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Chapter 98 - Cardiac Output (Page 575)

Muscle Pump (continued)

During contraction (Fig. 98.2 A):
  • The valve proximal to the contracting muscles opens.
  • Blood is pushed toward the heart.
  • The valve distal to the muscles closes, preventing backflow.
During relaxation (Fig. 98.2 B):
  • The valve proximal to the muscles closes, preventing backflow.
  • The valve distal to the muscles opens, allowing blood to flow upward.

iii. Gravity

  • Gravitational force reduces venous return.
  • When a person stands for a long time, gravity causes blood to pool in the legs - this is called venous pooling.
  • Venous pooling decreases the amount of blood returning to the heart.

iv. Venous Pressure

  • Venous pressure also affects venous return.
  • Pressure in venules = 12 to 18 mm Hg.
  • Pressure gradually decreases in smaller and larger veins.
  • In great veins (inferior vena cava, superior vena cava) = ~5.5 mm Hg.
  • At the junction of venae cavae and right atrium = ~4.6 mm Hg.
  • Pressure in the right atrium is low and changes during cardiac action.
  • It falls to zero during atrial diastole.
  • This pressure gradient throughout the venous tree acts as a driving force for venous return.

v. Sympathetic Tone

  • Venous return is helped by sympathetic (vasomotor) tone (Chapter 103).
  • It causes constriction of venules.
  • Venoconstriction pushes blood toward the heart.

2. Force of Contraction

  • Cardiac output is directly proportional to the force of contraction (other three factors remain constant).
  • According to Frank-Starling Law: force of contraction is directly proportional to the initial length of muscle fibers, before contraction begins.
  • Force of contraction depends on preload and afterload.

Preload

  • Preload = stretching of cardiac muscle fibers at the end of diastole, just before contraction.
  • Caused by increased ventricular pressure due to blood filling during diastole.
  • Stretching of muscle fibers → increases their length → increases force of contraction and cardiac output.
  • Therefore, cardiac output is directly proportional to preload.

Afterload

  • Afterload = the force the ventricles must work against to eject blood.
  • It is determined by arterial pressure.
  • At the end of isometric contraction, semilunar valves open and blood is ejected into the aorta and pulmonary artery → pressure increases in these vessels.
  • Ventricles must work against this pressure for further ejection.
  • Afterload for left ventricle = aortic pressure.
  • Afterload for right ventricle = pulmonary artery pressure.
  • Cardiac output is inversely proportional to afterload.

3. Heart Rate

  • Cardiac output is directly proportional to heart rate (other three factors remain constant).
  • Moderate change in heart rate → does not alter cardiac output.
  • Marked increase in heart rate → cardiac output increases.
  • Marked decrease in heart rate → cardiac output decreases.

4. Peripheral Resistance

  • Peripheral resistance = resistance to blood flow at the peripheral blood vessels.
  • It is the load against which the heart must pump.
  • Cardiac output is inversely proportional to peripheral resistance.

Page 576 - Cardiovascular System

  • Resistance is mainly offered at arterioles → arterioles are called resistant vessels.
  • Maximum peripheral resistance is at the splanchnic region.
  • More details in Chapter 102.

Measurement of Cardiac Output

  • Cardiac output is measured by direct and indirect methods.
  • Direct methods - used only in animals.
  • Indirect methods - used in both animals and humans.

Ultrasonic Doppler Flowmeter

Principle:
  • Uses ultrasound at high frequency (beyond the range of human hearing).
  • Ultrasound waves are transmitted through a blood vessel - these are called transmitted waves.
  • The probe encircles the blood vessel and connects to an electronic device to measure blood flow volume.
  • Advantage: the blood vessel does not need to be cut open.
Key Proportionality Summary:
FactorRelationship to Cardiac Output
Force of contractionDirect
PreloadDirect
AfterloadInverse
Heart rateDirect
Peripheral resistanceInverse
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Chapter 98 - Cardiac Output

Section 8 - Cardiovascular System (Pages 574-576)

Distribution of Cardiac Output

  • The fraction of cardiac output going to a particular region or organ depends on the metabolic activity of that region or organ.

