Heart Rate and Cardiac Output

• Heart rate and cardiac output are important physiological parameters
• Presented by Dr. Marwa Awad, Assistant Professor of Physiology

Lecture Objectives

• Identify factors that regulate heart rate
• Define cardiac output precisely
• Be able to describe the body's response to hemorrhage and shock scenarios

Understanding Heart Rate

• Normal resting heart rate typically falls between 60 to 90 beats per minute
• Bradycardia is characterized by a heart rate less than 60 beats per minute
• Tachycardia indicates a heart rate exceeding 100 beats per minute
• Methods used for counting HR include:
  • Counting radial pulse or apex beats per minute.
  • Counting cardiac cycles by stethoscope
  • The recording of an ECG

Methods of Counting Heart Rate

• Counting the radial pulse/min is a method to measure heart rate.
• Heart sounds are counted via the use of a stethoscope
• ECG, is a method used to analyze the electrical activity of the heart

Innervation of the Heart

• The vagal nucleus contains cardioinhibitory centers that slow down the heart
• Cardioacceleratory centers speed up the heart
• There are sympathetic and parasympathetic components to the spinal cord
• Sympathetic and parasympathetic preganglionic fibers are involved
• Cardiac plexus contain Synapses
• Cardiac nerves involved

Regulation of Heart Rate

• Heart rate regulation is influenced by:
  • Afferent impulses affecting medullary cardiac centers
  • Changes in the chemical composition of blood
  • Blood temperature

Intrinsic Reflexes

• Arterial baroreceptors (carotid sinus & aortic arch) will be discussed
  • Site: Aortic arch & carotid sinus
  • Activation: ABP above 60 mmHg
  • Deactivation: Activities diminish and disappear around 40 mmHg

Arterial Baroreceptors

• Stimulation of CAC & stimulation of CIC ⇒ H.R. (Marey's law)
• Inhibition of vasoconstrictor center & stimulation of vasodilator center ⇒ VD →↓ABP
• Inhibition of respiratory center⇒(temporary apnea).
• Inhibition of adrenaline secreting center

Mary's Law

• HR is inversely proportional to ABP, assuming all other factors affecting HR are constant
• HR and ABP both increase during muscular exercise and hyperthyroidism
• HR and ABP both decrease during sleep and in hypothyroidism
• Mechanism: arterial baroreceptors in carotid sinus & aortic arch ↑ABP, send afferent depressor impulses through aortic nerve & sinus nerve to inhibit CAC & stimulate CIC in medulla oblongata →↓H.R

Vagal Tone Defined:

• Continuous inhibitory effect exerted by the vagi on the heart during rest, decreasing inherent rhythm of SAN from ~100-110 to ~70 impulses/minute
• Vagal tone is a reflex mechanism involving:
  • Stimulus: Normal level of arterial blood pressure (above 60 mmHg)
  • Receptors: Baroreceptors in aortic arch & carotid sinus
  • Afferent: Impulses pass along aortic nerve and carotid sinus nerve
  • Center: CIC in medulla oblongata
  • Efferent: Vagal nerves
  • Response: Inhibitory impulses to SAN (i.e., vagal tone)

Arterial Chemoreceptors

• Arterial chemoreceptors, also known as peripheral chemoreceptors, include the carotid and aortic bodies
• Stimuli include hypoxia (↓ O₂), hypercapnia (↑ CO₂), and acidosis (↑ H+)
• Afferent nerve supply, the aortic nerve operates much like the aortic arch
• The carotid sinus nerve operates like the carotid sinus
• Stimulation leads to Hypertension, Hyperpnea, and Tachycardia

Bainbridge Reflex Definition

• It involves increased venous return (VR) & pressure in the right atrium, resulting in a reflex increase in heart rate, or, reflex ↑ H.R
• Mechanism: ↑ Right atrial pressure (filling pressure)stimulation of atrial mechanoreceptors with impulses to the afferent vagus nerve to the medullary cardiovascular centers stimulation of CAC & inhibition of CIC reflex ↑ in sympathetic tone to heart ↑ H.R

Bainbridge Reflex Effects

• Stimulation of atrial baroreceptors leads to:
  • Impulses to the medullary centers Stimulation of CAC & inhibition of CIC ↑ H.R. (Bainbridge reflex)
  • Stimulation of respiratory center ↑ respiratory rate (Harrison reflex)
  • Inhibition of vagal vasoconstrictor tone to coronary blood vessels⇒ coronary V.D. ↑ coronary blood flow (Anrep's reflex)

Extrinsic Reflexes (from CVS)

• Definition: It means that H.R. ↑ during inspiration & ↓ during expiration
• Mechanisms: During inspiration H.R.↑ due to Distention of lungs during inspiration is pressor afferent impulses along pulmonary vagi inhibition of CIC & stimulation of CAC.
• Bainbridge reflex: During inspiration↑ VR distention of right atrium afferent vagal impulses inhibition of CIC & stimulation of CAC. The active inspiratory center irradiates impulses to stimulation of CAC

Changes in Chemical Composition of Blood

• Asphyxia: Condition of excess CO₂, O₂ deficiency, high H⁺ concentration
• Asphyxia effects on HR include:
  • Initial Slowing: Stimulation of CIC before CAC, increased ABP, decreased Heart Rate (Mary's Law)
  • Acceleration: The CIC is paralyzed, and the CAC remains active, respiratory center irradiation & reflexes secondary to hyperpnea, with Secretion of adrenaline
  • Premortal slowing is paralysis of the CAC and depression of the heart's rhythmicity & conductivity

Effect of Blood Temperature

• An increase in blood temperature of 1°C increases the heart rate by about 15 beats per minute.
• Mechanism involves stimulation of CAC, inhibition of CIC, and direct stimulation of SAN.

