Cardio Vascular System Heart Rate Youtube Lecture Handout

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Image of Heart Rate

Image of Heart Rate

Image of Heart Rate

Heart Rate

  • Normal heart rate (Sinus Rhythm)

    • 70 to 80 beats/min. range is 50 to 100

  • Sinus Bradycardia Equation

  • Sinus Tachycardia Equation

  • Intrinsic Heart Rate when all nerves are cut 100/min

  • Heart rate is low in trained athelets

Sinus Arrhythmia

  • Sinus Arrhythmia HR varies with the phases of respiration

    • Increases during inspiration

    • Decreases during expiration

Explanations:

1. Central irradiation--- Impulses irradiates from Res. to Cardiac centre

2. Bainbridge Reflex

  • During insp. Venous return increases

  • Congestion of rt. Atrium & great veins

  • Mobilize the Reflex to heart rate

Inhibit Vagus

Stimulates sympathetic

3. Afferent impulses arise from the lungs during inspiration & reflexly inhibit the vagal tone

Factors affecting

  • Size of animal-

    • Elephant-----25 beats/min

    • Dog

    • Canary bird-1000/min.

  • Age

    • Foetus------140 to 150/min

    • At birth-----130 to 140 “

    • 12 yrs. ------100 to 110/ “

    • Adult---------70 to 80/ “

    • Old age------up to 100/ “

  • Sex-HR more in females as they have high sympathetic tone

  • Body temp-HR ↑ by 10 beats for every 1°F increase in temp.

  • Emotional stimuli

    • Fear, anger, excitement---- ↑ HR.

    • Grief, shock ----------------- ↓ HR.

  • Exercise------------↑HR

  • Pain

    • Superficial pain------ ↑HR

    • Deep pain------------ ↓HR

  • 8. Respiration-----------Sinus arrhythmia

Pathological cond

Diseases

  • Thyrotoxicosis ↑ HR

  • Hypothyroidism ↓ HR

Drugs

  • Ephinephrine ↑HR

  • Acetylcholine ↓HR

Heart Rate Regulation

Adjusted according to need of body

  • Increases during exercise

  • Falls during sleep

Two mechanisms

1. Local Mech.—any change in SAN function or junction tissue affect HR.

2. Nervous mechanism

Includes:

  • Cardio excitatory center (Vasomotor center) acts through sympathetic nerves

  • Cardio inhibitory center acts through Vagus

  • They are in reciprocal relation

  • Vagus exerts a Vagal tone

Sympathetic Innervation

  • From T3 & T4

  • Nerves of right side have more influence on SAN

  • Of left side have more on AVN

  • Neurotransmitter--Norepinephrine

  • Receptor β1 adrenergic receptor

  • Mechanism of action – acts by ↑ concentration of Cyclic AMP which leads to↑ influx of Na+ & Ca++

  • Blocker---Propranolol, atenolol

Actions

Sympathetic (Stimulation Causes)

  • + Chronotropic effect (↑rate)

  • + Inotropic effect (↑force)

  • + Bethmotropic effect (↑excitability)

  • + Dromotropic effect (↑conductivity)

Sympathetic Vasodilator fibers

  • Mainly supply skeletal muscles. They

    Cause vasodilatation only during exercise & this leads to anticipatory rise in blood flow

  • Is controlled by Anterior hypothalamus

Parasympathetic Innervation

  • Parasympathetic Rt.& Lt. Vagus

    • Right supplies to SAN

    • Lt. to AVN

  • Ventricles have no or very little parasympathetic innervation

  • Neurotransmitter-Acetylcholine

  • Receptors- Muscarinic M1

    • Mechanism of Action - ↓ Ca++ entry in nodal tissue

  • Blocker- Atropine

Parasympathetic stimulation

Causes

  • Chronotropic effect (↓rate)

  • Inotropic effect

  • Bethmotropic effect

  • Dromotropic effect

Vagal Tone

  • Vagal tone is reflexly produced through sinoaortic nerves & exerts a stronger inhibitory action

  • Action is suppressed by Atropine & it can increase HR up to 150/min.

  • Normal physiological variations in heart rate are by change in vagal tone

Vasovagal Syncope

  • Fainting associated with intense emotional disturbances

  • Strong inhibitory signals come to heart through vagus - intense stimulation of vasodilator fibers to skeletal muscles

  • Fall in BP. & ↓ Blood supply to brain

  • Person loses consciousness

Image of Sympathetic Nerve And Parasympathetic Nerve

Image of Sympathetic Nerve and Parasympathetic Nerve

Image of Sympathetic Nerve And Parasympathetic Nerve

Reflexes influencing HR

  • Sino-aortic reflex /Cardio inhibitory /Marey’s Reflex

  • ↑BP. Results in ↓ HR. through Vagus. Baroreceptors are involved

Marey’s Law: Heart rate & BP have an inverse relation

  • 2. Bainbridge reflex Cardio acceleratory reflexes

  • 3. Other reflexes

  • Sensory stimuli can alter H.R.

  • Stimulation of splanchnic nerves ↓ H.R.

  • Sudden blow on abdomen can stop heart

Other factors

  • Body temp. Affect HR. by

    • Direct action on SAN

    • Stimulate Cardio acceleratory centre

  • Impulses from higher centers—Frontal lobe of cerebral cortex & hypothalamus

    • Excitement increases H.R.

    • Sudden shock may even stop heart