Distribution of Blood Pumped Out of Left Ventricle

OrganBlood Flow (mL/min)Percentage
Liver1,50030%
Kidney1,30026%
Skeletal muscles90018%
Brain80016%
Skin, bone, GI tract3006%
Heart2004%
Total5,000100%
Note: The heart pumps blood to ALL organs but receives the least amount itself. The liver receives the maximum amount.

Factors Maintaining Cardiac Output

Cardiac output is maintained by four factors:
  1. Venous return
  2. Force of contraction
  3. Heart rate
  4. Peripheral resistance

1. Venous Return

  • Venous return = amount of blood returned to the heart from different parts of the body.
  • When venous return increases → ventricular filling increases → cardiac output increases.
  • Cardiac output is directly proportional to venous return (when force of contraction, heart rate, and peripheral resistance remain constant).
Venous return depends on five factors:
  • i. Respiratory pump
  • ii. Muscle pump
  • iii. Gravity
  • iv. Venous pressure
  • v. Sympathetic tone

i. Respiratory Pump

  • Respiratory pump = the respiratory activity that helps return blood to the heart during inspiration.
  • Also called the abdominothoracic pump.
During inspiration:
  • Thoracic cavity expands → intrathoracic pressure becomes more negative.
  • This increases the diameter of the inferior vena cava → venous return increases.
  • At the same time, the diaphragm descends → intra-abdominal pressure increases → compresses abdominal veins → pushes blood upward toward the heart → venous return increases further. (Fig. 98.1)
Respiratory pump is much stronger during forced respiration and severe muscular exercise.

ii. Muscle Pump

  • Muscle pump = muscular activity that helps return blood to the heart.
  • During muscular activity, veins are compressed/squeezed.
  • Due to valves in veins, compression moves blood toward the heart. (Fig. 98.2)
  • More muscular activity = more venous return.
During muscle contraction (Fig. 98.2 A):
  • Valve proximal to the contracting muscle opens.
  • Blood is pushed toward the heart.
  • Valve distal to the muscle closes → prevents backflow.
During muscle relaxation (Fig. 98.2 B):
  • Valve proximal to the muscle closes → prevents backflow.
  • Valve distal to the muscle opens → blood flows upward.

iii. Gravity

  • Gravitational force reduces venous return.
  • Long standing → gravity causes blood to pool in legs = venous pooling.
  • Venous pooling → less blood returns to the heart.

iv. Venous Pressure

  • Venous pressure also affects venous return.
  • Pressure in venules = 12 to 18 mm Hg.
  • Pressure gradually falls in smaller and larger veins.
  • In great veins (IVC, SVC) = ~5.5 mm Hg.
  • At the junction of venae cavae and right atrium = ~4.6 mm Hg.
  • Right atrial pressure is low and changes during cardiac action; falls to zero during atrial diastole.
  • This pressure gradient across the venous tree acts as a driving force for venous return.

v. Sympathetic Tone

  • Venous return is aided by sympathetic (vasomotor) tone (Chapter 103).
  • It causes constriction of venules (venoconstriction).
  • Venoconstriction pushes blood toward the heart.

2. Force of Contraction

  • Cardiac output is directly proportional to force of contraction (other three factors constant).
  • Frank-Starling Law: force of contraction is directly proportional to the initial length of muscle fibers before contraction.
  • Force of contraction depends on preload and afterload.

Preload

  • Preload = stretching of cardiac muscle fibers at end of diastole, just before contraction.
  • Caused by increased ventricular pressure from blood filling during diastole.
  • Stretching → longer fibers → greater force of contraction → higher cardiac output.
  • Cardiac output is directly proportional to preload.

Afterload

  • Afterload = force the ventricles must overcome to eject blood, determined by arterial pressure.
  • At end of isometric contraction → semilunar valves open → blood ejected into aorta and pulmonary artery → pressure rises in these vessels.
  • Ventricles must work against this raised pressure for further ejection.
  • Afterload for left ventricle = aortic pressure.
  • Afterload for right ventricle = pulmonary artery pressure.
  • Cardiac output is inversely proportional to afterload.

3. Heart Rate

  • Cardiac output is directly proportional to heart rate (other three factors constant).
  • Moderate change in heart rate → no significant change in cardiac output.
  • Marked increase in heart rate → cardiac output increases.
  • Marked decrease in heart rate → cardiac output decreases.

