power point of the cardiac cycle and heart rate lecture for 1st level MU dental students

1. Cardiac cycle (dental)
Def:
• “ The cardiac events that
occur from the beginning
of one heart beat to the
beginning of the next “.
Time:
• Average duration of each
cycle is 0.8 second (60
seconds/75 beats.min-)
Dr. Asaad Ahmed Elmileegy 1

2. cardiac cycle is divided into the
following phases:
I- Atrial systole:
II- Ventricular systole (0.3 sec),
which includes:
• a- Isometric (or isovolumetric)
contraction phase.
• b- Maximum ejection phase.
• c- Reduced ejection phase.
III- Ventricular diastole (0.5 sec),
which includes:
• a- Isometric (or isovolumetric)
relaxation phase.
• b- Rapid filling phase.
• c- Reduced filling phase.
Dr. Asaad Ahmed Elmileegy 2

3. I- Atrial systole:
Duration:
• 0.1 second.
During this phase:
• Atrial contraction  
the atrial pressure
(from zero to 2 mm Hg)
 pumping of the
blood into the
ventricles.
Dr. Asaad Ahmed Elmileegy 3

4. II- Ventricular systole:
Duration:
• 0.3 second.
During this phase:
1. The ventricle contract isometrically
(i.e., without change in the length
of the ms fibers)  
intraventricular pressure, from
zero to 80 mmHg in the left
ventricle, or from zero to 10 mm
Hg in the right ventricle.
2.  ventricular pressure  opens
the semilunar valves and the blood
is ejected at a high rate out of the
ventricles to the aorta and
pulmonary artery.
3. Then ejection of blood continues
but at a gradually slowing rate.
Dr. Asaad Ahmed Elmileegy 4

5. IV- Ventricular diastole:
Duration:
• 0.5 seconds.
During this phase:
• The ventricles relax isometrically
 the pressure   sudden
closure of the semilunar valves.
• The ventricular pressure falls
rapidly towards zero.
• Then, the atrial pressure  more
than the ventricular pressure 
opening of the AV valve and
rushing of blood by its weight
into the relaxed ventricle.
• This ventricular filling starts
rapidly and then continues
slowly.
Dr. Asaad Ahmed Elmileegy 5

6. Dr. Asaad Ahmed Elmileegy 6

7. HEART RATE (dental)
• In a normal adult male, the average heart beats
at rest is approximately 70 beats/ minute.
• Methods of counting the HR:
a) Counting the radial pulse or apex beats/min.
b) Counting the cardiac cycles or heart sounds by
stethoscope.
c) Recording the ECG.
By: Dr. Asaad Ahmed Elmileegy 7

8. HEART RATE (dental)
By: Dr. Asaad Ahmed Elmileegy 8

9. Regulation of the heart rate
• I- Nervous
– Impulses from higher centers: cerebral cortex,
hypothalamus, respiratory center
– Afferent impulses from right & left side of circulation
• II- Chemical:
– Blood gases, hormones, others
• III. Physical:
– Blood temperature
By: Dr. Asaad Ahmed Elmileegy 9

10. I- Nervous regulation of heart rate
The cardiac centers
[cardioinhibitory (CIC) &
cardioacceleratory (CAC)] in
the brain stem are
influenced by:
Impulses from higher centers:
1. Cerebral cortex,
2. Hypothalamus,
3. Respiratory center
Afferent impulses from:
1. Right &
2. left side of circulation
By: Dr. Asaad Ahmed Elmileegy 10

11. 1- Impulses from The cerebral cortex:
a- Conditioned reflex:
• Visual, smell or auditory stimuli → the cerebral
cortex →  or  heart rate .
b- Voluntarily:
• Some people can  or  their heart rate
voluntarily.
c- Emotions:
• Most emotions →  heart rate .
• Sudden shocking news →  heart rate .
By: Dr. Asaad Ahmed Elmileegy 11

12. 2- Impulses from The hypothalamus:
• Stimulation of anterior
hypothalamus (sleep) →
↑ parasympathetic
activities →  heart rate
due to stimulation of CIC.
• Stimulation of posterior
hypothalamus (ms
exercise) → ↑
sympathetic activities →
 heart rate due to
stimulation of CAC.
By: Dr. Asaad Ahmed Elmileegy 12

13. Parasympathetic (Vagal) Tone
Def:
• A continuous
parasympathetic
inhibitory impulses to
the heart at rest
It inhibits the inherited
high rhythm of the SAN
from 110 to 70 beats/
minute.
Dr. Asaad Ahmed Elmileegy 13

