Cardiac output
Cardiac Output
Stroke volume, minute volume, cardiac index, cardiac reserve, variations, distribution, measurement and cardiac catheterization.
1 Definitions and Normal Values
Cardiac output is expressed in three ways namely stroke volume, minute volume and cardiac index.
However, in routine clinical practice cardiac output refers to minute volume.
Stroke Volume
Stroke volume is the amount of blood pumped out by each ventricle during each beat.
Minute Volume
Minute volume is the amount of blood pumped out by each ventricle in one minute.
It is the product of stroke volume and heart rate.
Cardiac Index
Cardiac index is the minute volume expressed in relation to square meter of body surface area.
It is defined as the amount of blood pumped out per ventricle/minute/square meter of the body surface area.
Average body surface area in an adult is 1.734 sq. M and normal minute volume is 5 L/min.
2 Ejection Fraction
3 Cardiac Reserve
Cardiac reserve plays an important role in increasing cardiac output during conditions such as exercise and is essential to withstand the stress of exercise.
Values of Cardiac Reserve
Cardiac reserve is usually expressed in percentage:
| Condition | Cardiac Reserve |
|---|---|
| Normal young healthy adult | 300 to 400% |
| Elders | 200 to 250% |
| Trained athletes | 500 to 600% |
| Cardiac diseases | Minimum or nil |
4 Variations in Cardiac Output
Physiological Variations
- Age: In children, cardiac output is less because of less blood volume. Cardiac index is more than in adults because of less body surface area.
- Sex: In females, cardiac output is less than in males, because of less body surface area.
- Body build: Greater the body build, more is the cardiac output.
- Diurnal variation: Cardiac output is low in early morning and increases in daytime.
- Environmental temperature: Moderate change in temperature does not affect cardiac output. Increase in temperature above 30°C raises cardiac output.
- Emotional conditions: Anxiety, apprehension and excitement increase cardiac output about 50 to 100%.
- After meals: During the first 1 hour after taking meals, cardiac output increases.
- Exercise: Cardiac output increases during exercise.
- High altitude: In high altitude, cardiac output increases.
- Posture: While changing from recumbent to upright position, the cardiac output decreases.
- Pregnancy: During the later months of pregnancy, cardiac output increases by 40%.
- Sleep: Cardiac output is slightly decreased or unaltered during sleep.
Pathological Variations
Conditions When Cardiac Output Increases
- Fever
- Anemia
- Hyperthyroidism
Conditions When Cardiac Output Decreases
- Hypothyroidism
- Atrial fibrillation
- Heart block
- Congestive cardiac failure
- Shock
- Hemorrhage
5 Distribution of Cardiac Output
Whole amount of blood pumped out by right ventricle goes to lungs.
But 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 upon the metabolic rate of that region or organ.
Distribution of Blood Pumped Out of Left Ventricle
Distribution of blood pumped out of left ventricle to different organs and the percentage of cardiac output are given below.
| Organ | Amount of Blood (mL/min) | Percentage |
|---|---|---|
| Liver | 1,500 | 30 |
| Kidney | 1,300 | 26 |
| Skeletal muscles | 900 | 18 |
| Brain | 800 | 16 |
| Skin, bone and GI tract | 300 | 6 |
| Heart | 200 | 4 |
| Total | 5,000 | 100 |
6 Factors Maintaining Cardiac Output
Cardiac output is maintained by four factors:
- Venous return
- Force of contraction
- Heart rate
- Peripheral resistance
1. Venous Return
Venous return is the amount of blood, which is returned to heart from different parts of the body.
When venous return increases, the ventricular filling and cardiac output are increased.
Thus, cardiac output is directly proportional to venous return provided the other three factors (force of contraction, heart rate and peripheral resistance) remain constant.
Venous return in turn depends upon two pumps:
Respiratory Pump
Respiratory pump is the respiratory activity that helps return of blood back to heart during inspiration.
It is also called abdominothoracic pump.
During inspiration, thoracic cavity expands and makes the intrathoracic pressure more negative.
It increases the diameter of inferior vena cava resulting in increased venous return.
At the same time, descent of diaphragm increases the intra-abdominal pressure.
This compresses the abdominal veins and pushes the blood upward towards heart and thereby the venous return is increased.