4. Peripheral Resistance

  • Peripheral resistance = resistance to blood flow at peripheral blood vessels.
  • It is the load against which the heart pumps.
  • Cardiac output is inversely proportional to peripheral resistance.
  • Resistance is mainly at arterioles → called resistant vessels.
  • Maximum peripheral resistance is in the splanchnic region.
  • More details in Chapter 102.

Measurement of Cardiac Output

  • Measured by direct and indirect methods.
  • Direct methods - used only in animals.
  • Indirect methods - used in both animals and humans.

Ultrasonic Doppler Flowmeter

Principle:
  • Uses ultrasound at high frequency (beyond human hearing range).
  • Ultrasound waves are transmitted through a blood vessel = transmitted waves.
  • Probe encircles the blood vessel and connects to an electronic device to measure blood flow volume.
  • Advantage: blood vessel does not need to be cut open.

Quick Summary Table

FactorRelationship to Cardiac Output
Venous returnDirect
Force of contractionDirect
PreloadDirect
AfterloadInverse
Heart rateDirect
Peripheral resistanceInverse
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Chapter 95 - Cardiac Output

Chapter Outline

  • Definitions and Normal Values
  • Ejection Fraction
  • Cardiac Reserve
  • Variations in Cardiac Output
  • Pathological Variations
  • Distribution of Cardiac Output
  • Factors Maintaining Cardiac Output
  • Venous Return
  • Force of Contraction
  • Heart Rate
  • Peripheral Resistance
  • Measurement of Cardiac Output
  • Cardiac Catheterization

Definitions and Normal Values

  • Cardiac output = amount of blood pumped from each ventricle per minute.
  • It refers to left ventricular output.
  • It is the most important factor in the cardiovascular system because the rate of blood flow through different parts of the body depends on cardiac output.
  • Cardiac output is expressed in three ways: stroke volume, minute volume, and cardiac index.
  • In clinical practice, cardiac output refers to minute volume.

1. Stroke Volume

  • Stroke volume = amount of blood pumped out by each ventricle during each beat.
  • It is the difference between end-diastolic volume and end-systolic volume (Chapter 88).
Stroke volume = End-diastolic volume - End-systolic volume

2. Minute Volume

  • Normal value: 70 mL (60-80 mL) when heart rate is normal (72 per minute).
  • Minute volume = amount of blood pumped by each ventricle in 1 minute.
  • It is the product of stroke volume and heart rate.
Minute Volume = Stroke Volume × Heart Rate

3. Cardiac Index

  • Cardiac index = minute volume expressed in relation to body surface area.
  • It is defined as the amount of blood pumped out per ventricle per minute/square meter of body surface area.
  • In adults, body surface area = 1.73 square meters.
  • Normal value: 2.8 ± 0.3 L/m²/min (i.e., per square meter of body surface area per minute, normal minute volume = 5 L/min).

Ejection Fraction

  • Ejection fraction = fraction of end-diastolic volume that is ejected out by each ventricle.
  • Normal ejection fraction = 60 to 65% (refer Chapter 88 for details).

Cardiac Reserve

  • Cardiac reserve = maximum amount of blood that can be pumped by the heart above the normal value.
  • Cardiac reserve plays an important role in increasing cardiac output during conditions like exercise.
  • It is essential to withstand the stress of exercise.

Values of Cardiac Reserve:

  1. Normal young healthy adult: 300 to 400%
  2. Old aged person: 200 to 250%
  3. Trained athletes: 500 to 600%
  4. In cardiac diseases: minimum or nil

Physiological Variations in Cardiac Output

1. Age

  • In cardiac children, cardiac index is more than in adults because of less blood volume.

2. Sex

  • In females, cardiac output is less than in males because of less body surface area.
  • Cardiac index is more in females because of less body surface area (less denominator).

3. Body Build

  • Greater the body build, more is the cardiac output.

4. Diurnal Variation

  • Cardiac output is low in early morning and increases in daytime.
  • It depends on the basal conditions of the individuals.

5. Environmental Temperature

  • Moderate change in temperature does not affect cardiac output.
  • Increase in temperature above 30°C raises cardiac output.

6. Emotional Conditions

  • Anxiety, apprehension, and excitement increase cardiac output about 50 to 100% through release of catecholamines which increase heart rate and force of contraction.

7. After Meals

  • During the first 1 hour after taking meals, cardiac output increases.

