Cardio Vascular System Heart Rate YouTube Lecture Handout Medical Science
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Heart Rate
- Normal heart rate (Sinus Rhythm)
- 70 to 80 beats/min. range is 50 to 100
- Sinus Bradycardia
- Sinus Tachycardia
- 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
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
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