Respiratory pump is much stronger in forced inspiration and in muscular exercise.
Muscle Pump
Muscle pump is the muscular activity that helps return of blood back to heart.
When muscular activity increases, the venous return is more.
When skeletal muscles contract, the vein located in between the muscles is compressed.
Valve of the vein proximal to contracting muscles is opened and blood is propelled towards the heart.
Valve of the vein distal to the muscle is closed by the back flow of blood.
2. Force of Contraction
Cardiac output is directly proportional to the force of contraction provided the other three factors remain constant.
Force of contraction depends upon diastolic period and ventricular filling.
Frank-Starling law of heart is applicable to this.
According to Frank-Starling law, the force of contraction of heart is directly proportional to initial length of muscle fibers before the onset of contraction.
Force of contraction also depends upon preload and afterload.
Preload
Preload is the stretching of the cardiac muscle fibers at the end of diastole just before contraction.
Preload depends upon venous return and ventricular filling.
During diastolic period due to the ventricular filling, ventricular pressure increases.
This causes stretching of muscle fibers resulting in increase in their length.
Increase in length of the muscle fibers increases the force of contraction and cardiac output.
Afterload
Afterload is the force against which the ventricles must contract and eject the blood.
This force is determined by the arterial pressure.
At the end of isometric contraction period, semilunar valves are opened and blood is ejected into the aorta and pulmonary artery.
So, the pressure increases in these two vessels.
The ventricle has to work against this pressure for further ejection.
Thus, the afterload for left ventricle is determined by aortic pressure and afterload for right ventricle is determined by pressure in pulmonary artery.
3. Heart Rate
Cardiac output is directly proportional to heart rate provided other three factors remain constant.
Moderate change in heart rate does not alter the cardiac output.
If there is a marked increase in heart rate, cardiac output is increased.
If there is marked decrease in heart rate, cardiac output is decreased.
4. Peripheral Resistance
Peripheral resistance is the resistance offered to blood flow at the peripheral blood vessels.
It is the resistance or load against which heart has to pump the blood.
Resistance is offered at arterioles.
So, the arterioles are called resistant vessels.
In the body, the maximum peripheral resistance is offered at the splanchnic region.
7 Measurement of Cardiac Output
Cardiac output is measured by direct methods and indirect methods.
Direct methods are used only in animals.
Indirect methods are used both in animals and human beings.
Several indirect methods are available to measure cardiac output in human beings.
Each method has got its own advantages and disadvantages.
Generally, the safe and accurate method is preferred.
In view of safety, always non-invasive methods are preferred to measure cardiac output.
Invasive and Non-Invasive Methods
8 Measurement by Using Fick’s Principle
Modification of Fick Principle to Measure Cardiac Output
Fick’s principle is modified to measure the cardiac output or a part of cardiac output (amount of blood to an organ).
Thus, cardiac output or amount of blood flowing through an organ in a given unit of time is determined by the formula given below.
By using Fick’s principle, cardiac output is measured in two ways:
- By using oxygen consumption.
- By using carbon dioxide given out.
Measurement of Cardiac Output by Using Oxygen Consumption
Fick’s principle is used to measure cardiac output by determining the amount of oxygen consumed in body in a given period of time and dividing this value by arteriovenous difference across the lungs.
Thus, amount of oxygen moved from lungs to blood is 250 mL/min.
Measurement of Cardiac Output by Using Oxygen Consumption
Amount of oxygen consumed is measured by using a respirometer or BMR apparatus (Benedict-Roth apparatus).
Oxygen Content in Arterial Blood
Blood is collected from any artery to determine the oxygen content in arterial blood.
Oxygen content is determined by blood gas analysis.
Oxygen Content in Venous Blood
Only mixed venous blood is used to determine the oxygen content of venous blood, since oxygen content is different in different veins.
Mixed venous blood is collected from right atrium.
From this blood oxygen content is measured by blood gas analysis.
Calculation
5 mL of oxygen is taken by 100 mL of blood while passing through the lungs.
Thus, 250 mL of oxygen is taken by 5,000 mL of blood.
Since, cardiac output is equivalent to the amount of blood passing through pulmonary circulation, the cardiac output = 5 L/min.
Measurement of Cardiac Output by Using Carbon Dioxide
Cardiac output is also measured by knowing the arteriovenous difference of carbon dioxide and amount of carbon dioxide given out from lungs.