8. Exercise

  • Cardiac output increases during exercise because of increase in heart rate and force of contraction.

9. High Altitude

  • In high altitude, cardiac output increases because of hypoxia.

10. Posture

  • While changing from recumbent to upright position, cardiac output decreases.

11. Pregnancy

  • During later months of pregnancy, cardiac output increases by 40%.

12. Sleep

  • Cardiac output is slightly decreased or unchanged during sleep.

Pathological Variations - Applied Physiology

Increase in Cardiac Output:

  1. Fever - due to increased oxidative processes.
  2. Anemia - due to hypoxia.
  3. Hyperthyroidism - due to increased basal metabolic rate.

Decrease in Cardiac Output:

  1. Hypothyroidism - due to decreased basal metabolic rate.
  2. Atrial fibrillation - because of incomplete filling of ventricles.
  3. Incomplete heart block with coronary sclerosis - because of defective pumping of the heart.
  4. Congestive cardiac failure - because of less contractions of the heart.
  5. Shock - due to poor pumping and circulation.
  6. Hemorrhage - because of decreased blood volume.

Distribution of Cardiac Output

  • The whole amount of blood pumped out by right ventricle goes to lungs.
  • Blood pumped by left ventricle is distributed to different parts of the body.
  • Fraction of cardiac output distributed to a particular region or organ depends on the metabolic activities of that region or organ.

Distribution of Blood Pumped Out of Left Ventricle:

OrganBlood Flow (mL/min)Percentage
Liver1,50030%
Kidney1,30026%
Skeletal muscles90018%
Brain80016%
Skin, bone, GI tract3006%
Heart2004%
Total5,000100%
Heart pumps blood to all other organs but receives the least amount. Liver receives the maximum amount.

Factors Maintaining Cardiac Output

Cardiac output is maintained by four factors:
  1. Venous return
  2. Force of contraction
  3. Heart rate
  4. Peripheral resistance

1. Venous Return

  • Venous return = amount of blood returned to the heart from different parts of the body.
  • When it increases → ventricular filling and cardiac output increase.
  • Cardiac output is directly proportional to venous return (when force of contraction, heart rate, and peripheral resistance remain constant).
Venous return depends on five factors:
  • i. Respiratory pump
  • ii. Muscle pump
  • iii. Gravity
  • iv. Venous pressure
  • v. Sympathetic tone

i. Respiratory Pump

  • Respiratory pump = respiratory activity that helps return blood to the heart.
  • Also called abdominothoracic pump.
During inspiration:
  • Thoracic cavity expands → intrathoracic pressure becomes more negative.
  • Diameter of inferior vena cava increases → venous return increases.
  • At the same time, diaphragm descends → intra-abdominal pressure increases → compresses abdominal veins → pushes blood upward toward the heart → venous return increases. (Fig. 95.1)
  • Respiratory pump is much stronger in forced respiration.

ii. Muscle Pump

  • Muscle pump = muscular activity that helps return blood to the heart.
  • During muscular activity, veins are compressed/squeezed.
  • Due to valves in veins, compression moves blood toward the heart. (Fig. 95.2)
  • More muscular activity = more venous return.
  • Muscle pump is much stronger in severe muscular exercise.
During muscle contraction (Fig. 95.2 A):
  • Valve proximal to contracting muscles opens.
  • Blood is pushed toward the heart.
  • Valve distal to the muscle closes → prevents backflow.
During muscle relaxation (Fig. 95.2 B):
  • Valve proximal to muscles closes → prevents backflow.
  • Valve distal to muscles opens → blood flows upward.

iii. Gravity

  • Gravitational force reduces venous return.
  • Long standing → gravity causes blood to pool in legs = venous pooling.
  • Venous pooling → less blood returns to the heart.

iv. Venous Pressure

  • Venous pressure also affects venous return.
  • Pressure in venules = 12 to 18 mm Hg.
  • Pressure gradually decreases in smaller and larger veins.
  • In great veins (IVC, SVC) = ~5.5 mm Hg.
  • At the junction of venae cavae and right atrium = ~4.6 mm Hg.
  • Right atrial pressure is low; falls to zero during atrial diastole.
  • This pressure gradient acts as a driving force for venous return.

v. Sympathetic Tone

  • Venous return is aided by sympathetic (vasomotor) tone (Chapter 10).
  • It causes constriction of venules (venoconstriction).
  • Venoconstriction pushes blood toward the heart.