Hemorrhage

• Hemorrhage is the loss of blood from the cardiovascular system
• Types:
  • External: Bleeding from an open wound.
  • Internal: Intra-abdominal, intrathoracic or intracranial
• Losing 30-40% of total blood volume is life-threatening and blood transfusion is needed

Compensatory Reactions

• Immediate compensatory mechanism occurs
• Local responses: Enhanced blood coagulation aims to prevent blood loss
• Cardiovascular changes elevate arterial pressure stimulated by VCC which begins within 30 seconds to 1 minute

The VCC

• VCC Stimulation result in:
  • Inhibitory effect release of arterial baroreceptors that decrease arterial blood pressure
  • Peripheral chemoreceptors stimulated due to O₂ lack, which increases H+
  • Sympathetic stimulation of the adrenal medulla increases secretions of catecholamines
• All effects lead to:
  • ↑ stroke volume (inotropic effect) with ↑ systolic blood pressure
  • ↑ heart rate (chronotropic effect) ↑ diastolic blood pressure
  • vasoconstriction of the skin, sphlanchnic area increases blood pressure
  • venous return & arterial blood pressure rises due tovenoconstriction
  • Contraction of the spleen adds some blood to the circulation

Renal Response to Hemorrhage

• A drop in arterial blood pressure causes renal ischemia
  • Leading to renin secretion which converts to angiotensinogen from the liver
  • Which results to angiotensin I and finally to angiotensin II by ACE enzyme
• Angiotensin II triggers:
  • Generalized vasoconstriction causing increased aldosterone secretion promoting renal Na+ absorption
  • Anti-diuretic (ADH) to stimulate water retention and also a potent to increase arterial blood pressure

Respiratory changes.

• Acceleration of the respiration which helps in efficient arterialization of blood.
• Delayed compensatory reactions aim to regenerate blood volume using blood elements to return the arterial blood pressure back to normal.
• Mechanism:
  • Increase water intake will then cause subsequent thirst sensations
  • Decrease water loss occurs from posterior pituitary of antidiuretic hormone
  • Salt & water retention will cause secretion of angiotensin II & aldosterone

Restoration of components

• Restoration synthesized by the liver from tissue, protein, and diet.
• Protein, especially proteins with biological value
• Red blood cells- stimulated within 4-8 weeks
• Stimulate the bone marrow

Understanding Shock

• Shock is a clinical syndrome characterized by tissue hypoperfusion
• Tissue hypoperfusion with blood due to a decrease in the cardiac output and arterial blood pressure
• Causes:
  • Blood Loss (Hypovolemic Shock): Hemorrhagic
  • The blood volume is normal (Normovolemic Shock): Distributive, Cardiogenic, Obstructive

Shock Types

• Shock can be classified into types:
  • Hypovolemic: Hemorrhage
  • Distributive: Shock without blood loss but with vascular capacity, i.e. Anaphylactic shock or vasometer tone
  • Cardiogenic: Heart failure
  • Obstructive: Obstruction to blood flow in the lungs or heart as pneumothorax, pulmonary embolism, and cardiac tumors

Treatment for Shock

• Head-down position to increase venous return
• Vasopressor (sympathomimetic) drugs should be given
• In neurogenic and anaphylactic shock cases
• Glucocorticoids increase the strength of the heart and stabilize lysosomes
• Specific measures for each type of shock will be assessed
• Oxygen therapy will be given to help assist
• Blood transfusion with hypovolemic shock

Specific Shock Cases

• Septic shock requires prompt antibiotics and large amounts of fluids to maintain blood pressure.
• Anaphylactic shock includes epinephrine, steroids, antihistamines, oxygen, and H2 blockers.
• Cardiogenic shock is treated by addressing the underlying cause, medications, or possibly a heart transplant.
• Hypovolemic shock can be treated by intravenous saline and blood transfusions with not giving vasopressor
• Neurogenic shock involves fluids, immobilization, steroids, and sometimes surgery

Question 1

• Regulation of cardiac output by venous return is regulated by which law?
  • Correct answer: b. Marye's

Question 2

• A 53-year-old female patient with a history of 3 days of headaches showed arterial BP of 150/90; her heart rate was 60 bpm. What receptors caused the slowing of heart rate?
  • Correct answer: A. Baroreceptors

Question 3

• During asphyxia, what is the change of the heart rate?
  • Correct answer: C. Initial slowing, acceleration, slowing

Question 4

• Which of the following is one of renal changes that occur in response to hemorrhage?
  • Correct answer: A. Renal ischemia leads to secretion of renin

Question 4

• Which one is the criteria for shock?
  • Correct answer: C. Anaphylactic shock occurs as result of VD due to histamine