Since, cardiac output is equal to amount of blood passing through lungs (pulmonary circulation), the cardiac output is 5 L/min.
Advantages and Disadvantages of Measurement of Cardiac Output by Fick Principle
However, this is an invasive method and involves insertion of catheter through subject’s vein.
9 Other Methods of Measurement of Cardiac Output
1. Indicator (Dye) Dilution Method
Indicator dilution technique is described in detail elsewhere.
Marker substance used to measure cardiac output is lithium chloride.
Advantage and Disadvantage
Results are accurate.
But it is an invasive method and involves injection of marker substance.
2. Thermodilution Technique
Cardiac output can also be measured by thermodilution technique or thermal indicator method.
This method is the modified indicator dilution method.
It is the popular method to measure cardiac output.
In this method, a known volume of cold sterile solution is injected into the right atrium by using a catheter.
Cardiac output is measured by determining the resultant change in the blood temperature in pulmonary artery.
For this purpose, two thermistors (temperature transducers) are used.
One of them is placed in the inferior vena cava and the second one is placed in pulmonary artery.
A pulmonary artery catheter is used to place the thermistors in their positions.
Thermistors determine the temperature of blood entering the heart via inferior vena cava and temperature of blood leaving the heart via pulmonary artery.
From the values of temperature, cardiac output is measured by applying indicator dilution technique.
Continuous Cardiac Output Measurement Catheter
Cardiac output can be measured continuously by using a modified pulmonary artery catheter called continuous cardiac output measurement catheter (CCO catheter).
CCO catheter works on thermodilution principle.
Instead of injecting cold saline, a heating filament which delivers heat directly to blood is used.
The heating filament is fitted to the ventricular portion of the catheter.
Cardiac output is measured as done in thermodilution technique.
This method is commonly used in intensive care unit (ICU).
Advantages and Disadvantages
Results are accurate in this method.
Even low cardiac output can be measured.
Saline is also harmless.
Catheter is also used to determine hemodynamic pressures and to collect mixed venous blood.
Thermodilution technique is an invasive method and it requires catheterization.
3. Esophageal Doppler Transducer Technique
Principle: Doppler Effect
Ultrasound which has very high frequency is beyond the audible range of human ears.
Waves of ultrasound are transmitted through a blood vessel.
While passing through blood vessels, the sound waves hit against blood cells, particularly red blood cells, and are reflected back.
Frequency of reflected waves is different from that of transmitted waves.
This effect is called the Doppler effect.
Alteration in the frequency of reflected waves depends upon velocity of blood flowing through blood vessel.
By detecting the differences between frequencies of transmitted and reflected sound waves, velocity of blood flow and then the volume of blood are determined.
Procedure
A flexible probe is inserted into midthoracic part of esophagus.
A pulse wave ultrasonic Doppler transducer is fixed at the tip of the probe.
This transducer calculates the velocity of blood flow in descending aorta.
Diameter of aorta is determined by echocardiography.
Cardiac output is calculated by using the values of velocity of blood flow and diameter of aorta.
Advantages and Disadvantages
Cardiac output can be measured continuously.
This can be used during cardiac surgery.
Esophageal ultrasonic Doppler transducer is an invasive method and results are less accurate.
4. Doppler Echocardiography
Doppler echocardiography is a method for detecting the direction and velocity of moving blood within the heart.
It is also a popular method to measure cardiac output.
Echocardiography is a diagnostic procedure, which uses ultrasound waves (>20,000 Hz) to produce the image of heart.
Ultrasound waves which reflect or echo off heart can determine the size, shape, movement of the valves and chambers and the flow of blood through the heart.
During echocardiographic examination, the patient lies bare-chested on examination table.
A special gel is spread over chest on which the transducer can make good contact and slide smoothly over the skin.
Transducer is a small hand operated probe attached to the machine by a flexible cable.
The transducer is placed against chest.
The transducer produces and directs ultrasound waves into chest.
Some of the waves get reflected back to transducer.
Reflection of sound waves depends upon type of tissues and blood.
Reflected sound waves are received by transducer and converted into an image of heart and displayed on a monitor or recorded on paper or tape.
Echocardiography may also show the abnormalities in functioning of heart or damage to myocardium from an earlier heart attack.