2. Force of Contraction

  • Cardiac output is directly proportional to force of contraction (other three factors constant).
  • Frank-Starling Law: force of contraction is directly proportional to the initial length of muscle fibers before contraction.
  • Force of contraction depends on preload and afterload.

Preload

  • Preload = stretching of cardiac muscle fibers at end of diastole, just before contraction.
  • Caused by increased ventricular pressure from blood filling during diastole.
  • Stretching → longer fibers → greater force of contraction and cardiac output.
  • Cardiac output is directly proportional to preload.

Afterload

  • Afterload = force the ventricles must overcome to contract and eject blood.
  • Determined by arterial pressure.
  • At end of isometric contraction → semilunar valves open → blood ejected into aorta and pulmonary artery → pressure rises in these vessels.
  • Ventricles must work against this raised pressure for further ejection.
  • Afterload for left ventricle = aortic pressure.
  • Afterload for right ventricle = pulmonary artery pressure.
  • Cardiac output is inversely proportional to afterload.

3. Heart Rate

  • Cardiac output is directly proportional to heart rate (other three factors constant).
  • Moderate change in heart rate → no significant change in cardiac output.
  • Marked increase in heart rate → cardiac output increases.
  • Marked decrease in heart rate → cardiac output decreases.

4. Peripheral Resistance

  • Peripheral resistance = resistance offered to blood flow at peripheral blood vessels.
  • It is the load against which the heart pumps.
  • Cardiac output is inversely proportional to peripheral resistance.
  • Resistance is mainly at arterioles → called resistant vessels.
  • Maximum peripheral resistance is in the splanchnic region.

Measurement of Cardiac Output

  • Cardiac output is measured by direct and indirect methods.
  • Direct methods - used only in animals.
  • Indirect methods - used in both animals and humans.

Measurement by Direct Methods

Two main instruments:

1. Mechanical Flowmeter

  • Used to measure the amount of blood flow to any organ or the artery entering any organ.
  • The flowmeter is inserted into the cut ends of a blood vessel.
  • When blood passes through the flowmeter, the measuring device determines the amount of blood flow.
  • Flowmeter is of different types.
  • Other details are given in Chapter 99.

2. Electromagnetic Flowmeter

Principle:
  • This flowmeter has two coils of wire.
  • When placed around a blood vessel, an electromagnetic field is produced around the vessel.
  • When blood flows through the vessel, there is an alteration in the electromagnetic field.
  • By connecting appropriate electrodes, the changes in the electromagnetic field can be detected.
  • From the basis of changes in the magnetic field, the volume of blood flow is calculated.
Instrument:
  • An electromagnetic probe is used with the magnetic coils and electrodes.
  • The probe is fixed in such a way that the blood vessel passes through the cleft.
  • The probe is connected to an electronic device to measure blood flow volume.
  • Advantage: blood vessel does not need to be cut open.

3. Ultrasonic Doppler Flowmeter

Principle:
  • Uses ultrasound at very high frequency (beyond the audible range of human ears).
  • These waves are transmitted through a blood vessel = transmitted waves.
  • While passing through blood vessels, the sound waves hit against blood cells and are reflected = reflected waves.
  • Alteration in the velocity of these waves is detected by the electronic equipment to determine the volume of blood flow.
Disadvantages of Ultrasonic Doppler Flowmeter:
  • i. Can be used only for large blood vessels.
  • ii. Blood vessel size affects the reading.
  • iii. Diameter of the vessel must be equivalent to the size of the cardiometer(?).

Invasive and Non-Invasive Methods

  • Invasive methods: involve invasion or penetration of the body by injection or catheter.
  • Non-invasive methods: do not involve invasion of the body.
  • Generally, a safe method is also a useful method.
  • Each method has its own advantages and characteristics.
  • In addition to providing measurements, these methods provide useful information about valves and chambers.

Quick Summary Table

FactorRelationship to Cardiac Output
Venous returnDirect
Force of contractionDirect
PreloadDirect
AfterloadInverse
Heart rateDirect
Peripheral resistanceInverse
Cardiac ReserveValue
Normal young healthy adult300-400%
Old aged person200-250%
Trained athletes500-600%
Cardiac diseaseMinimum or nil
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