
• In left ventricular failure, inadequate systolic ejection leaves an increased end-systolic volume within the ventricle.
• This raises the end-diastolic volume before the next cardiac cycle.
• The increased ventricular filling enhances the force of the subsequent contraction through the Frank–Starling mechanism, producing alternating strong and weak beats.