CARDIAC OUTPUT
& VENOUS RETURN
Lecture – 7
Dr. Zahoor Ali Shaikh
1
CARDIAC OUTPUT
What is Cardiac Output?
• It is volume of the blood pumped out by each
ventricle per minute .It is about 5 – 5.5 Lit/min
• Cardiac Output [COP ]
= Heart rate × Stroke volume
= 70 beats/min × 70ml/beat
= 4900 ml/min ≈ 5liters/min
• COP of each ventricle is same.
2
CARDIAC OUTPUT [COP]
• COP increases during exercise, and depending
on exercise, it can increase to 20–25 liters/min
[up to 35 liters/min is recorded in trained
athlete during heavy exercise].
How ?
- By increasing stroke volume and heart rate.
3
CARDIAC INDEX
What is Cardiac Index ?
• It is cardiac output per minute per square meter
of body surface area.
• Normal Cardiac Index = 3.2 Liter /min/ sq meter
body surface area.
What is Cardiac Reserve ?
• It is the difference between cardiac output at rest
and maximum volume of blood that heart can
pump per minute.
4
REMEMBER THE FOLLOWING
DEFINITIONS WHICH WE WILL USE
DURING DISCUSSION OF COP
• Stroke Volume: It is a volume of blood pumped out by each
ventricle per beat. It is about 70 - 80 ml.
Stroke volume (SV) = EDV – ESV
• End Diastolic Volume: Volume of blood in each ventricle at
the end of diastole.
It is about 120 – 130 ml.
• End Systolic Volume: Volume of blood in each ventricle at the
end of Systole. It is about 50 to 60 ml
5
• Ejection fraction (EF) is the percentage of
ventricular end diastolic volume (EDV) which
is ejected with each stroke.
SV (EDV – ESV)
X 100
EF =
EDV
75
X 100 = 62.5%
120
Normal ejection fraction is about 60 – 65 %.
Ejection fraction is good index of ventricular function.
6
CARDIAC OUTPUT [COP]
• As cardiac output depends on heart rate and
stroke volume, we will discuss each one.

•
•
•
•
HEART RATE [No. of beats per minute]
Normal heart rate = 70 beats/min [60 to 100].
Heart rate above 100 is called Tachycardia.
Heart rate below 60 is called Bradycardia.
Heart rate is determined by autonomic nervous
system effect on SA – Node.
7
CARDIAC OUTPUT [COP]
Effect of Autonomic Nervous System on Heart
8
CARDIAC OUTPUT [COP]
 Control of Heart Rate
• Heart rate is 70 beats/min, when there is
sympathetic and parasympathetic ANS effect
on SA –Node.
• If all autonomic nerves to the heart are
blocked, Heart Rate [HR] at rest will increase
to 100 beats/min, which is inherent rate of SA
– Node spontaneous discharge when there is
no influence of ANS.
9
CARDIAC OUTPUT [COP]
 Control of Heart Rate
• Our Heart Rate is 70/min as normal rate of SA
– Node discharge because of dominant effect
of parasympathetic ANS on SA – Node.
10
CARDIAC OUTPUT [COP]
We will discuss Stroke Volume:
 STROKE VOLUME
• It is amount of blood pumped out by each
ventricle per beat.
• Stroke volume can be increased by TWO
mechanism:
1. INTRINSIC CONTROL – by increasing venous
return to the heart
2. EXTRINSIC CONTROL – due to the sympathetic
stimulation of the heart
11
12
CARDIAC OUTPUT [COP]
 STROKE VOLUME [cont]
• Both factors( Intrinsic and Extrinsic ) increase
stroke volume by increasing the strength of heart
contraction.
• First, we will see mechanism of INTRINSIC control
– when there is increase in end – diastolic
volume, it results in increased stroke volume.
• It is due to length – tension relationship of
cardiac muscle.
13
FRANK STARLING LAW OF THE HEART
• When there is increase in initial length of cardiac
muscle fiber [within physiological limits], there
will be increased force of contraction.
OR
• When there is increased end- diastolic volume
[EDV], there is increased stroke volume [SV].
• This is INTRINSIC relationship between EDV and
SV, it is known as ‘Frank Starling Law of the
Heart’.
14
FRANK STARLING LAW OF THE HEART
Mechanism of Cardiac Length – Tension
Relationship
• When there is increase in the length of cardiac
muscle fiber to the optimal length, there is
maximum sliding of actin and myosin and we
get maximum contraction.
15
16
CARDIAC OUTPUT [COP]
 EXTRINSIC CONTROL [factors outside the heart]
• Extrinsic control is through sympathetic
stimulation.
• Sympathetic stimulation and epinephrine
increases heart contractility, at any given end –
diastolic volume.
• Increased contractility results from increased Ca2+
influx triggered by nor- epinephrine and
epinephrine.
17
CARDIAC OUTPUT [COP]
 EXTRINSIC CONTROL [cont]
• Example :
Normally EDV 135ml
ESV 65ml
Therefore, SV
70ml
• Under sympathetic stimulation
EDV 135ml
ESV 35ml
SV 100ml
• Frank Starling Curve shifts to the left by sympathetic
stimulation.