Advantage and Disadvantage
Doppler echocardiography is a non-invasive technique.
It also provides other useful information about structures and movements of valves and chambers of the heart.
Doppler echocardiography method provides less accurate results.
It requires well trained operator.
5. Ballistocardiographic Method
Ballistocardiography is the technique to record the movements of body caused by ballistic recoil associated with contraction of heart and ejection of blood.
It is based on Newton’s third law of motion (for every action there is an equal and opposite reaction).
When heart pumps blood into aorta and pulmonary artery, a recoiling force is exerted against heart and the body.
It is similar to that of ballistic recoil when a bullet is fired from a riffle.
Pulsations due to this ballistic recoil can be recorded graphically by making the subject to lie on a suspended bed movable in the long axis of the body.
Cardiac output is determined by analyzing the graph obtained.
Advantage and Disadvantage
It is a noninvasive method.
But it is not a commonly used technique because it involves cumbersome procedures for calibrating the equipment and analyzing the graph.
It also does not provide accurate results.
6. Transthoracic Electrical Bioimpedance Method
Transthoracic electrical bioimpedance (TEB) or impedance cardiography (ICG) is a noninvasive method for continuous cardiac output monitoring.
A high-frequency, low amplitude current passed across the thorax by placing electrodes on neck and sides of thorax.
Pulsatile changes in blood flow through thoracic blood vessels causes changes in electrical conductivity in thorax.
By measuring the changes in voltage, stroke volume and continuous cardiac output are determined.
Advantage
It is a noninvasive technique and useful for continuous cardiac output monitoring.
10 Cardiac Catheterization
Cardiac catheterization is an invasive procedure in which a catheter is inserted intravascularly into any chamber of the heart or a blood vessel.
Cardiac catheterization is helpful to study different variables of hemodynamics, both in normal and diseased conditions.
Cardiac catheterization was discovered by a German medical student Werner Forssmann, who practiced this technique first on himself.
Conditions When Cardiac Catheterization is Performed
- When clinical assessments indicate rapid deterioration of patient’s health and immediate treatment. This is the most common condition when cardiac catheterization is needed.
- Whenever there is a need to confirm the suspected cardiac disease of a patient.
- Whenever there is a need to determine anatomical structures of heart and blood vessels.
Procedure
Cardiac catheterization is performed by insertion of catheter into the peripheral blood vessel through skin, by needle puncture.
This procedure is called percutaneous insertion of catheter.
Uses of Cardiac Catheterization
Cardiac catheterization is useful for both diagnostic and therapeutic purposes.
It gives crucial information about the need for cardiac surgery, coronary angioplasty and other therapeutic procedures.
It also gives information about anticipated risks and reversibility in the patient’s condition during cardiac surgery or other therapeutic interventions.
Diagnostic Uses of Cardiac Catheterization
- Blood samples are collected during cardiac catheterization to measure oxygen saturation and the concentration of ischemic metabolites like lactate.
- Cardiac output is measured by using Fick principle, indicator dilution technique or thermodilution technique during cardiac catheterization.
- Angiography is done with the help of catheterization. Angiography or arteriography is the diagnostic or therapeutic radiography (imaging technique).
- In this technique, the fluoroscopic picture is used to visualize the blood-filled structures such as cardiac chambers, arteries, veins of heart and other blood vessels, by using a radiopaque contrast medium.
- It is used to determine the obstruction or occlusion of coronary blood vessels or other blood vessels.
- It is also used to determine the anomalies of coronary blood vessels.
- Various pressures are determined by attaching a pressure transducer to the cardiac catheter.
Right Heart Catheterization
Right heart catheterization is used to measure:
- Right atrial pressure
- Right ventricular pressure
- Pulmonary arterial pressure
- Pulmonary capillary wedge pressure
Left Heart Catheterization
Left heart catheterization is used to measure:
- Aortic pressure
- Left ventricular pressure
- Left atrial pressure
Therapeutic Uses of Cardiac Catheterization
Interventional cardiology is a branch of cardiology that deals with nonsurgical cardiovascular treatment by using intravascular catheter-based techniques.
Interventional cardiology helps in following procedures:
- Thrombolysis
- Percutaneous transluminal coronary angioplasty
- Laser coronary angioplasty
- Catheter ablation