14. Mechanism of vagal tone: a reflex in
which:
1. Stimulus: The normal level of
arterial blood pressure.
2. Receptors: Baroreceptors in the
aortic arch and carotid sinus.
3. Afferent fibers: The impulses
pass along: a- glossopharyngeal
nerve (IX) from the carotid
sinus b- vagus nerve (X) from
the aortic arch
4. Center: Cardioinhibitory center
(CIC) in the medulla.
5. Efferent fibers: In the vagus
nerves.
6. Effect: Inhibitory impulses to
the SAN.
Dr. Asaad Ahmed Elmileegy 14

15. Vagal (ventricular) Escape
Phenomenon
1) Strong vagal stimulation
 marked inhibition of
the SAN   the heart
rate and then the heart
stops completely.
2) After that the ventricles
start to beat according to
their own rhythm i.e.
idioventricular rhythm
(25-40 beats/min).
• This proves that the
vagus does not supply
the ventricles.
Dr. Asaad Ahmed Elmileegy 15

16. 3- Impulses from The respiratory
center:
Respiratory sinus
arrhythmia:
• Inspiration   heart
rate,
• Expiration   heart
rate .
By: Dr. Asaad Ahmed Elmileegy 16

17. B- Impulses from the circulatory
system :
Impulses from the right side (Bainbridge
reflex):
• “ right atrial pressure  reflex  heart
rate ”.
Impulses from the left side:
• Marey's law: “The heart rate is inversely
proportional to the arterial blood pressure
provided that other factors remain constant”.
By: Dr. Asaad Ahmed Elmileegy 17

18. Bainbridge reflex:
• "  right atrial pressure
 stretch receptors 
stimulation of the CAC 
sympathetic fibers  
heart rate.
• Significance:
1. Pumping of excess
venous return
2. Prevents stagnation of
blood in veins.
By: Dr. Asaad Ahmed Elmileegy 18

19. Marey's law:
• The heart rate is inversely
proportional to the arterial
blood pressure provided that
other factors remain
constant.
• Mechanism:
•  arterial blood pressure
(ABP)  the arterial
baroreceptors in the carotid
sinus and aortic arch 
impulses by the IX and X
nerves  inhibition of the
CAC and stimulation of CIC
  heart rate.
By: Dr. Asaad Ahmed Elmileegy 19

20. Carotid sinus syndrome:
• A pathological condition in which the carotid
sinus is abnormally hypersensitive.
• Mechanism:
• A slight pressure on the carotid sinus 
baroreceptors  reflex  vagal tone and a 
of the sympathetic vasoconstrictor tone  
heart rate and hypotension  cerebral
ischemia and temporary loss of consciousness.
• Treatment:
• Denervation of the hypersensitive carotid
sinus.
By: Dr. Asaad Ahmed Elmileegy 20

21. II- Chemical regulation of heart rate
1) Changes in blood gases:
2) Changes in blood hormones:
3) Chemical substances affecting the H.R.:
By: Dr. Asaad Ahmed Elmileegy 21

22. 1) Changes in blood gases:
• Mild or moderate hypoxia (O2 lack): 
heart rate due to ++ of CAC and -- of CIC both:
• Severe hypoxia   heart rate, hypotension
and finally death due to:
• Damage of Medullary centers and Nodal
tissue of the heart.
By: Dr. Asaad Ahmed Elmileegy 22

23. 2) Changes in blood hormones:
Adrenaline:
• Small dose   HR due to direct stimulation
of the SAN.
• Large dose  reflex  HR (Marey's law).
Noradrenaline:
• Small or large doses  reflex  HR
(generalized VC   ABP).
Thyroxine:
•  HR
By: Dr. Asaad Ahmed Elmileegy 23

24. 3) Chemical substances affecting the
H.R.:
By: Dr. Asaad Ahmed Elmileegy 24
Substances that  HR Substances that  HR
1) Atropine:
Inhibits the vagus nerve 
 HR
1) Bile salts:
Inhibit the SAN
2) Histamine:
 VD   ABP  reflex 
HR (Marey's law).
2) Morphine:
Stimulates the CIC
3) Sympathomimetic drugs: 3) Parasympathomimetic
drugs:

25. III. Physical regulation of HR
↑ blood temperature:
• A rise of blood temperature 1°C   HR by
10 beats/min.
↓ blood temperature:
• Opposite effect.
By: Dr. Asaad Ahmed Elmileegy 25

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