Heart Rate and Arterial Pulse PDF

# Chapter 49: Heart Rate and Arterial Pulse ## Learning Objectives - Give the normal range of heart rate and physiological variations of heart rate. - Understand the mechanisms of regulation of heart rate. - Draw the graph of arterial pulse tracing. - List the causes of bradycardia and tachycardia....

# Chapter 49: Heart Rate and Arterial Pulse ## Learning Objectives - Give the normal range of heart rate and physiological variations of heart rate. - Understand the mechanisms of regulation of heart rate. - Draw the graph of arterial pulse tracing. - List the causes of bradycardia and tachycardia. - Name the causes of common abnormal pulses and understand their physiological basis. - Explain the mechanism of genesis of water-hammer pulse, pulsus alternans and pulsus paradoxus. - Explain the physiological basis and mechanism of alterations of all abnormal pulses. ## Heart Rate - The normal heart rate is 60-100 beats/min in adults. - 60 beats/min is called **bradycardia** and above 100 beats/min is called **tachycardia**. - Normally, heart rate is more in infants and children and less in geriatric age group. - Heart rate reflects the rate of discharge of the cardiac pacemaker. As sinoatrial (SA) node is the natural primary pacemaker, heart rate is the rate of discharge of SA node. ## Physiological Variations Heart rate is easily influenced by various physiological factors. Some of the common and important factors are described below: - **Age**: Heart rate is more in infants and children. After the age of 60 years, heart rate decreases. - **Gender**: Heart rate is comparatively less in females due to their relatively high parasympathetic tone and less basal metabolism. - **Diurnal Variation**: Heart rate is more in the day, especially in the afternoon, and less in the night, especially during sleep. This difference is due to less physical activity and sympathetic discharge in the night, and due to the low level of stress in sleep than in awakened state. - **Respiration**: Heart rate is more during inspiration and less during expiration (sinus arrhythmia). This is mainly due to alteration in vagal activity, which is more in expiration and less in inspiration. - **Body Temperature**: Increase in body temperature increases heart rate and decrease in temperature decreases heart rate. - **Environmental Temperature**: Heart rate is more in summer and less in winter. - **Food intake**: Food intake increases heart rate by increasing body metabolism. - **Posture**: Change in posture from supine to standing increases heart rate due to decreased stimulation of baroreceptors. - **Exercise**: Heart rate increases in exercise due to sympathetic stimulation. Heart rate may even increase before starting the exercise due to psychological effects that acts through limbic system. ## Regulation of Heart Rate Heart rate is one of the physiological parameters of the body which is influenced easily by external and internal factors. 1. Heart rate is primarily controlled by autonomic nervous system. 2. Vagus (parasympathetic) nerve inhibits and sympathetic nerves stimulate heart rate. 3. However, heart rate is primarily a vagal function. ## Mechanisms regulating heart rate can be divided broadly into two categories: 1. Neural mechanisms 2. Humoral mechanisms ## Neural Control Mechanisms - Neural regulating mechanisms are divided into three categories: 1. Autonomic regulation 2. Reflex regulation 3. Regulation by higher centers ## Autonomic Control - Both parasympathetic and sympathetic divisions of autonomic nervous system influence heart rate. - Normally, parasympathetic control of SA node dominates the sympathetic control. Therefore, the basal heart rate is less than the intrinsic heart rate. - The intrinsic heart rate is the rate of discharge of SA node when the heart is completely denervated. It is about 110/ min. This indicates that the heart rate is more in the absence of neural influences. - With intact autonomic innervation, due to more vagal influence on SA node (as vagus is inhibitory), the normal heart rate is less than the intrinsic heart rate. - **Parasympathetic Control**: The parasympathetic fibers supplying the heart originate in the nucleus tractus solitarius, dorsal motor nucleus of vagus, and the nucleus ambiguous, in the medulla. The fibers travel in the vagus nerve to supply the SA node. - The right vagus nerve predominantly supplies SA node and the left vagus nerve predominantly supplies AV node. - Stimulation of vagus nerve results in decrease in heart rate. This occurs due to the secretion of acetylcholine at vagal nerve endings that suppresses SA node. However, cholinesterase, the enzyme that hydrolyzes acetylcholine, is present in higher concentration in the nodal tissues. - Therefore, the effect of a single stimulation of vagus nerve on SA node remains for a short period as acetylcholine is rapidly hydrolyzed by cholinesterase. The action of cholinesterase is so fast that it allows beat-to-beat control of SA node by the vagus nerve. - **Sympathetic Control**: The sympathetic fibers supplying the pacemaker tissue originate from the lower two cervical and upper six thoracic segments of spinal cord. 1. The stimulation of sympathetic fibers results in increase in heart rate. 2. In contrast to the effect of parasympathetic stimulation on SA node, the effect of sympathetic stimulation persists longer as there is no specific enzyme to degrade norepinephrine, which is released at the sympathetic nerve endings. 