18
19
EJECTION FRACTION
• Ejection Fraction is ratio of Stroke Volume to
End – Diastolic Volume.
EF = [SV ÷ EDV] × 100
• Normal healthy heart has Ejection Fraction of
50 – 75% [55 – 65%] under resting conditions
and may go up to 90% during strenuous
exercise.
• A failing heart (cardiac failure)  EF maybe
30% or less.
20
CARDIAC OUTPUT [COP]
Control of Cardiac Output
21
Frank Starling Curve In Heart Failure
22
VENOUS RETURN TO THE HEART
• EDV depends on Venous Return and Venous
Return is increased to the heart by:
i). Blood Volume
ii). Skeletal Muscle Pump
iii). Respiratory Pump
iv). Increased Sympathetic Veno constriction
v). Cardiac suction effect
vi). Venous Valves
23
VENOUS RETURN TO THE HEART
i). Increased Blood Volume
• Veins are capacitance vessels and hold about 60
to 70% of blood, when veins store less blood,
more blood is returned to the heart.
ii). Skeletal Muscle Pump
• Muscle contraction compresses the veins.
• This external venous compression decreases
venous capacity and increases venous pressure
and moves blood towards the heart.
24
VENOUS RETURN TO THE HEART
iii). Respiratory Pump
• During respiration, intra-thoracic pressure
decreases and is less than atmospheric
pressure [-5 mmHg].
• This negative chest cavity pressure squeezes
blood from the lower veins to the chest,
increasing venous returns.
25
VENOUS RETURN TO THE HEART
iv). Increased Sympathetic Vasoconstriction
• Sympathetic Stimulation causes vasoconstriction,
which increases venous pressure and drives more
blood to right atrium, therefore, more venous returns
and increase EDV.
v). Cardiac suction effect
• Heart plays role in its own filling. During ventricular
contraction, AV valves are pulled downward enlarging
atrial cavities.
• Atrial pressure drops below 0 mmHg and increases
venous returns.
26
VENOUS RETURN TO THE HEART
vi). Venous Valves
• In the veins, blood can be driven forward only
as large veins have one way valve placed at 2
to 4 cm intervals.
• These valves prevent back flow of blood that
tends to occur when a person stands up.
27
28
Summary of Factors Affecting Venous
Returns
29
30
MEASUREMENT OF CARDIAC OUTPUT
• Cardiac Output can be measured
1. Fick Principle
2. Dye Dilution Method
3. Doppler Combined with Echocardiography
31
FICK PRINCIPLE
Output of Left Ventricle
Oxygen Uptake by lungs ml/min
=
AO2 - VO2
=
200 ml / min
200 ml / L – 160 ml / L
Art blood – Venous blood
[Pul artery]
= 200 ml/min
40ml / liter
= 5 L/min
32
CARDIAC OUTPUT IN EXERCISE
• Cardiac Output increases to 25 to 30 liters/min
because there is
- increased muscle pump
- increased respiratory pump
- increased sympathetic stimulation
- increased venomotor tone
- increased venous return
- increased force of contraction of heart [SV]
- increased heart rate
33
APPLIED
HEART FAILURE
What is Heart Failure ?
• It is inability of heart to give cardiac output,
sufficient to keep pace with body’s demand.
• There may be left ventricular failure or right
ventricular failure or bi – ventricular failure.
• Most common cause heart failure is
1. Heart Attack or Myocardial Infarction
2. Working against Increased after load e.g. hyper
tension or aortic valve stenosis
34
PRE LOAD & AFTER LOAD
• PRE LOAD – load on the heart before
contraction i.e. end – diastolic volume.
• AFTER LOAD – load against which ventricle has
to pump i.e. pressure in the artery or arterial
blood pressure.
35
SIGNS OF HEART FAILURE
• In Left Ventricular Failure – pulmonary congestion or
pulmonary edema occurs which causes decrease exchange
of O2 and CO2 in the lungs.
• In Right Ventricular Failure – due to back pressure, there is
engorgement of neck veins, peripheral edema, liver
enlargement.
• Heart failure is treated by
-- positive Inotropic drugs e.g. digitalis
-- diuretics - to get rid of salt and water
-- ACE Inhibitors [Angiotensin Converting Enzyme] inhibitors
which decrease preload and after load.
36
LIST OF POSITIVE INOTROPIC &
NEGATIVE INOTROPIC
 Positive Inotropic

- Epinephrine
- Nor Epinephrine
- Sympathetic Stimulation
- Calcium
- Caffeine
- Digitalis
Negative Inotropic
- Acetylcholine
- Parasympathetic Stimulation
- Potassium
- Hypoxia
- Hypercapnia
- Acidosis
- Drugs Beta Blocker
37
WHAT YOU SHOULD KNOW FROM
THIS LECTURE
•
•
•
•
•
•
•
•
•
•
Definition of Cardiac Output [COP]
Factors Affecting Heart Rate & Stroke Volume
Cardiac Index
Cardiac Reserve
Intrinsic & Extrinsic Control For Stroke Volume
Frank Starling Law of Heart
Ejection Fraction
Factor Affecting Venous Return
Heart Failure
Positive Inotropic & Negative Inotropic
38
THANK YOU
39