Introduction to Cardiac
Cycle and Cardiac
Output
Objectives
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Explain the ECG waves and correlate them
with mechanical events.
Describe the phases of cardiac cycle.
Identify the origin of heart sounds.
Define heart rate, stroke volume, venous return
and cardiac output.
Explain the Starling’s law of the heart.
List the function of autonomic nervous system
on the heart.
Electrocardiogram ECG (EKG)
Electrocardiogram ECG (EKG)
• Surface electrodes record electrical activity deep within
body
• Reflects electrical activity of whole heart not of single
cell!
• EC fluid = “salt solution” (NaCl)  good conductor of
electricity to skin surface

ECG tracing =  of all electrical potentials generated by
all cells of heart at any given moment
Electrocardiogram (ECG, EKG)

Electrodes placed on arms and
legs (limb leads) + six
positions on chest (chest
leads).

Each lead records different
electrical activity (each looks
at heart from a different
position)
3 waves appear with each
heart beat:
P wave: represents atrial
depolarization
QRS complex: represents
ventricular depolarization
T wave: represents ventricular
repolarization
Since:
Depolarization = signal for contraction
Segments of ECG reflect mechanical heart events
Correlation of ECG waves with Atrial and
Ventricular Systole
ECG cont.
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Comparing records with one
another & normal records,
allows determination :
 if conductivity pathway is
normal
 if the heart is enlarged
 if certain regions of heart
are damaged
ECG time intervals or segments:
 P-Q interval
 S-T interval
 Q-T interval
ECG cont.
Cardiac Cycle

One cardiac cycle (CC): all events associated with one
heart beat.
(Systole and diastole of atria + systole and diastole of
ventricles)
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At HR 75 beats /min, CC lasts 0.8 secs.

In each CC, atria & ventricles alternatively contract &
relax

Force blood from areas of higher pressure

Areas of lower pressure
Cardiac Cycle cont.
Cardiac Cycle comprises the following
phases:

Atria systole:
atria contracting but Ventricles relaxing.
Lasts 0.1 sec
Ventricular systole:
Ventricular Contracting while atria relax
(atrial diastole). Lasts 0.3 sec.

 Relaxation Phase
(Complete cardiac diastole ), Both atria
and ventricular relax. Lasts about 0.4 sec
Heart Sounds

Auscultation: act of listening to
sounds within the body, usually done
with a stethoscope

sounds are caused by vibrations set
up within the walls of ventricles and
major arteries during valve closure.

During each CC4 heart sounds
Only first & second (S1 &S2) heard
in normal heart
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First heard sound (S1) lubb:
- louder & longer than second
- caused by closure of A-V
valves
(soon after beginning of
ventricular systole)
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Second heart sound (S2) dupp:
- shorter & not as loud as first
- caused closure of SL valves
(beginning of ventricular
diastole)
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S3 is due to turbulance during
ventric filling
S4 is due to turbulance during
atrial systole
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Heart sounds
S1 &S2 are
best heard at
the surface of
chest in
specific
locations
Cardiac output (CO)
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CO is the volume of blood ejected from the left ventricle
(or the right ventricle) into the aorta (or pulmonary trunk)
each minute.
CO equals the stroke volume (SV) multiplied by heart
rate (HR)
CO = SV X HR
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SV volume of blood ejected by the ventricle during each
contraction
HR number of heart beats per minute.
CO = SV X HR = 70 ml/beat X 75 beats/minute
= 5.25 L/min
Cardiac Output cont.
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CO may  to meet demands
During exercise CO can increase to 20-25 L/ min
Cardiac reserve (CR): The difference between the
maximum CO and CO at rest.
Average CR: 4 -5 X resting value (20-25 L/m)
higher in athletes (35 L/m)
little or no reserve in heart disease
Regulation of cardiac output
CO= SV x HR
Depends on the regulation of SV and HR
Cardiac Terminology
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End diastolic volume (EDV):
blood in the ventricles at the end of diastole.
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Ejection fraction:
fraction of EDV that is ejected (%), used to measure
heart efficiency.
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End systolic volume (ESV):
blood that remains in the blood at the end of systole.
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Preload:
the degree of stretch on the heart before it contracts.
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Afterload:
the pressure that must be exceeded before ejection of
blood from the ventricles can occur.
Regulation of stroke volume
THREE factors regulate SV:
1.Preload (EDV)
2.Contractility
3.Afterload
1. Preload: effect of stretching
Frank-Starling (F-S) law of the
heart:

Within limits, the more the
heart fills during diastole
(preload – stretching), the
greater the force of contraction
during systole.

the preload is proportional to
EDV, the greater the EDV, the
more forcefull the next
contraction
“The heart will pump what it receives”Starling’s law of the heart
Preload cont.
Two factors determine EDV:
Duration of ventricular diastole (HR)
Ventricular filling occur during diastole
Tachycardia ( HR)
diastole duration
ventricular filling
end diastolic volume
CO
2. Venous return (volume of blood flowing back to the
heart through systemic veins)
1.
2. Contractility
Contractility: strength of contraction at any given preload
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+ve inotropism:  contractility
Include: - sympathetic stimulation
- Hormones; adrenaline and noradrenaline
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-ve inotropism:  contractility
May result from:
- inhibition of the sympathetic system
- anoxia
- acidosis
3. Afterload
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pressure in pulmonary tract is a bout 20 mm.Hg and in the
aorta is about 80 mm Hg. This pressure must be overcome
before the semilunar valves open.
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Depend on:
 Elasticity of large arteries
 Peripheral resistance of arterioles
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An  in afterload   SV  more blood remains in ventricle
at end of systole (ESV)
Conditions that  afterload include:
 Hypertension
 Narrowing of arteries by atherosclerosis
Regulation of heart rate
Several factors, the most important are
1. Nervous factors
2. Chemical factors
3. Others
Autonomic regulation of HR
Chemical regulation of HR
1. Hormones:

Adrenaline and noradrenaline (adrenal medulla) HR & 
contractility
Adrenal medulla stimulated by:
exercise
stress
excitement

Thyroid hormones   HR & Contractility
Hyperthyroidism  tachycardia
2. Cations
  Na+ & K+   HR & Contractility
 Ca2+   HR & Contractility
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Other factors in HR regulation
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Age: Newborn HR ~120 beats/min
Old people may also develop  HR
Gender: Adult females  higher HR than
males
Exercise: Athletic bradycardia (60 or
under) (more efficient heart)
Body temperature (BT):
 BT (fever or exercise)  HR
BT  HR & contractility