3. Norepinephrine is mostly taken up by the nerve terminals, and the remaining amount is slowly absorbed into circulation. Moreover, the effect of norepinephrine is mediated by adenylyl cyclase system, which takes longer time to exert its effect. 4. Thus, the effect of sympathetic stimulation on heart rate remains for a longer period than the vagal stimulation. ## Reflex Control - Cardiovascular reflexes that regulate blood pressure also control heart rate, which is part of the integrated control mechanisms. - Details of these reflexes are described in the regulation of blood pressure. - **Baroreceptor Reflex**: Baroreceptors located in the carotid sinus and aortic arch are stimulated when blood pressure rises, which in turn stimulates the nucleus tractus solitarius (NTS) in medulla via 9th and 10th cranial nerves. NTS inhibits the vasomotor center (VMC). - Inhibition of VMC decreases sympathetic activity via bulbospinal pathway. Decreased sympathetic discharge decreases heart rate. - Stimulation of NTS also directly stimulates the vagus nerve that causes bradycardia. - Heart rate increases in conditions in which baroreceptors are less stimulated as occurs in hypotension. - **Chemoreceptor Reflex**: Chemoreceptors are stimulated by change in chemical composition of blood as occurs in hypoxia, hypercapnia and acidosis. Activation of chemoreceptors primarily produces bradycardia, but heart rate may remain unchanged or even slightly increased by secondary effects. - **Bainbridge Reflex**: Bainbridge, in 1915, demonstrated that in dogs, rapid infusion of saline or blood increases heart rate, if the initial heart rate is less. This is known as Bainbridge reflex. - The receptors are present in the atria at the venoatrial junction, and are known as tachycardia producing receptors (TPR). - This reflex accounts for tachycardia produced following saline infusion or blood transfusion. - The effect of Bainbridge reflex on heart rate is more observable if the initial heart rate is less. This reflex competes with baroreceptors reflex and tries to increase heart rate. - **Cushing's Reflex**: Cushing's reflex is activated in gross hypovolemia and hypotension that decreases blood flow to the VMC in the medulla. Direct stimulation of VMC produces vasoconstriction and tachycardia. But, the consequent increase in pressure stimulates the baroreceptors that finally result in bradycardia. ## Control by Higher Centers - Stimulation of motor cortex, frontal lobe and thalamus increases heart rate. Increase in heart rate in emotional states, anxiety and excitement is due to stimulation of limbic system. ## Humoral Control Mechanisms - **Hormonal Control**: Thyroxine and catecholamines increase heart rate. - **Chemical Control**: Hypoxia increases heart rate, which is partly mediated by release of catecholamines from adrenal medulla. Hypercapnia and acidosis decrease heart rate. ## Arterial Pulse - **Physiological Aspects** - **Definition**: Arterial pulse is defined as the rhythmic expansion of the arterial wall due to transmission of pressure waves along the walls of the arteries that are produced during each systole of the heart. - **Clinical Importance**: Clinically, radial pulse is examined for the assessment of arterial pulse. It is an important and essential part of the clinical examination of a patient as pulse is one of the vital signs of a living being (other vital signs are blood pressure, respiration and temperature). Examination of arterial pulse also provides valuable information regarding the functioning of the heart, condition of hemodynamics, such as blood pressure, and the condition of blood vessels. - **Normal Pulse Rate**: Normally, pulse rate is same as heart rate. Thus, the normal pulse rate is 60-100/min. - The deficit between pulse rate and heart rate is called pulse deficit (Clinical Box 49.1). - There is no pulse deficit in normal conditions. That means the pulse rate exactly coincides with the rate of heart beat. - Pulse rate more than 100 is called tachycardia, and less than 60, is called bradycardia. - Normally, the heart rate is more in children and less in elderly people. - **Normal Pulse Tracing**: Arterial pulse tracing shows two waves and one notch. The waves are percussion (p) wave, also called as tidal wave, and dicrotic (d) wave (Fig. 49.1), and the notch is dicrotic notch (n). - Percussion wave occurs due to ejection of blood during ventricular systole. - Dicrotic wave occurs due to rebound of blood against the closed aortic valve during diastole. - Dicrotic notch represents closure of aortic valve. ### Variations in Heart Rate - **Causes of Tachycardia** - **Physiological**: Exercise, after eating - **Pathological**: Fever, Anemia, Thyrotoxicosis, Beriberi, Paget's disease, Arteriovenous fistula, Heart failure, Paroxysmal atrial tachycardia, Ventricular or supraventricular tachycardia, Other tachyarrhythmias. - **Causes of Bradycardia** - **Physiological**: Athletes, Fear, Grief, Very old age, Yogis - **Pathological**: Myxedema, Increased intracranial pressure, e.g., brain tumors, Obstructive jaundice, Different types of heart block, Drugs, e.g., digitalis. ## Common Abnormal Pulses - Some of these common abnormal pulses are anacrotic pulse, dicrotic pulse, water-hammer pulse, pulsus bisferiens, pulsus paradoxus, and pulsus alternans. - **Anacrotic Pulse**: This is also called as anacrotic pulse, which means two up beats. A secondary wave occurs in the upstroke of the pulp b It is commonly found in aortic stenosisk The upstroke is slow and sloping (Fig. 49.2A). - **Dicrotic Pulse**: It is also called as twice-beating pulse. The dicrotic wave is prominent and gives the feeling of two beats (Fig. 49.2B). It is commonly seen in febrile states, especially in typhoid fever. - **Water-Hammer Pulse**: This is also called as collapsing pulse or Corrigan's pulse. It is typically seen in aortic regurgitation. The collapsing pulse is characterized by a rapid upstroke (ascent) and a rapid downstroke (descent) of the pulse wave (Fig. 49.2C). - **Pulsus Bisferiens**: Pulsus bisferiens is a combination of the low rising pulse (anacrotic pulse) and the collapsing pulse. This is typically seen in aortic stenosis associated with aortic incompetence. - **Pulsus Paradoxus**: This is a misnomer. There is nothing paradoxical in this pulse. Actually, this is an accentuation of the normal phenomenon. Normally, the amplitude of the pulse decreases in inspiration and increases in expiration. In pulsus paradoxus, in inspiration the volume of the pulse is grossly decreased, or may be absent in severe cases (Fig. 49.2E). - **Common Causes**: Constrictive pericarditis, Pericardial effusion. - **Less Common Causes**: Emphysema, Asthma, Massive pleural effusion, A mass in the thorax, Advanced right ventricular failure. - **Pulsus Alternans**: The pulse is regular, but the alternating beats are strong and weak (Fig. 49.2D). - It is difficult to appreciate pulsus alternans by palpating the artery. Diagnosis is confirmed while measuring blood pressure. A difference of 5-20 mm Hg in the systolic pressure is marked between two alternate beats. When the column of mercury in the manometer is being lowered, the stronger beats are heard first, and on further lowering, the weaker beats also become audible. Thus, suddenly the number of audible beats is doubled. - **Causes**: Left ventricular failure (most common cause), Toxic carditis. - **Physiological Basis**: In left ventricular failure, due to decreased myocardial contractility the stroke volume is decreased. This results in low volume pulse. - Due to inadequate ejection of blood during systole, the end-systolic volume of the left ventricle increases. - Therefore, EDV increases before the onset of next ventricular contraction. - This increases the force of contraction of the ventricle in the next beat by Frank-Starling mechanism. - Consequently, the second beat becomes stronger. Likewise, the strong and weak beats alternate. ## Chapter Summary - Heart rate is primarily a parasympathetic function. Tachycardia reflects decreased vagal tone and bradycardia indicated increased vagal tone. - Pulsus alternans is observed in heart failure and water-hammer pulse seen in aortic regurgitation. ## Key Concepts - Importance of pulse rate - Factors regulating what that is the normal HR, What are the physiological variations of HR (tachycardia and bradycardia), What are the factors regulating HR, What are the neural control mechanisms of HR, What are the autonomic nervous system's control mechanisms of HR, What are the humoral control mechanisms of HR, Define pulse deficit, What is the clinical importance of pulse rate, What are the waves of a normal pulse tracing, List the factors causing tachycardia, List the factors causing bradycardia, What are the humoral control mechanisms of HR, What is the clinical importance of pulse rate. - Importance of abnormal pulses, What are the causes of Water-Hammer pulse. What is pulsus paradoxus, What are the causes of pulsus paradoxus, What is the physiological basis of Water-Hammer pulse. What is pulsus alternans, What are the causes of pulsus alternans, What are the physiological bases of pulsus alternans, what is pursus bistenient, what is pulsus alternans, What are the causes of pulsus what are the causes of pulisysiologisi basa de ## Multiple Choice Questions 1. The range of normal pulse rate is